Bridge-Rectifier LED Indicator

Using a few diodes and a LED, you can make a nice indicator as shown in associated schematic diagram that can be used for a lot of applications (with a bit of luck). It’s quite suitable for use in series with a doorbell or thermostat (but don’t try to use it with an electronically con-trolled central-heating boiler!). This approach allows you to make an attractive indicator for just a few pennies.


Bridge-Rectifier-LED-Indicator image


The AC or DC current through the circuit causes a voltage drop across the diodes that is just enough to light the LED. As the voltage is a bit on the low side, old-fashioned red LEDs are the most suitable for this purpose. Yellow and green LEDs require a somewhat higher forward voltage, so you’ll have to first check whether it works with them. Blue and white LEDs are not suitable. You also don’t have to use modern high-efficiency types (sometimes called ‘2-mA LEDs’ or ‘3-mA LEDs’). If a DC current flows through the circuit and the LED doesn’t light up, reverse the plus and minus leads.


Circuit diagram :




Bridge-Rectifier  LED Indicator Circuit Diagram


When building the circuit, you’ll notice that despite its simplicity it involves fitting quite a few components to a

small printed circuit board or a bit of prototyping board. That’s why we’d like to give you the tip of using a bridge rectifier, since that allows everything to be made much more compact, smaller and more tidy, and it eliminates the need for a circuit board to hold the components. Besides that, you can surprise friend and foe alike, because even an old hand in the trade won’t understand the trick at first glance and will likely mumble something like “Huh?

That’s impossible.”


A bridge rectifier contains four diodes, which is exactly what you need. If you short the + and – terminals of the bridge, you create a circuit with two pairs of diodes connected in parallel with oppo-site polarity. Select a bridge rectifier that can handle the current that will flow through it. In the case of a doorbell, for example, that can easily be 1 A. Select a voltage of 40 or 80 V.


Never use this circuit in combination with mains voltage, due to the risk of contact with a live lead.


Author: Karel Walraven - Copyright: Elektor

Stable USB Power Supply

A common problem when an AC mains adapter is used to power a USB device is that the voltage does not match the nominal 5 V specified by the USB standard. The circuit shown here accepts an input voltage in the range of 4-9 V and converts it into a 6-V output voltage, which is then stabilized to a clean 5-V level by a series regulator. The combined boost/buck converter used here operates on the SEPIC principle. That principle is quite similar to the operating principle of the Cuk converter, but without the disadvantage of a negative output voltage.


Circuit diagram :



Stable USB Power Supply Circuit Diagram


The circuit is built around a MAX668, which is intended to be used as a controller for boost converters. The difference between a SEPIC converter and a standard boost (step-up) converter is that the former type has an additional capacitor (in this case C2) and a second inductor (in this case, the secondary winding of transformer L1). If C2 is replaced by a wire bridge and the secondary winding of L1 is left open, the result is a normal boost converter. In that case, a current can always flow from the input to the output via L1 and D1, even when the FET is not driven by IC1. Under these conditions, the output voltage can never be less than the input voltage less the voltage drop across the diode.


The operation of a SEPIC converter can be explained in simple terms by saying that C2 prevents any DC voltage on the input from appearing at the output, so the output voltage can easily be made lower than the input voltage. The second coil causes a defined voltage to be present at the anode of D1. It is also possible to replace the transformer by two separate coils that are not magnetically coupled. However, the efficiency of the circuit is somewhat higher if coupled coils are used as shown here. The value of resistor R4 is chosen to limit the maximum current to 500mA, which is also the maximum current that a USB bus can provide according to the specifications. Resistors R1 and R2 cause the voltage across C3 and C7 to be regulated at a value of around 6 V. A low-drop regulator (LM2940) is used to generate a stabilized 5V from the 6V output (with ripple voltage). The efficiency should be somewhere between 60% and 80%.


Author : Unknown – Copyright : Elektor

Colour Sensor

Colour sensor is an interesting project for hobbyists. The circuit can sense eight colours, i.e. blue,green and red (primary colours); magenta, yellow and cyan (secondary colours); and black and white. The circuit is based on the fundamentals of optics and digital electronics.


Circuit diagram :

Colour-Sensor -Circuit-Diagram


Colour Sensor  Circuit Diagram


The object whose colour is required to be detected should be placed in front of the system. The light rays reflected from the object will fall on the three convex lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. This helps to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2 and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass plates determine which of the LDRs would get triggered. The circuit makes use of only ‘AND’ gates and ‘NOT’ gates.

When a primary coloured light ray falls on the system, the glass plate corresponding to that primary colour will allow that specific light to pass through. But the other two glass plates will not allow any light to pass through. Thus only one LDR will get triggered and the gate output corresponding to that LDR will become logic 1 to indicate which colour it is. Similarly, when a secondary coloured light ray falls on the system, the two primary glass plates corresponding to the mixed colour will allow that light to pass through while the remaining one will not allow any light ray to pass through it. As a result two of the LDRs get triggered and the gate output corresponding to these will become logic 1 and indicate which colour it is.

When all the LDRs get triggered or remain untriggered, you will observe white and black light indications respectively. Following points may be carefully noted:


    1. Potmeters VR1, VR2 and VR3 may be used to adjust the sensitivity of the LDRs.
    2. Common ends of the LDRs should be connected to positive supply.
    3. Use good quality light filters.


The LDR is mounded in a tube, behind a lens, and aimed at the object. The coloured glass filter should be fixed in front of the LDR as shown in the figure. Make three of that kind and fix them in a suitable case. Adjustments are critical and the gadget performance would depend upon its proper fabrication and use of correct filters as well as light conditions.


Author : Tony Gladvin George - Copyright : EFY

Relay Coil Energy Saver

Some relays will become warm if they remain energized for some time. The circuit shown here will actuate the relay as before but then reduce the ‘hold’ current through the relay coil current by about 50%, thus considerably reducing the amount of heat dissipation and wasted power. The circuit is only suitable for relays that remain on for long periods.


Circuit diagram:

Relay Coil Energy Saver-Circuit-Diagram


Relay Coil Energy Saver Circuit Diagram


The following equations will enable the circuit to be dimensioned for the relay on hand: R3 = 0.7 / I Charge time = 0.5 × R2 × C1 Where I is the relay coil current. After the relay has been switched off, a short delay should be allowed for the relay current to return to maximum so the relay can be energized again at full power. To make the delay as short as possible, keep C1 as small as possible. In practice, a minimum delay of about 5 seconds should be allowed but this is open to experimentation.


The action of C2 causes the full supply voltage to appear briefly across the relay coil, which helps to activate the relay as fast as possible. Via T2, a delay network consisting of C1 and R2 controls the relay coil current flowing through T1 and R3, effectively reducing it to half the ‘pull in’ current. Diode D2 discharges C1 when the control voltage is Low. Around one second will be needed to completely discharge C1. T2 shunts the bias current of T1 when the delay has elapsed.


Diode D1 helps to discharge C1 as quickly as possible. The relay shown in the circuit was specified at 12 V / 400 ohms. All component values for guidance only.


Author: Myo Min - Copyright: Elektor

Tuned Radio Frequency (TRF) Receiver

Super heterodyne receivers have been mass-produced since around 1924, but for reasons of cost did not become successful until the 1930s. Before the second world war other, simpler receiver technologies such as the TRF receiver and the regenerative receiver were still widespread.


Circuit diagram :

 Tuned Radio Frequency (TRF) Receiver-Circuit-Diagram

Tuned Radio Frequency (TRF) Receiver Circuit Diagram


The circuit described here is based on the old technology, but brought up-to-date a The most important part of the circuit is the input stage, where positive feedback is used to achieve good sensitivity and selectivity. The first stage is adjusted so that it is not quite at the point of oscillation. This increases the gain and the selectivity, giving a narrow bandwidth. To achieve this, the potentiometer connected to the drain of the FET must be adjusted very carefully: optimal performance of the receiver depends on its setting. In ideal conditions several strong stations should be obtainable during the day using a 50 cm antenna. At night, several times this number should be obtainable.


The frequency range of the receiver runs from 6 MHz to 8 MHz. This range covers the 49 m and the 41 m shortwave bands in which many European stations broad-cast. Not bad for such a simple circuit! The circuit employs six transistors. The first stage is a selective amplifier, followed by a transistor detector. Two low-frequency amplifier stages complete the circuit. The final stage is a push-pull arrangement for optimal drive of the low-impedance loud-speaker. This circuit arrangement is some-times called a ‘1V2 receiver’ (one preamplifier, one detector and two audio frequency stages).


Setting-up is straightforward. Adjust P1 until the point is reached where the circuit starts to oscillate: a whistle will be heard from the loudspeaker. Now back off the potentiometer until the whistle stops. The receiver can now be tuned to a broad-caster. Occasional further adjustment of the potentiometer may be required after the station is tuned in.  The receiver operates from a supply volt-age of between 5 V and 12 V and uses very little current. A 9 V PP3 (6F22) battery should give a very long life.


Author : Gert Baars - Copyright :  Elektor

Stereo Widening

Although the principle is quite old, ‘widening’ of the sound image is still done these days in many portable devices, ghetto blasters and PC loudspeakers, even though it is usually called something else in these applications. To generate the stereo image, the left channel also contains part of the sound from the right channel, shifted a little in phase compared to the right channel. The same is true for the right channel, where the signal from the left channel is slightly shifted in phase. To make the stereo image ‘wider’, you can amplify the difference signals of both channels.

Circuit diagram :

Stereo Widening-Circuit-Diagram

Stereo Widening Circuit Diagram

To do this you generate a sum- and a difference signal from the left and right channels. With a couple of opamps you can real-ise a ‘left+right’ signal and a ‘left-right’ signal. So the (left–right) signal needs to be made stronger with respect to the (left+right) sig-nal. Expressed as a formula: (L+R) + (L–R) = 2L and (L+R) – (L–R) = 2R

With a suitable circuit, the left signal in the left channel is increased and the right signal is decreased. Similarly, in the right channel the right signal is increased if the left signal reduces. To maintain a constant volume, we also have to make sure that the total signal strength remains the same. From the schematic you can see how this problem was solved. IC1 and IC2 are the input buffers. After the buffer, the left and right signals are combined with the other channel respec tively. IC3 generates the (L–R) signal and IC4 the (L+R) signal. With t wo times six resistors and a multi-position switch, the amount of the effect can be adjusted. The values of resistors R7–R12 and R14 – R21 are selec ted such that the total volume remains about the same when changing the switch. IC5 and IC6 generate the final left and right signal from the (L+R) and (L–R) signals.

Fo r additional protection, electro ly tic coupling capacitors of 10 μF 16 V can be added to the inputs and outputs. Each of the inputs of IC1 and IC2 will then also need a 10 kΩ resistor to ground, otherwise the opamp outputs will run up against power supply rail. The power supply requires a symmetrical voltage of ±12 V. This voltage can usually be found in an existing amplifier, so normally there is no need to build a special power supply.

Author : Huub Smits - Copyright : Elektor

12 V AC Dimmer

The circuit described here is derived from a conventional design for a simple lamp dimmer, as you can see if you imagine a diac connected between points A and B. The difference between this circuit and a normal diac circuit is that a diac circuit won’t work at 12 V. This is the fault of the diac. Most diacs have a trigger voltage in the range of 30 to 40V, so they can’t work at 12 V, which means the dimmer also can’t work.

Circuit diagram :

The portion of the circuit between points A and B acts like a diac with a trigger voltage of approximately 5.5 V. The network formed by R1, P1 and C1 generates a phase shift relative to the supply voltage. The ‘diac equivalent’ circuit outputs a phase-shifted trigger pulse to the triac on each positive and negative half-cycle of the sinusoidal AC voltage.

This works as follows. First consider the positive half of the sine wave. C1 charges when the voltage starts to rise, with a time constant determined by C1, R1 and P1. T1 does not start conducting right away. It waits until the voltage across D2 reaches 4.7 V and the Zener diode starts to conduct. Then current starts to flow, driving T1 and T3 into conduction. This produces a pulse at point B. The same principle applies to the negative half of the sine wave, in this case with D1, T2 and T4 as the key players.

The trigger angle can be adjusted with P1 over a range of approximately 15 degrees to 90 degrees. C2 provides a certain amount of noise decoupling. Depending on the load, the triac may need a heat sink. You can use practically any desired transistors; the types indicated here are only examples. If the circuit does not dim far enough, you can change the value of P1 to 25 kΩ. This allows the trigger angle to be increased to 135 degrees.

Note: this circuit works fine with normal transformers, but not with ‘electronic ’ transformers.

Author :Peter Jansen - Copyright : Elektor

Backlight Delay

Lots of devices are fitted with a liquid crystal display(LCD). Now LCD implies backlighting that rather useful option that enables us to read the message being dis-played! For devices where there’s no need to read the display continuously, the backlight doesn’t need to stay lit up all the time-several seconds is often all you need to read the display. This saves a little power and lengthens the life of the backlight.

Circuit diagram :

Devices fitted with an LCD also have a processor, and so it’s possible to employ a function to control the backlight directly from within the processor software. But some-times it’s not possible to implement this sort of function within the microcontroller, because all the controller’s pins are already in use, or because you don’t have the source codes or tools needed to modify the software.The circuit described here has been designed for just such cases.

A device using an LCD usually has at leas tone button that, in most cases, pulls one of the microcontroller inputs down to 0 V when it is pressed. If no such button exists, one can always be added. We can use the signal from this button to control the backlight. As soon as the button is pressed,the backlight is activated, then extinguished a few seconds later by the timer. Using an OR gate, it ’s possible to use several different buttons to trigger the timer.

It doesn’t take many components to build a timer like this. The OR gate consists of a pull-up resistor R1+R2 and as many diodes as there are but-tons. Thanks to these diodes, transistor T1 conducts while the button is pressed, and hence capacitor C1 is charged, the MOSFET T2 conducts, and the backlight comes on. Because R3 has a very low value, capacitor C1 charges very rapidly, so even a very brief press of one of the buttons is enough to trig-ger the timer. Once the button is released, T1 turns off, and C1 then discharges slowly through R4 alone, since T2 has a very high input impedance. When T2’s gate voltage falls low enough, it turns off and the back-light goes out. The time the backlight stays lit after all the buttons have been released is roughly R4 (O) × C1 (F) seconds.

Of course, this circuit can be used for other applications too, and can be used to switch things other than an LED for example, a relay.The value of R5 depends on the load being switched. For an LED running off a 5 V supply, a value of around 300 O will be about right.

Author : Clemens Valens - Copyright : Elektor

Dog Whistle for Ronja

Ronja is the author’s dog, a beagle-mongrel,  who seems increasingly often to need to be  called to heel either with a shout or with a  whistle. And so the idea came about for an  electronic dog whistle that could produce  two alternating high-frequency tones. A  design like this has several advantages over  conventional whistles or calling.


Circuit diagram :

Dog Whistle for-Ronja-Circuit-Diagram

Dog Whistle for Ronja Circuit Diagram


  • You can continue to carry on a conversation with your friends without having to  stop to whistle or call to your dog.
  • Using high frequencies means that  the whistle sound is barely audible to  (especially older) humans and so is less  annoying to other people than conventional whistles or calls. As is well known,  dogs have rather better hearing than  we do and can hear frequencies of up to  40 kHz.
  • The two alternating pitches mean that the  dog can more easily distinguish it from  other whistles.


The dog whistle is constructed from two  standard 555 timer ICs (or a single 556 IC),  both wired as astable multivibrators. The  first 555 oscillates at around 1.5 Hz and modulates the frequency of the second, which thus  switches between two different frequencies  every 0.7 seconds or so. The output of the second 555 is connected to a piezo sounder. If the  volume from the sounder used is insufficient, a small transistor amplifier can be added  between it and the output of the second 555. The circuit draws current only when activated by pressing S1. An optional green  LED indicates that the circuit is functioning.  When S2 is pressed the output frequencies  are reduced, making them more audible to  human ears for test purposes.


R1, R2 and C1 set the frequency of astable  multivibrator IC1. Diode D1 ensures that the  output is a symmetrical squarewave, by making C1 charge only via R1 and discharge only  via R2. Turning to IC2, where there is no diode in the  circuit, capacitor C2 is charged via R3 and R4  and discharged only via R4. With C2 = 22nF  the 555 oscillates at about 10 kHz; with S2  pressed, and hence C3 in parallel with C2, this  falls to about 1.8 kHz. Changing C2 to 10 nF  results in an even higher frequency (about  22 kHz), which can only be heard by dogs  and certain other animals. Setting C2 to 15 nF gives an output frequency of about 15 kHz. IC1 modulates the frequency of IC2 via R5. The green LED D2 is connected to the output  of IC1 via a series resistor and thus flashes at  the modulation frequency. The output from the piezo sounder at 10 kHz  (C2 = 22 nF) should be loud enough to verify  by ear. If desired, a more efficient piezo horn  tweeter can be used instead.


Author : Stefan Hoffmann - Copyright : Elektor Electronics

Analogue LED Chaser Light

The circuit shown here is formed of nine inverting transistor amplifier stages connected in series with an LED connected  between emitter and ground.The output of the final stage connected to the input of the  first stage.The principle is similar to the ring oscillator described by the author elsewhere in this edition. Similar but not identical, since the stages in this circuit have additional delay elements formed of a 33 kΩ resistor and a 47 μF electrolytic.

Circuit diagram :

Analogue LED Chaser-Light-Circuit-Diagram

Analogue LED Chaser Light Circuit Diagram


The circuit operates with any odd number of LED stages of your choice, for instance  with nine (as shown here). The project oscillates very reliably and the  way the twinkling LEDs fade in and out is quite  a novelty. If you watch just two LEDs, they  look like a simple blinker as there is always  one lit LED next to a dark one. But with the  lights circulating it looks a lot more complicated. Any disturbance will also travel round  the ring. To watch the effect take a look at this  You Tube Video: Video.


Author :Burkhard Kainka - Copyright : Elektor Electronics

Cell-Phone-Controlled Audio/Video Mute Switch

This cell-phone-controlled audio/ video mute switch is highly useful in automobiles. The circuit automatically disconnects power supply to the audio/video system whenever the mobile handset is lifted off the holder for making or receiving a call. You can use any readily available cell-phone holder with some mi-nor alterations or fabricate it yourself as shown in Fig. 1. 

Proposed cell-phone holder Fig. 1: Proposed cell-phone holder


The circuit is wired around IC LM555 (IC1), the CMOS version of timer NE555, as shown in Fig. 2. IC1 is used as a medium current line driver with either an inverting or non-inverting output. It can sink (or source) current of up to 50 mA only, so take care while handling it. The audio/video system is connected to the circuit via normally opened (N/O) contacts of the relay.


circuit of the cell phone-controlled audio video mute switch Fig. 2: The circuit of the cell phone-controlled audio/video mute switch


When the cell phone is in its holder, LDR1 does not receive any light from white LED1 and its resistance is high. As a result, the voltage at pin 2 of IC1 re-mains high to provide a low output at pin 3. The low output of IC1 activates relay RL1 and the audio/video system gets power supply via its N/O contacts. LED3 glows to indicate that the audio/video system is ‘on.’  When the handset is taken off the holder, light rays from LED1 fall on LDR1 and its resistance decreases. As a result, the voltage at pin 2 of IC1 de-creases to provide a high output at its pin 3. The high output of IC1 deactivates relay RL1 and the audio/video system does not get power supply. LED2 glows to indicate that the audio/video system is ‘off.’   Preset VR1 is used to control the sensitivity of the circuit. Zener diode ZD1 is used for protecting white LED1 from the higher voltage. The circuit works off a 12V car battery. Switch S1 can be used to manually switch on/off the audio/video system.


Author : T.K. Hareendran - Copyright:  Electronics For You

Precision Headphone Amplifier

Designs for good-quality headphone amplifiers abound, but this one has a few special features that make it stand out from the crowd. We start with a reasonably conventional input stage in the form of a differential amplifier constructed from dual FET T2/T3. A particular point here is that in the drain of T3, where the amplified signal appears, we do not have a conventional current source or a simple resistor. T1 does indeed form a current source, but the signal is coupled out to the base of T5 not from the drain of T3 but from the source of T1. Notwithstanding the action of the current source this is a low impedance point for AC signals in the differential amplifier.


Circuit diagram:

Precision Headphone Amplifier-Circuit-Diagram

Precision Headphone Amplifier Circuit Diagram


Measurements show that this trick by itself results in a reduction in harmonic distortion to considerably less than –80 dB (much less than 0.01 %) at 1 kHz. T5 is connected as an emitter follower and provides a low impedance drive to the gate of T6: the gate capacitance of HEXFETs is far from negligible. IC1, a volt-age regulator configured as a current sink, is in the load of T6. The quiescent current of 62 mA (determined by R11) is suitable for  an output power of 60 mWeff into an impedance of 32 Ω, a value typical of high-quality headphones, which provides plenty of volume.


Precision Headphone Amplifierw


Using higher-impedance headphones, say of 300 Ω, considerably more than 100 mW can be achieved. The gain is set to a useful 21 dB (a factor of 11) by the negative feedback circuit involving R10 and R8. It is not straightforward to change the gain because of the single-sided supply: this voltage divider also affects the operating point of the amplifier. The advantage is that excellent audio quality can be achieved even using a simple unregulated mains supply.  Given the relatively low power output the power supply is considerably overspecified. Noise and hum thus remain more than 90 dB below the signal (less than 0.003 %), and the supply can also power two amplifiers for stereo operation.


The bandwidth achievable with this design is from 5 Hz to 300 kHz into 300 Ω, with an output voltage of 10 Vpp. The damping factor is greater than 800 between 100 Hz and 10 kHz. A couple of further things to note: some-what better DC stability can be achieved by replacing D1 and D2 by low-current red LEDs (connected with the right polarity!). R12 prevents a click from the discharge of C6 when headphones are plugged in after power is applied. T6 and IC1 dissipate about 1.2 W of power each as heat, and so cooling is needed. For low impedance headphones the current through IC1 should be increased. To deliver 100 mW into 8 Ω, around 160 mA is required, and R11 will need to be 7.8 Ω (use two 15 Ω resistors in parallel).


To keep heat dissipation to a reasonable level, it is recommended to reduce the power supply volt-age to around 18 V (using a transformer with two 6 V secondaries). This also means an adjustment to the operating point of the amplifier: we will need about 9V between the positive end of C6 and ground. R4 should be changed to 100 Ω, and R8 to 680 Ω. The gain will now be approximately 6 (15 dB). The final dot on the ‘i’ is to increase C7 by connecting another 4700 µF electrolytic in parallel with it, since an 8 Ω load will draw higher currents.


Author : Hergen Breitzke - Copyright : Elektor

White Light For Refrigerator

Normally, the refrigerator lights  are  yellow in colour  and  go  bad  very  often. If you want a long-lasting white light  for  your  refrigerator, this circuit is especially for you.The circuit is easy to install inside the refrigerator. Also, it consumes very  little power compared to the traditional yellow bulb.


Circuit diagram :

White Light For Refrigerator-Circuit-Diagram

White Light For Refrigerator Circuit Diagram


The circuit is transformerless and uses a capacitor for providing the power supply to the white LEDs. Mains input is fed to capacitor C1 and resistor R1, rectified by a bridge rectifier comprising diodes D1 through D4, and filtered by capacitor C2 to provide sufficient voltage to drive the white LEDs. Resistor R2 limits the current flowing through the LEDs. The circuit uses 16 bright white LEDs in 2×8 parallel connection configuration.


Assemble the circuit on a general purpose PCB along with 16 white  LEDs. Connect the LEDs in series with resistor R2. Take out the two wires from the usual bulb connection and connect to the circuit. After connecting the supply wires, wrap electrical tape around the wires properly. Enclose the entire PCB (excluding LEDs) in a plastic box and wrap the box properly with tape to seal it against moisture.


Author : Uday Shende – Copyright : EFY

USB Power Booster

Power shortage problems arise when too many USB devices connected to PC are working simultaneously. All USB devices, such as scanners, modems, thermal printers, mice, USB hubs, external storage devices and other digital devices obtain their power from PC. Since a PC can only supply limited power to USB devices, external power may have to be added to keep all these power hungry devices happy. This circuit is designed to add more power to a USB cable line. A sealed 12-V 750 mA unregulated wall cube is cheap and safe. To convert 12 V to 5 V, two types of regulators, switching and linear are available with their own advantages and drawbacks. The switching regulator is more suitable to this circuit because of high efficiency and compactness and now most digital circuits are immune to voltage ripple developed during switching.


Circuit diagram :

USB Power Booster-Circuit-Diagram

USB Power Booster Circuit Diagram


The simple switcher type LM2575-5 is chosen to provide a stable 5V output voltage. This switcher is so simple it just needs three components: an inductor, a capacitor and a high-speed or fast-recovery diode. Its principle is that internal power transistor switch on and off according to a feedback signal. This chopped or switched voltage is converted to DC with a small amount of ripple by D1, L1 and C2.  The LM2575 has an ON/OFF pin that is switched on by pulling it to ground. T1, R2, and R1 (pull-up resistor) pull the ON/OFF pin to ground when power signal from PC or +5 V is received. D2, a red LED with current resistor R3, serves to indicate ‘good’ power condition or stable 5V. C3 is a high-frequency decoupling capacitor. The author managed to cut a USB cable in half without actually cutting data wires. It is advisable to look at the USB cable pin assignment for safety.


Author : Myo Min - Copyright : Elektor

Auto Turn-Off Battery Charger

This charger for series-connected 4-cell AA batteries automatically disconnects from mains to stop charging when the batteries are fully charged. It can be used to charge partially discharged cells as well. The circuit is simple and can be divided into AC-to-DC converter, relay driver and charging sections. In the AC-to-DC converter section, transformer X1 steps down mains 230V AC to 9V AC at 750 mA, which is rectified by a full-wave rectifier comprising diodes D1 through D4 and filtered by capacitor C1. Regulator IC LM317 (IC1) provides the required 12V DC charging voltage.


Circuit diagram :

Auto Turn-Off Battery Charger-Circuit-diagram

Auto Turn-Off Battery Charger Circuit Diagram


When you press switch S1 momentarily, the charger starts operating and the power-on LED1 glows to indicate that the charger is ‘on.’ The relay driver section uses pnp transistors T1, T2 and T3 (each BC558) to energise electromagnetic relay RL1. Relay RL1 is connected to the collector of transistor T1. Transistor T1 is driven by pnp transistor T2, which, in turn, is driven by pnp transistor T3. Resistor R4 (10-ohm, 0.5W) is connected between the emitter and base of transistor T3.
When a current of over 65 mA flows through the 12V line, it causes a voltage drop of about 650 mV across resistor R4 to drive transistor T3 and cut off transistor T2. This, in turn, turns transistor T1 ‘on’ to energise relay RL1.


Now even if the pushbutton is released, mains is still available to the primary of the transformer through its normally open (N/O) contacts.  In the charging section, regulator IC1 is biased to give about 7.35V. Preset VR1 is used for adjusting the bias voltage. Diode D6 connected between the output of IC1 and battery limits the output voltage to about 6.7V, which is used for charging the battery.  Pushing switch S1 latches relay RL1 and the battery cells start charging. As the voltage per cell increases beyond 1.3V, the voltage drop across resistor R4 starts decreasing. When it falls below 650 mV, transistor T3 cuts off to drive transistor T2 and, in turn, cuts off transistor T3.


As a result, relay RL1 de-energises to cut off the charger and red LED1 turns off.  You may determine the charging voltage depending on the NiCd cell specifications by the manufacturer. Here, we’ve set the charging voltage at 7.35V for four 1.5V cells. Nowadays, 700mAH cells are available in the market, which can be charged at 70 mA for 10 hours. The open-circuit voltage is about 1.3V. The shut-off voltage point is determined by charging the four cells fully (at 70 mA for 14 hours). After measuring the output voltage, add the diode drop (about 0.65V) and bias LM317 accordingly.


Author : Y.M. Anandavardhana  - Copyright : electronicsforu

Step-Up Converter For 20 LEDs

The circuit described here is a step-up converter to drive 20 LEDs, designed to be used as a home-made ceiling night light for a child’s bedroom. This kind of night light generally consists of a chain of Christmas tree lights with 20 bulbs each consuming 1 W, for a total power of 20 W. Here, in the interests of saving power and extending operating life, we update the idea with this simple circuit using LEDs. Power can be obtained from an unregulated 12 V mains adaptor, as long as it can deliver at least about 330 mA.  The circuit uses a low-cost current-mode controller type UCC3800N, reconfigured into voltage mode to create a step-up converter with simple compensation. By changing the external components the circuit can easily be modified for other applications. To use a current-mode controller as a voltage-mode controller it is necessary to couple a sawtooth ramp (rising from 0 V to 0.9 V) to the CS (current sense) pin, since this pin is also an input to the internal PWM comparator.


Circuit diagram :

Step-up Converter For 20 LEDs -Circuit-Diagram

Step-up Converter For 20 LEDs Circuit Diagram


The required ramp is present on the RC pin of the IC and is reduced to the correct voltage range by the voltage divider formed by R3 and R2. The RC network formed by R4 and C6 is dimensioned to set the switching frequency at approximately 525 kHz. The comparator compares the ramp with the divided-down version of the output voltage produced by the potential divider formed by R6 and R7. Trimmer P1 allows the output voltage to be adjusted. This enables the current through the LEDs to be set to a suitable value for the devices used. The UCC3800N starts up with an input voltage of 7.2 V and switches off again if the input voltage falls below 6.9 V. The circuit is designed so that output voltages of between 20 V and 60 V can be set using P1.


This should be adequate for most cases, since the minimum and maximum specified forward voltages for white LEDs are generally between 3 V and 4.5 V. For the two parallel chains of ten LEDs in series shown here a voltage of between 30 V and 45 V will be required. The power components D1, T1 and L1 are considerably over specified here, since the circuit was originally designed for a different application that required higher power. To adjust the circuit, the potentiometer should first be set to maximum resistance and a multimeter set to a 200 mA DC current range should be inserted in series with the output to the LEDs. Power can now be applied and P1 gradually turned until a constant current of 40mA flows. The step-up converter is now adjusted correctly and ready for use.

Author : Dirk Gehrke - Copyright : Elektor

Efficient Current Source for High-power LEDs

To get the maximum brightness and working life out of a high-power LED, it needs to be driven at the optimum specified current. Allowing the current to exceed the permitted value is to be avoided at all costs, since it will severely affect the life of the device. A power supply or a battery with a small current-limiting resistor is not really an ideal solution, since not only is energy wasted in heating the resistor, but, if a small value is chosen to minimise this wastage, small changes in the applied voltage will lead to large changes in the current that flows. It is well known that LEDs have a small dynamic resistance in the neighbourhood of their optimal operating point. We will there fore need more in the way of electronics than a simple series resistor to meet our requirements.


Circuit diagram:

Efficient Current Source for High-power LEDs Circuit-Diagram

Efficient Current Source for High-power LEDs Circuit Diagram


The most direct way to provide a highly constant current in the face of relatively small changes in supply voltage is to use a conventional regulated current source. Unfortunately this type of circuit unnecessarily wastes energy in its series transistor, which rather detracts from the charm of using a semiconductor-based circuit. The inefficiency can be mitigated by using a modern device such as a power MOS-FET as the series component. Power loss is then limited to that in any current sense resistor that might be used and the dissipation in the relatively small ‘on’ resistance of the switching transistor.


The circuit suggested here drives a commercially available Luxeon LED using a BUZ71. The 5 W version of this LED draws 0.7 A. This means that 0.175 V is dropped across R9, making for a power dissipation of 122 mW. T1 has a typical resistance in the ‘on’ state of 85 mΩ. In the ideal scenario this means that about 60 mV is dropped across it, for a dissipation of at least 42 mW. The supply voltage therefore needs to be about 230 mV higher than the nominal voltage of the LED (6.85 V). To have something in reserve, 7.2 V, allowing 0.35 V for T1 and R9, is a good compromise. Serendipitously, a series of six NiCd or NiMH cells will give almost exactly this value under load! A further happy coincidence is that an unregulated mains power supply with a 6V transformer with bridge rectifier and smoothing capacitor will also give us almost exactly our target voltage when loaded. A 7.5 VA transformer is suitable, along with a 2200 µF/16 V electrolytic smoothing capacitor.


Now to how it works. D1 acts as a reference, with a voltage of 2.5 V being dropped across it. IC1b, together with T1, form a current source, whose current can be set between 360 mA and 750 mA using P2. The otherwise unused opamp IC1a is connected to form an under-volt-age cutout switch which prevents a connected battery from being discharged too deeply. The threshold point is set using P1. IC1a is configured as a comparator with a small hysteresis. If its output is high, IC1b is fooled into thinking that the cur-rent through R9 is too high, whereupon it switches off the LED. The same happens if R1 is not shorted by a switch. For the purposes of this circuit only opamps with input stages constructed using PNP transistors should be used.


One last look at the energy budget: if six cells are used, the average voltage during discharge will be around 7.4 V. Sub-tract the nominal voltage of the LED and 0.55 V is left to be converted into heat. About 0.4 W will be dissipated by T1, which therefore will not require cooling. Efficiency is very good, at over 90 %.

Music-On-Hold for Telephones

Here is a simple circuit for music-on-hold with automatic shut off facility. During telephone conversation if you are reminded of some urgent work, momentarily push switch S1 until red LED1 glows, keep the telephone handset on the cradle, and attend to the work on hand. A soft music is generated and passed into the telephone lines while the other-end subscriber holds. When you return, you can simply pick up the handset again and continue with the conversation. The glowing of LED1, while the music is generated, indicates that the telephone is in hold position. As soon as the handset is picked up, LED1 is turned off and the music stops.


Circuit diagram :

Music-On-Hold for-Telephones-Circuit-Diagram

Music-On-Hold for Telephones Circuit Diagram


Normally, the voltage across telephone lines is about 50 volts. When we pick up the receiver (handset), it drops to about 9 volts. The minimum voltage required to activate this circuit is about 15 volts. If the voltage is less than 15 volts, the circuit automatically switches off. However, initially both transistors T1 and T2 are cut off. The transistor pair of T1 and T2 performs switching and latching action when switch S1 is momentarily pressed, provided the line voltage is more than 15 volts, i.e. when the handset is placed on the cradle. Once the transistor pair of TI and T2 starts conducting, melody generator IC1 gets the supply and is activated. The mu-sic is coupled to the telephone lines via capacitor C2, resistor R1, and the bridge rectifier.


With the handset off-hook after a ring, momentary depression of switch S1 causes forward biasing of transistor T2. Mean-while, if the handset is placed on the cradle, the current passing through R1 (connected across the emitter and base terminals of pnp transistor T1) develops enough voltage to forward bias transistor T1 and it starts conducting. As a consequence, output voltage at the collector of transistor T1 sustains for-ward biasing of transistor T2, even if switch S1 is released. This latching action keeps both transistors T1 and T2 in conduction as long as the output of the bridge rectifier is greater than 15 volts. If the handset is now lifted off-hook, the rectifier output drops to about 9 volts and hence latching action ceases and the circuit automatically switches off.


EFY lab note. The value of resistor R2 determines the current through resistor R1 to develop adequate voltage (greater than 0.65 volts) for conduction of transistor T1. Hence it may be test selected between 33 kilo-ohms and 100 kilo-ohms to obtain instant latching.) The total cost of this circuit is around Rs 50.


Author : SIBIN K. ZACHARIAH - Copyright : Electronicsforu

Automatic Night Lamp with Morning Alarm

This circuit automatically turns on a night lamp when bedroom light is switched off. The lamp remains ‘on’ until the light sensor senses daylight in the morning. A super-bright white LED is used as the night lamp. It gives bright and cool light in the room. When the sensor detects the daylight in the morning, a melodious morning alarm sounds. The circuit is powered from a standard 0-9V transformer. Diodes D1 through D4 rectify the AC voltage and the resulting DC voltage is smoothed by C1. Regulator IC 7806 gives regulated 6V DC to the circuit. A battery backup is provided to power the circuit when mains fails. When mains supply is available, the 9V rechargeable battery charges via diode D5 and resistor R1 with a reasonably constant current. In the event of mains failure, the battery automatically takes up the load without any delay. Diode D5 prevents the battery from discharging backwards following the mains failure and diode D6 provides current path from the battery.

Circuit diagram :

Automatic Night Lamp with Morning Alarm-Circuit-Diagram

Automatic Night Lamp with Morning Alarm Circuit Diagram

The circuit utilises light-dependant resistors (LDRs) for sensing darkness and light in the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning. The circuit is designed around the popular timer IC NE555 (IC2), which is configured as a monostable. IC2 is activated by a low pulse applied to its trigger pin 2. Once triggered, output pin 3 of IC2 goes high and remains in that position until IC2 is triggered again at its pin 2. When LDR1 is illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2 of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the device increases.

In total darkness, the specified LDR has a resistance in excess of 280 kilo-ohms. When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC2 via resistor R2 (150 kilo-ohms). This activates the monostable and its output goes high, causing the white LED to glow. Low-value capacitor C2 maintains the monostable for continuous operation, eliminating the timer effect. By increasing the value of C2, the ‘on’ time of the white LED can be adjusted to a predetermined time. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC UM66 (IC3) gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone generated by IC3 is standard 0-9V transformer. Diodes D1 through D4 rectify the AC voltage and the resulting DC voltage is smoothed by C1. Regulator IC 7806 gives regulated 6V DC to the circuit.

A battery backup is provided to power the circuit when mains fails. When mains supply is available, the 9V rechargeable battery charges via diode D5 and resistor R1 with a reasonably constant current. In the event of mains failure, the battery automatically takes up the load without any delay. Diode D5 prevents the battery from discharging backwards following the mains failure and diode D6 provides current path from the battery.

The circuit utilises light-dependant resistors (LDRs) for sensing darkness and light in the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning. The circuit is designed around the popular timer IC NE555 (IC2), which is configured as a monostable. IC2 is activated by a low pulse applied to its trigger pin 2. Once triggered, output pin 3 of IC2 goeshigh and remains in that position until IC2 is triggered again at its pin 2. When LDR1 is illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2 of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the device increases.

In total darkness, the specified LDR has a resistance in excess of 280 kilo-ohms. When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC2 via resistor R2 (150 kilo-ohms). This activates the monostable and its output goes high, causing the white LED to glow. Low-value capacitor C2 maintains the monostable for continuous operation, eliminating the timer effect. By increasing the value of C2, the ‘on’ time of the white LED can be adjusted to a predetermined time. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC UM66 (IC3) gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone generated by IC3 is amplified by single-transistor amplifier T2. Resistor R7 limits the current to IC3 is amplified by single-transistor amplifier T2. Resistor R7 limits the current to IC3 and zener diode ZD limits the voltage to a safer level of 3.3 volts.

The circuit can be easily assembled on a general-purpose PCB. Enclose it in a good-quality plastic case with provisions for LDR and LED. Use a reflective holder for white LED to get a spotlight effect for reading. Place LDRs away from the white LED, preferably on the backside of the case, to avoid unnecessary illumination. The speaker should be small so as to make the gadget compact.

Author : D. Mohan Kumar - Copyright : Electronicsforu

Electronic Code Lock Schematic Circuit

Nowadays, electronic code locks are usually based on microcontrollers. However, if you like your electronics discrete, you will enjoy the battery-operated circuit shown here. Since the circuit automatically switches off after the door has been opened and draws no current in the idle state, three alkaline batteries (mignon, AA or R6 cells) are good for around 5,000 door openings. The main advantage is that the door opener can also be powered from the battery, so it’s not necessary to run any extra cables.


Project  Image :

Electronic Code Lock-Schematic-Circuit-Image

Electronic Code Lock Schematic Circuit  Image


Figure 1 shows the schematic diagram of the circuit, which is split into two parts. The first part is the control panel, which consists of a 12-position keypad and two LEDs. The second part is the programming and evaluation logic, which contains only standard logic ICs. The control panel is connected to the logic board by a 16-way flat cable. The keypad circuit is laid out with separate connections to the individual switches, instead of a matrix. The code is programmed using the two pin connector strips K1 and K2.  The circuit allows any desired combination of numbers to be used for the code, up to a maximum of 9 positions. Press-ing a particular button, which in principle is random but which naturally must be specified in advance, awakens the circuit from the zero-current idle state. This Start button cannot be used in the subsequent code sequence. The Start button is programmed by connecting a wire bridge from the associated pin of K2 to pin 1 of K1.  The code sequence is programmed in a similar manner. The first numeral of the code is programmed by connect-ing the associated pin of K2 to pin 2 of K1, the connection for the second numeral is made to pin 3 of K1, the third to pin 4 and so on. Numerals that are not used in the code do not actually have to be connected. However, if the unused buttons are connected to VDD, the code lock will assume that an error has occurred if any of these buttons is pressed and will reset the circuit. Pressing the Start button switches on transistor T1, which connects the supply voltage source to the code lock. This is indicated by the yellow LED (D20).


Circuit diagram :

Electronic Code Lock-Schematic-Circuit -Diagram

Electronic Code Lock Schematic Circuit Diagram

Part List :

Resistors :

R1,R2 = not fitted
R3 = 220kΩ
R4,R5 = 1MΩ
R6 = 220kΩ
R7,R9,R10,R17 = 100kΩ
R8,R12,R14 = 2MΩ2
R11 = 560Ω
R13,R15,R20 = 1kΩ5
R16 = 100kΩ
R18 = 120Ω
R19 = 10k
R21-R24 = 3Ω3
R25-R35 = 22kΩ

Capacitors :

C1,C6,C7,C8,C10 = 100nF
C2,C3,C5 = 10nF
C4 = 1µF
C9 = 330nF
C11 = 47µF 16V radial

Semiconductors :

D18 = 1N4148
D10,D12 = zener diode 1V2
D16 = 1N4001
D19 = LED, green
D20 = LED, yellow
T1 = BC327
T2,T3,T4 = BC337
T5 = BD140
IC1 = 4017
IC2,IC3 = 4069 or 40106

Miscellaneous :

JP1 = jumper
K1,K2 = 12-way pinheader or
wire links
K3,K4 = not required (ribbon
cable )
K5, K6 = 2-way PCB terminal
block, lead pitch 5mm
S1-S12 = pushbutton with
make contact


Since the logic ICs are now enabled, the output of IC3f will be High, so T2 also conducts and pulls the base of T1 to ground. This means that the Start button can be released without affecting the circuit. However, C11 can now slowly charge via the high resistance of R12 until the voltage at the inverter input is high enough to cause its output to go Low, which interrupts the supply voltage to the circuit and puts it back into the idle state. The valid code must therefore be entered during the time interval determined by this R–C time constant. Once the supply voltage is disconnected, C11 discharges rapidly via D18. This is important, since other-wise C11 could retain its charge for a long time. This would make the time allowed for entering the code significantly shorter the next time the lock is used.  Pressing the Start button also has other consequences. Via the Start switch, ground potential arrives at IC2d, where it causes a pulse to be generated that places counter IC1 in a defined state (Q0 = 1) prior to the entry of the first code numeral. The first code numeral can now be entered. If the correct button is pressed, the High potential from Q0 passes through the closed switch to reach IC2d–IC2a. This net-work generates a positive pulse at the instant that the but-ton is released. This pulse clocks the counter, so that the High level from Q0 moves by one position to Q1. This process repeats itself until all code numerals have been entered.


PCB Layout :

Pcb Lyout

Electronic Code Lock Schematic PCB Layout


After the ninth numeral has been entered, the positive volt-age jumps to Q9, where it charges C4 (if jumper JP1 is installed). While C4 is charging, the output of IC2e goes Low for approximately two seconds, and the output of IC3d goes high for the same interval. Power transistor T5 is switched on via R19 and T4 to supply current to the door opener. At the same time, IC3a switches on the green LED (D19) to indicate that the door can be opened. T3 limits the current through the door opener to around 700 mA. Once C4 is sufficiently charged, the output of IC2e changes to High. Not only does this switch off the door opener, but the positive edge also generates a pulse in the network IC2f/IC3c that passes through D14 to reach IC1 as a reset pulse (D14, D17 D13 and R7 together form a ‘wired-or’ gate). Inverter IC3b also provides the power-up reset to the counter. The reset signal places the circuit back into its initial state. What happens if an incorrect button is pushed? In such a case a Low level is passed through in place of the High level from the counter output. This has the same consequence as the Low level from the Start button: the counter is reset.


Note that you can also modify the circuit to use fewer than nine numerals for the code. All that is necessary is to connect C4 via a jumper to another counter output in place of Q9 (for example, to Q4 for a four-position code). The diode at the selected output of the counter can be replaced by a wire jumper, and the ‘higher level’ diodes can also be omit-ted. The active ‘on’ time of the door opener is 2 s. If this seems to be too short, the value of R8 or C4 can be increased. However, this also increases the amount of power drawn from the battery, especially considering that the door opener is by far the biggest power glutton in the circuit. In order to integrate the circuit into an existing door opener or to use it to operate an ac door opener, you should connect a relay to K5. Before assembling the circuit using the printed circuit board shown in Figure 2, you should separate the two sections by sawing between K3 and K4. The logic board should not be fitted directly behind the pushbuttons for entering the code. Instead, it is better to separate the entry pushbuttons, the LEDs and the door opener from the logic circuit board with a length of cable. Otherwise, a screw driver or a bit of wire connected between the emitter and collector of T5 is all that is needed to outfox the code lock and open the door.


Fitting the components to the circuit board should not be difficult. The ICs can be mounted in sockets. The author used 4049 inverter ICs, but in the Elektor Electronics lab prototype we used 4069’s, which are functionally compatible but not pin-compatible, and we also tried a 40106, which has Schmitt-trigger inputs. With a 4069, normal 1N4148 diodes can be used for D10 and D12. The best solution is to use the relatively noise-immune 40106. However, it is then necessary to use Zener diodes for D10 and D12, due to the higher threshold voltage. A 3.3-V type is ideal with an operating voltage of 15 V. There is one thing you should not overlook: with low-voltage zener diodes, the band on the package marks the anode instead of the cathode, as you would normally expect. At least, this is true in most cases, but not always.


Author : R. Heimann - Copyright :  Elektor Electronics

Two-button Digital Lock

Now here’s a digital lock unlike any other, as  it has only two buttons instead of the usual  numeric keypad. The way it works is as simple  as its keypad. Button S1 is used to enter the  digits of the secret code in a pulsed fashion-i.e. the number of times you press the but-ton is determined by the digit to be entered.  A dial telephone uses the same type of coding (now maybe there’s an idea?). Press four  times for a 4, nine times for a 9, etc. Pressing button S2 indicates the end of a digit. 


Project Image :

Two-button Digital Lock Project-Image 

 Two-button Digital Lock Project Image


For example, to enter the code 4105, press  S1 four times, then press S2, then S1 once, S2  once, then without pressing S1 at all, press S2  again, then finally S1 five times and S2 once  to finish. If the code is correct, the green LED D1 lights for 2 seconds and the relay is energised for 2 seconds. If the code is wrong, the  red LED D2 lights for 2 seconds, and the relay  is not energised. To change the code, fit a jumper to J1 and  enter the current code. When the green LED  D1 has flashed twice, enter the new 4-digit  code. D1 will flash three times and you will  need to confirm the new code. If this confirmation is correct, D1 will flash four times.  If the red LED D2 flashes four times, some-thing’s wrong and you’ll need to start all over  again. To finish the operation, remove the  jumper and turn the power off and on again the digital lock is now ready for use with  the new code.


Circuit diagram :

Two-button Digital Lock Circuit-diagram

Two-button Digital Lock Circuit Diagram


The software can be found on the webpage for the project [1]. Don’t forget to erase the microcontroller’s EEPROM memory before  programming  it,  so  you  can  be  sure  that  the  default  code  is  1234  and  not  some -thing unknown that was left behind in the  EEPROM. A little exercise for our readers: convert this  project into a single-button digital lock for  example, by using a long press on S1 instead  of pressing S2 to detect the end of a digit.


Author : Francis Perrenoud  - Copyright : Elektor

Low-drop Regulator with Indicator

Even today much logic is still powered from 5 volts and it then seems obvious to power the circuit using a standard regulator from a rectangular 9-V battery. A disadvantage of this approach is that the capacity of a 9-V battery is rather low and the price is rather high. Even the NiMH revolution, which has resulted in considerably higher capacities of (pen-light) batteries, seems to have escaped the 9-V battery generation. It would be cheaper if 5 volts could be derived from 6 volts, for example. That would be 4 ‘normal’ cells or 5 NiMH- cells. Also the ‘old fashioned’ sealed lead- acid battery would be appropriate, or two lithium cells.


Circuit diagram : 

Low-drop Regulator with Indicator-Circuit-Diagram

Low-drop Regulator with Indicator Circuit Diagram


Using an LP2951, such a power supply is easily realised. The LP2951 is an ever- green from National Semiconductor, which you will have encountered in numerous  Elektor Electronics designs already. This IC can deliver a maximum current of 100 mA at an input voltage of greater than 5.4 V. In addition to this particular version, there are also versions available for 3.3 and 3 V output, as well as an adjustable version.  In this design we have added a battery indicator, which also protects the battery from too deep a discharge. As soon as the IC has a problem with too low an input voltage, the ERROR output will go low and the regulator is turned off via IC2d, until a manual restart is provided with the RESET pushbutton.


The battery voltage is divided with a few resistors and compared with the reference voltage (1.23 V) of the regulator IC. To adapt the indicator for different voltages you only need to change the 100-k resistor. The comparator is an LP339. This is an energy-friendly version of the LM339. The LP339 consumes only 60 µA and can sink 30 mA at its output. You can also use the LM339, if you happen to have one around, but the current consumption in that case is 14 times higher (which, for that matter, is still less than 1 mA).


Finally, the LP2951 in the idle state, consumes about 100 µA and depend- ing on the output current to be deliv- ered, a little more.

Author : Karel Walraven - Copyright : Elektor

Add-On Stereo Channel Selector

The add-on circuit presented here is useful for stereo systems. This circuit has provision for connecting stereo outputs from four different sources/channels as inputs and only one of them is selected/ connected to the output at any one time. When power supply is turned ‘on’, channel A (A2 and A1) is selected. If no audio is present in channel A, the circuit waits for some time and then selects the next channel (channel B), This search operation continues until it detects audio signal in one of the channels. The inter-channel wait or delay time can be adjusted with the help of preset VR1. If still longer time is needed, one may replace capacitor C1 with a capacitor of higher value.


Suppose channel A is connected to a tape recorder and channel B is connected to a radio receiver. If initially channel A is selected, the audio from the tape recorder will be present at the output. After the tape is played completely, or if there is sufficient pause between consecutive recordings, the circuit automatically switches over to the output from the radio receiver. To manually skip over from one (selected) active channel, simply push the skip switch (S1) momentarily once or more, until the desired channel inputs gets selected. The selected channel (A, B, C, or D) is indicated by the glowing of corresponding LED (LED11, LED12, LED13, or LED14 respectively).


Circuit diagram :

Add-On Stereo Channel Selector Circuit-diagram

Add-On Stereo Channel Selector Circuit Diagram


IC CD4066 contains four analogue switches. These switches are connected to four separate channels. For stereo operation, two similar CD4066 ICs are used as shown in the circuit. These analogue switches are controlled by IC CD4017 outputs. CD4017 is a 10-bit ring counter IC. Since only one of its out-puts is high at any instant, only one switch will be closed at a time. IC CD4017 is configured as a 4-bit ring counter by connecting the fifth output Q4 (pin 10) to the reset pin. Capacitor C5 in conjunction with resistor R6 forms a power-on-reset circuit for IC2, so that on initial switching ‘on’ of the power supply, output Q0 (pin 3) is always ‘high’. The clock signal to CD4017 is pro-vided by IC1 (NE555) which acts as an astable multivibrator when transistor T1 is in cut-off state.


IC5 (KA2281) is used here for not only indicating the audio levels of the selected stereo channel, but also for for-ward biasing transistor T1. As soon as a specific threshold audio level is detected in a selected channel, pin 7 and/ or pin 10 of IC5 goes ‘low’. This low level is coupled to the base of transistor T1, through diode-resistor combination of D2-R1/D3-R22. As a result, transistor T1 conducts and causes output of IC1 to remain ‘low’ (disabled) as long as the selected channel output exceeds the preset audio threshold level.


Presets VR2 and VR3 have been included for adjustment of individual audio threshold levels of left stereo channels, as desired. Once the multivibrator action of IC1 is disabled, output of IC2 does not change further. Hence, searching through the channels continues until it receives an audio signal exceeding the preset threshold value. The skip switch S1 is used to skip a channel even if audio is present in the selected channel. The number of channels can be easily extended up to ten, by using additional 4066 ICs.


Author : Prabhash K.P- Copyright : EFY

Video Switch for Intercom System

Nowadays a lot of intercom units are  equipped with video cameras so that you can  see as well as hear who is at the door. Unfortunately, the camera lens is perfectly placed  to serve as a sort of support point for people  during the conversation, with the result that  there’s hardly anything left see in the video  imagery.  One way to solve this problem is to install two cameras on the street side instead only  one, preferably some distance apart. If you  display the imagery from the two cameras  alternately, then at least half of the time you  will be able to see what is happening in front  of the door. Thanks to the video switch module described  here, which should be installed on the street  side not too far away from the two cameras,  you need only one monitor inside the house and you don’t need to install any additional video cables.


Circuit diagram :

Video Switch for Intercom System-Circuit-Diagram

Video Switch for Intercom System Circuit Diagram


Along with a video switch, the circuit includes  a video amplifier that has been used with  good results in many other Elektor projects,  which allows the brightness and the contrast  to be adjusted separately. This amplifier is  included because the distance between the  street and the house may be rather large, so it is helpful to be able to compensate for cable attenuation in this manner.  The switch stage is built around the well  known 4060 IC, in which switches IC2a and  IC2d alternately pass one of the two signals to  the output. They are driven by switches IC2b and IC2c, which generate control signals that  are 180 degrees out of phase. The switching rate for the video signals is  determined by a clock signal from an ‘old  standby’ 555 IC, which causes the signals to  swap every 2 seconds with the specified com ponent values.


Naturally, this circuit can also used in many other situations, such as where two cameras are needed for surveillance but only one video cable is available.


Author :Jacob Gestman Geradts - Copyright : Elektor

Midnight Security Light

Most thefts happen after midnight hours when people enter the second phase of sleep called ‘paradoxical’ sleep. Here is an energy-saving circuit that causes the thieves to abort the theft attempt by lighting up the possible sites of intrusion (such as kitchen or backyard of your house) at around 1:00 am. It automatically resets in the morning. The circuit is fully automatic and uses a CMOS IC CD 4060 to get the desired time delay. Light-dependent resistor LDR1 controls reset pin 12 of IC1 for its automatic action. During day time, the low resistance of LDR1 makes pin 12 of IC1 ‘high,’ so it doesn’t oscillate. After sunset, the high resistance of LDR1 makes pin 12 of IC1 ‘low’ and it starts oscillating, which is indicated by the fashing of LED2 connected to pin 7 of IC1. The values of oscillator components (resistors R1 and R2 and capacitor C4) are chosen such that output pin 3 of IC1 goes ‘high’ after seven hours, i.e., around 1 am.

This high output drives triac 1 (BT136) through D5 and R3. Bulb L1 connected between the phase line and M2 terminal of triac 1 turns on when the gate of triac 1 gets the trigger voltage from pin 3 of IC1. It remains ‘on’ until pin 12 of IC1 becomes high again in the morning. Capacitors C1 and C3 act as power reserves, so IC1 keeps oscillating even if there is power interruption for a few seconds. Capacitor C2 keeps trigger pin 12 of IC1 high during day time, so slight changes in light intensity don’t affect the circuit.

Circuit diagram:

Midnight Security Light Circuit Diagram

Midnight Security Light Circuit Diagram

Using preset P1 you can adjust the sensitivity of LDR1. Power supply to the circuit is derived from a step-down transformer T1 (230V AC primary to 0-9V, 300mA secondary), rectifed by a full-wave rectifer comprising diodes D1 through D4 and fltered by capacitor C1. Assemble the circuit on a general-purpose PCB with adequate spacing between the components. Sleeve the exposed leads of the components. Using switch S1 you can turn on the lamp manually. Enclose the unit in a plastic case and mount at a location that allows adequate daylight.


Since the circuit uses 230V AC, many of its points are at AC mains voltage. It could give you lethal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. We will not be responsible for any kind of resulting loss or damage.

Author : D. Mohan Kumar – Copyright : electronics for you

Simple Alarm System

The circuit presented here is a very simple and yet highly effective alarm system for protecting an object. The circuit requires no special devices and can be built using components that you will no doubt be able to find in the junk box. The alarm-triggering element is a simple reed switch. To generate the alarm signal itself any optical or acoustic device that operates on 12 V can be used: for example a revolving light, a siren, or even both. In the quiescent state the reed switch is closed. As soon as the reed switch opens, the input to IC1.B will go low (previously the potential divider formed by R2 and R3 held the input at 5.17 V, a logic high level). A turn-on delay of between 0 and approximately 90 s can be set using P1, and a turn-off delay of between 0 and approximately 20 s can be set using P2. When the system is turned on (using S1), the turn-on delay is activated, giving the user of the system at most 90 s to leave the object alone before the system goes into the armed state, and the object is then protected.


Project image :

Simple Alarm System-Project-image

 Simple Alarm System Project image


Once the reed switch opens the turn-off delay of at most 20 s starts: this allows the rightful owner of the object to turn the system off before the alarm is triggered. If some unauthorised per-son causes the reed switch to open, the alarm will be triggered after the turn-off delay. Also, even if the reed switch is briefly opened and then closed again, the alarm will still be triggered. Once the alarm is triggered, T3 will conduct for about 45 s (because of R8 and C5). The turning off of the alarm is necessary to avoid the nuisance caused by a permanently sounding alarm system. The system then returns to the armed state, which means that the next time the reed switch is opened the alarm will trigger again. If it is not desired that the duration of the alarm be limited, for example if a visual indication is used, D5 should not be fitted. The system can be extended by fitting multiple reed switches in series. As soon as any one is opened, the alarm is triggered.


Circuit diagram: 

Simple Alarm System-Circuit-Diagram

Simple Alarm System Circuit Diagram


When S1 is closed C3 charges via P1. Depending on the potentiometer setting, it takes between 0 and 90 s to reach the input threshold voltage of IC1.A. The output of IC1.A then goes low and D3 stops conducting. Assuming the reed switch is closed, the inputs of IC1.B stay high and the output therefore low. If the reed switch is opened after the turn-on delay expires the output of the gate will change state and turn on T1. This ensures that the output of the gate remains high even after the reed switch is closed again. C4 now starts charging via P2, reaching the input threshold voltage of IC1.C after between 0 and 20 s, again according to the potentiometer setting.  The output of IC1.C goes low, and T2 and T3 are turned on — and the siren sounds. Any Darlington transistor can be used for T3. At  the same time, C5 charges via R8, reaching the input threshold of IC1.D in about 45 s. When the output of IC1.D swings low, it pulls the inputs of IC1.A low via diode D5: the siren stops and the system returns to the armed state.


If the potentiometers P1 and P2 are replaced by fixed resistors it is possible to build the circuit small enough to fit in a match-box, without the need to resort to SMD components. This is ideal if the circuit is to be used to protect a motorbike. If the alarm system is to be used in a car, an existing door switch contact can be used instead of the reed switch. In this case an RC combination needs to be added to prevent false triggering. Use a 10 µF/25 V electrolytic for C6, a 100 kΩ resistor for R9 and a 1N4001 for D7. It is again possible to wire multiple door switch contacts in parallel: as soon as one contact closes, IC1.B will be triggered.


Author : L. Libertin – Copyright : Elektor