Car Cigar Lighter to USB Power Socket Circuit

Nowadays, almost all computer systems have logic blocks for working with a USB port. A USB port, in practice, is capable of supplying more than 100 mA of continuous electric current at 5V to the peripherals which are hooked up with the bus. So a USB port could be utilized, without having any problems, for powering 5V DC operated tiny electronic devices.

Cigar-Plug-USB-Power-Socket Circuit diagram

Today, a lot of handheld gadgets (for example, portable reading lamps) utilise this resource of the USB port to recharge their built-in battery pack using the support of an internal circuitry. Typically 5V DC, 100mA electric current is needed to satisfy the input electrical power demand.

The above diagram shows the circuit of a versatile USB power socket that properly converts the 12V battery voltage into stable 5V. This circuit can make it possible to power / recharge any USB power-operated device, working with in-dash board cigar lighter socket of the car.

 

usb qwe

The DC supply presented from the cigar lighter socket is fed to an adjustable, three-pin regulator LM317L (IC1).

Usb Pin 

Capacitor C1 buffers any disorder in the input supply. Resistors R1 and R2 regulate the output of IC1 to constant 5V, that is accessible at the ‘A’ type female USB socket. Red LED1 signifies the output condition and zener diode ZD1 acts as a protector against excessive voltage.

Assemble the circuit on a general purpose PCB and enclose inside a slim plastic cabinet as well as the indicator and USB socket. Whilst wiring the USB outlet, make sure proper polarity of the supply. For interconnection between the cigar plug pin as well as the device, use a long coil cord as shown in second image.

Vest Pocket VHF FM Test Generator

After licensing restrictions were relaxed in many countries for VHF FM band transmitters with 50 nW transmit power, several small, inexpensive FM transmitter modules appeared on the  market. In the author's view, such a module forms a good basis for a small FM-band test generator. This only requires a sine-wave modulation signal, which can come from an  existing audio generator. lf you don't have a suitable audio generator available, you can build the Wien bridge oscillator described here.

Circuit diagram :

Vest Pocket VHF FM Test Generator-Circuit Diagram

Vest Pocket VHF FM Test Generator Circuit Diagram

FET T1 provides amplitude stabilisation in order to keep the distortion low.The generated signal is fed to the transmitter module via a 3.5-mm stereo headset socket, which mates with the usual 3.5-mm stereo plug of the FM transmitter (the left and right terminals of the socket are wired together). Adjust the output level of the audio oscillator with potentiometer Pl to avoid overdriving the transmitter.

ln the transmitter module used by the author, the HF stage is built  around a Rohm BH1418FV lC. You can easily find the data sheet for this lC with a Google search, and it can help you identify the HF output on the transmitter circuit board. You can then use a length of coax cable to tap off the FM signal and feed it to the antenna connector of the receiver under test. Here you must pay attention to the maximum rated input level of the receiver and impedance matching, and if necessary you should use an attenuator at the receiver input. You can use an oscilloscope to trace the signal in the receiver and analyse the receiver output signal.

Author : Kai Riedel  - Copyright : Elektor

Simple Bass Treble Tone Control

The LM1036 is a DC controlled tone (bass/treble), volume and balance circuit for stereo applications in car radio, TV and audio systems. An additional control input allows loudness compensation to be simply effected. Four control inputs provide control of the bass, treble, balance and volume functions through application of DC voltages from a remote control system or, alternatively, from four potentiometers which may be biased from a zener regulated supply provided on the circuit.

Circuit diagram :

bass-treble-tone-control-circuit

Bass Treble Tone Control Circuit Diagram

Each tone response is defined by a single capacitor chosen to give the desired characteristic.

Features:

  • Wide supply voltage range, 9V to 16V
  • Large volume control range, 75 dB typical
  • Tone control, ±15 dB typical
  • Channel separation, 75 dB typical
  • Low distortion, 0.06% typical for an input level of 0.3 Vrms
  • High signal to noise, 80 dB typical for an input level of 0.3 Vrms
  • Few external components required

Note:

Vcc can be anything between 9V to 16V and the output capacitors are 10uF/25V electrolytic

Network Wiring Tester Schematic Circuit

Like all its counter Parts, this network wiring tester comprises two elements, a transmitter unit, powered and fitted at the network start point, and a receiver unit, passive, which can be moved around from socket to socket. Each of these units carries eight LEDS, identically labelled 1 to 8. By operating a push-button in manual mode, or using a clock in automatic, the eight LEDs light up in sequence on the transmitter unit and obviously they should do the same on the receiver unit. In this way, just by watching the LED lighting cycle on the receiver unit, you can immediately spot any crossed wires, as well as any open circuits (the relevant LED never lights up) or shorts (two or more LEDs light at the same time).

Circuit diagram :

Network Wiring Tester-circuit Daigram

Network Wiring Tester Circuit Diagram

The transmitter unit circuit is simple' The Schmitt-input NAND gate lCl.A is wired as a multi-vibrator, whose speed can be adjusted using Pl, while lCl.B is wired as a simple debounce circuit for button 52, used in manual mode. Switch 51 lets you apply the output of one or the other of these to the input of lC3' a decade counter lC, which here we force to count up to eight by connecting its Q8 output back to its reset input. Its out Puts are not capable of driving LEDs, especially over wiring that be 'dangerous' for them (a short, for example), so a ULN2803 is used to drive the outputs. This integrated network of eight Darlington transistors, each capable of switching up to 500 mA, drives the eight LED5 fitted to the transmitter unit (D12-D19) and feeds its signals to the socket comprising contacts 01-08, to which the wiring to be tested must be connected. At the other end of the cable' via the socket comprising contacts l1-18, is the receiver unit which contains just eight LEDs (D20-D27) and their current limiting resistors.

For the latter to work, there obviously needs to be a common connection between transmitter and receiver. In the case of screened network wiring, the screen can be used for this purpose. Another solution consists of using the earth wire of the electrical installation to fulfilthe same function. But if neither of these solutions is feasible, then you'll have to resign your self to running a flying lead for this purpose. The transmitter unit power supply is obtained from a 'plugtop' adapter supplying around 9V at around 10 mA or so. The supply to lCl and lC3 is regulated at 5 V, even though it's not strictly necessary. For occasional short use, a 9 V battery could be used.

lf the project is intended solely for testing network wiring, 01-08 and l1-18 will be in the form of RJ45 sockets and COM will be connected to their screening contact. Take care to stick to the same numbering for the LEDs on the transmitter and receiver units, and if the project is going to be used in automatic mode, that the LEDs are in the correct order.

Author : Christian Tavernier -  Copyright : Elektor

Simple Low Power AM Transmitter

This transmitter is basic but allows transmission of audio to an AM radio. It consists of an RF oscillator operating in the AM broadcast band, together with a modulator stage, which mixes the incoming audio and the RF. A signal appears on the output, which has an AM component that can be picked up on a nearby AM radio receiver.

Circuit diagram :

simple-am-transmitter.Circuit daigram

Basic Low Power AM Transmitter Circuit Diagram

The transmitter consists of oscillator stage Q1 and modulator/buffer stage Q2. Q1 is biased via R1, R2, and R3. L1, C3, and C4 form the tank circuit with feedback network C3-C4 providing feedback to the emitter of Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator stage Q2. Q2 is biased by base current produced by RF rectification in the base emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2 form the tank circuit for the collector of Q2. RF is taken from the junction of C9 and C10 and fed to a short-wire antenna. Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes with the RF signal in the collector circuit of Q2, producing a signal that has sum and difference frequencies if the RF carrier and AF input  along with the carrier signal.

An AM signal appears at the collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collector voltage of Q2 is needed for full modulation of the output. Because of the high level of audio needed, the modulation obtained from this circuit is somewhat limited with conventional audio sources because several volts of audio into a few hundred ohms is needed. The circuit demonstrates the principle of an AM transmitter, however, and with a suitable audio drive level, produces a well modulated AM signal.

Green-Red Multi flasher

This circuit can be made to produce interesting and attractive light effects using just a cluster of red LEDs and one of green LEDs. One effect is first alternating bet ween red  and green, and then lighting red and green together. With the exception of the triple LED devices (Rapid Electronics # 56 - 0205  for green, # 56 - 0200 for red) all parts are cheap and easy to find, possibly even in your junkbox.

Circuit diagram :

Green-Red Multi flasher-Circuit Diagram

Green-Red Multi flasher Circuit Diagram

The values of networks R3/C3, R4/C4 and R5/ C5 govern the length of the flashes. Using the  indicated values, these are about 18 seconds  with a 0.5 second interval.

Because the colours used do not have equal  luminous intensity (expressed in millican-delas) D1 and D2 are silicon diodes and D3 and D4, germanium, with Schottky devices (BAT82) as an alterative because they also exhibit a low forward drop of about 0.3 V. As germanium devices, look for the OA91, OA85 or AA119. If D1 and D2 are omitted, Green and Red are brighter by themselves than when on simultaneously.

MOSFE T T2 switches both LED devices on simultaneously arranging for roughly equal luminous output. The display has an integrated LDR that causes  the LEDs brightness to adapt automatically to darkness and bright light conditions. The circuit has lots of openings for experimentation and adaptation, for example, the flash rate is determined by the value of C1, while the link between the counter’s R (reset)  input and O3 output determines if a space is inserted after the last flash, or not. Colour-ful and lively effects may also be obtained by  using tri-colour LEDs with a common anode.

The power consumption of the circuit depends  largely on the LEDs used. With the Rapid LED  types shown, about 70 mA may be expected  at a 6 volts supply voltage.

Author : Ken Barry – Copyright : Elektor

Automatic Temperature Controlled Fan

Here is a circuit through which the speed of a fan can be linearly controlled automatically, depending on the room temperature. The circuit is highly efficient as it uses thyristors for power control. Alternatively, the same circuit can be used for automatic temperature controlled AC power control.

Circuit diagram :

Automatic Temperature Controlled Fan-Circuit Diagram

Automatic Temperature Controlled Fan Circuit Diagram

In this circuit, the temperature sensor used is an NTC thermistor, i.e. one having a negative temperature coefficient. The value of thermistor resistance at 25°C is about 1 kilo-ohm. Op-amp A1 essentially works as I to V (current-to-voltage) converter and converts temperature variations into voltage variations. To amplify the change in voltage due to change in temperature, instrumentation amplifier formed by op-amps A2, A3 and A4 is used. Resistor R2 and zener diode D1 combination is used for generating reference voltage as we want to amplify only change in voltage due to the change in temperature.

Op-amp μA741 (IC2) works as a comparator. One input to the comparator is the output from the instrumentation amplifier while the other input is the stepped down, rectified and suitably attenuated sample of AC voltage. This is a negative going pulsating DC voltage. It will be observed that with increase in temperature, pin 2 of IC2 goes more and more negative and hence the width of the positive going output pulses (at pin 6) increases linearly with the temperature. Thus IC2 functions as a pulse width modulator in this circuit. The output from the comparator is coupled to an optocoupler, which in turn controls the AC power delivered to fan (load).

The circuit has a high sensitivity and the output RMS voltage (across load) can be varied from 120V to 230V (for a temp. range of 22°C to 36°C), and hence wide variations in speed are available. Also note that speed varies linearly and not in steps. Besides, since an optocoupler is used, the control circuit is fully isolated from power circuit, thus providing added safety. Note that for any given temperature the speed of fan (i.e. voltage across load) can be adjusted to a desired value by adjusting potmeters VR1 and VR2 appropriately. Potmeter VR1 should he initially kept in its mid position to realise a gain of approximately 40 from the instrumentation amplifier. It may be subsequently trimmed slightly to obtain linear variation of the fan speed.

Author : Umesh Shah – Copyright : EFY

3 Transistor Audio Amp (80 milliwatt)

This circuit is similar to the one above but uses positive feedback to get a little more amplitude to the speaker. I copied it from a small 5 transistor radio that uses a 25 ohm speaker. In the circuit above, the load resistor for the driver transistor is tied directly to the + supply. This has a disadvantage in that as the output moves positive, the drop across the 470 ohm resistor decreases which reduces the base current to the top NPN transistor. Thus the output cannot move all the way to the + supply because there wouldn't be any voltage across the 470 resistor and no base current to the NPN transistor.

Circuit diagram :

3 Transistor Audio Amp-Circuit Daigram

3 Transistor Audio Amp Circuit Diagram

This circuit corrects the problem somewhat and allows a larger voltage swing and probably more output power, but I don't know how much without doing a lot of testing. The output still won't move more than a couple volts using small transistors since the peak current won't be more than 100mA or so into a 25 ohm load. But it's an improvement over the other circuit above.  In this circuit, the 1K load resistor is tied to the speaker so that as the output moves negative, the voltage on the 1K resistor is reduced, which aids in turning off the top NPN transistor. When the output moves positive, the charge on the 470uF capacitor aids in turning on the top NPN transistor.

The original circuit in the radio used a 300 ohm resistor where the 2 diodes are shown but I changed the resistor to 2 diodes so the amp would operate on lower voltages with less distortion. The transistors shown 2n3053 and 2n2905 are just parts I used for the other circuit above and could be smaller types. Most any small transistors can be used, but they should be capable of 100mA or more current. A 2N3904 or 2N3906 are probably a little small, but would work at low volume.

The 2 diodes generate a fairly constant bias voltage as the battery drains and reduces crossover distortion. But you should take care to insure the idle current is around 10 to 20 milliamps with no signal and the output transistors do not get hot under load.  The circuit should work with a regular 8 ohm speaker, but the output power may be somewhat less. To optimize the operation, select a resistor where the 100K is shown to set the output voltage at 1/2 the supply voltage (4.5 volts). This resistor might be anything from 50K to 700K depending on the gain of the transistor used where the 3904 is shown.

Solid-State Dark-Room Light

Light-emitting diodes are perfectly suitable for dark-room light, because they (a) obviate the need of filters; (b) emit cold light; (c) have a life that is not shortened by continuous on-off switching; and (d) do not radiate infrarays. The types used must, of  course, have a high light output; fortunately, there are nowadays LEDs with a luminous intensity of hundreds of millicandela.

Circuit diagram :

Solid-State Dark-Room Light-Circuit Diagram

Solid-State Dark-Room Light Circuit Diagram

The sensitivity of photographic paper lies between wavelengths 300 nm and about 550 nm, whereas the wavelength of the light emitted by green LEDs is about 565 nm; that by amber types around 585 nm; and that by red LEDs about 640 nm. From  this, it is clear that all three types of LED may be used with impunity. None the less, in practice, it is best not to use green ones. Because of the special composition and high sensitivity of colour negative paper, only yellow LEDs with reduced light output should be used when processing this paper. The proposed light, therefore, has provision for reducing the emitted light. Note that since colour reversal paper is sensitive to all colours, it can only be processed  in  total  darkness. When working with orthochromatic paper, only red LEDs should be used. With reference to the diagram, each group of three LEDs is fed from a current source, Ti to T6 respectively. The current level, and consequently the light output of the LEDs, is determined by the setting of  Pti Zener diode D,9 provides the reference voltage for the current sources, ensuring that the light out-put of the lighting unit remains virtually constant over the life of the PP3 battery.

Maximum light output is set with the aid of P2. To this end, both Pi and P2 are first set to maximum resistance; after this, P2 is adjusted until a potential of 0.2 V is measured at point A. The maximum current through the LEDs is then about 20 mA. As the photograph shows, the unit has been constructed so that Si is easily operated. Since this switch is a press-to-make type, the light will switch off as soon as it is put aside, thus preserving the battery. It is possible to have the light on continuously by connecting an external battery to Uext. In that  case, Rio must be matched to this source according to Rio=(Uext-9)l0 [k4] but only if NiCd batteries are used. If standard cells are used, D2o and Rio must be omitted.

If a variety of photographic paper is processed, it may be useful to be able to switch between red and amber LEDs. For that purpose, each of the eighteen original yellow LEDs is duplicated by a red LED, shown in dashed lines. Switch S2 may be used to select the relevant bank of LEDs (red or yellow) as required for the specific application.

Simple Mini Running Text Display

This charming little circuit is a genuine four-digit running-text display, complete with a Christmas / New Year’s greeting. Naturally, any competent programmer can easily arrange to have a different text scroll across the display. The associated soft-ware, including the source code, can be downloaded from the Free Downloads section of our website or obtained from Readers Services on diskette (order number 020365-11).

 Simple Mini Running Text Display-Image

As can be seen from the schematic, the hardware consists of little more than an AT90S1200 microcontroller, a 4-digit LED display and a 5-V voltage regulator. The only external circuitry needed by the microcontroller consists of a reset circuit and a 4-MHz crystal, and the remainder of the components are limited to a few decoupling capacitors.

Circuit diagram :

Simple Mini Running Text Display-Circuit Daigram

Simple Mini Running Text Display Circuit Diagram

In the prototype model, an Osram SLO2016 display module was used. Although this four-digit module measures only 10×20 mm, it provides an especially clear and bright display. In order to give the 7805 voltage regulator sufficient ‘breathing room’, the supply voltage should be at least 8 V. A standard 9-V mains adapter should thus be perfectly adequate. The supply voltage does not have to be stabilised, and the adapter does not have to provide an especially large amount of current, since the running-text display draws scarcely more than 50 mA.

 

Author : R. van Arem - Copyright : Elektor

Mini Amplifier with IC LM1895N

This little amplifier, operating from 3...9 V, and providing 1 W output into a 4 4 loudspeaker, is one of those circuits of which you never have enough. The amplifier is based on one 8-pin DIL IC type LM1895N. Electrolytic capacitors C2 and C6 de- couple the supply lines; C7 prevents d.c. reaching the loudspeaker; and C3 and C5 provide a low- impedance path to earth for audio frequencies. The input  signal is applied to pin 4 of the LM1895N via Pi and C4. Resistor R4 and capacitor C8 suppress any tendency to oscillation, i.e., improve the stability.

Circuit diagram :

Mini Amplifier-Circuit Diagram

Mini Amplifier Circuit Diagram

The amplification is determined by Ri and R3: it is of the order of 50. Capacitor C,, in parallel with R,, ensures that the amplification drops off for frequencies above about 20 kHz. If the amplifier is intended for use with a small AM receiver, it is desirable that the amplification starts falling off at a lower frequency. This is brought about by enlarging C,; for instance, if its value is doubled, the amplification starts dropping at 20/2 =10 kHz.

On the printed circuit board shown in figure 2 (which is not available ready made), Pi may be replaced by a wire link; the volume control is then carried out by an external logarithmic potentiometer connected to the PCB via a short length of screened audio cable.

Current consumption is 2.5 mA at 3 V or 7.5 mA  at 9 V under no-signal conditions, and 80 mA at 3 V or 270 mA at 9 V under fully driven conditions: in the latter condition, the output power is 100 mW or 1 W respectively into 4 ohms. The output power for different supply voltages and loudspeaker impedances can be estimated by deducting I V from the supply voltage, and raising the result to the power 2. Divide the number obtained by 8 and then again by the loudspeaker impedance. The sensitivity of the amplifier is about 50 mV. This can be reduced by lowering the value of R1.

TDA1521-Integrated Stereo Amplifier

The Type TDA1521 from Valvo/Mullard is an integrated HiFi stereo power amplifier designed for mains fed applications such as stereo TV. The device works optimally when fed from a ± 16 V supply, and delivers a maximum output power of 2 x 12 W into 8 Q. The gain of the amplifiers is fixed internally at 30 dB with a spread of 0.2 dB to ensure optimum gain balance between the channels.

Circuit diagram :

Integrated Stereo Amplifier-Circuit Diagram

Integrated Stereo Amplifier Circuit Diagram

A special feature of the chip is its built-in mute circuit, which disconnects the non-inverting inputs when the supply voltage is less than ± 6 V, a level at which the amplifiers are still correctly biased. This arrangement ensures the absence of unwanted clicks and other noise when the amplifier is switched on or off. The TDA1521 is protected against output short circuits and thermal overloading. The SIL9 package should be bolted onto a heatsink with a thermal resistance of no  more than 3.3 K/W (RL = 852;  Vs = ± 16 V;  Pd =14.6 W; Ta = 65 °C). Note that the metal tab on the chip package is internally connected to pin 5. The accompanying photograph shows that this high quality stereo amplifier has a very low component count, and is readily constructed on a piece of Veroboard.

The following technical data are stated as typical in the datasheets for the TDA1521 (RL=852; Vs = ± 16 V):

  • Distortion at P°=12 W:           0.5%
  • Quiescent current:               40 mA
  • Gain balance:                        0.2 dB
  • Supply ripple rejection:        60 dB
  • Channel separation:             70 dB
  • Output offset voltage:         20 mV
  • 3 dB power bandwidth:       20-20,000 Hz

Light Gate with Counter

The circuit described here counts the number of times that an infrared beam is interrupted. It could be used to count the number of people entering a room, for instance, or how often a ball or another object passes through an opening (handy for playing shuffleboard). The heart of the circuit consists of you guessed it a light gate! Diode D1 is an IR diode that normally illuminates IR transistor T1. The light falling on T1 causes it to conduct to a certain extent. The resulting voltage on the collector of T1 should be just low enough to prevent the following transistor (T2) from conducting. This voltage can be adjusted within certain limits using P1.

Circuit diagram :

Light Gate with Counter-Circuit Diagram

Light Gate with Counter Circuit Diagram

As soon as an object comes between D1 and T1, the light shining on T1 will be partially or fully blocked, causing the IR transistor to conduct less current. As a result, the voltage on its collector will increase, producing a brief rise in the voltage on the base of T2. This will cause T2 to conduct and generate a negative edge at IC1. This negative edge will trigger the monostable multivibrator, which will then hold the output signal on pin 3 ‘high’ for a certain length of time (in this case, one second). Atthis point, two things will occur. First, a buzzer will be energised by the output of IC1 and produce a tone for approximately one second. When the buzzer stops, a negative edge will be applied to the clock input of IC2, causing the counter in IC2 to be incremented by 1. IC2 is conveniently equipped with an internal binary-to-BCD decoder, so its outputs only have to be buffered by IC3 and T3 to allow the state of the counter to be shown on the 7-segment display. Switch S1 can be used to reset the counter to zero.

If a one-second interval does not suit your wishes, you can modify the values of R3 or C1 to adjust the time. Increasing the value of R3 lengthens the interval, and decreasing it naturally shortens the interval. The same is true of C1. When building the circuit, make sure that T1 is well illuminated by the light from D1, while at the same time ensuring that T1 ‘sees’ as little ambient light as possible. This can best be done by fitting T1 in a small tube that is precisely aimed toward D1. The longer the tube, the less ambient light will reach T1. The sensitivity of the circuit can be adjusted using P1.

 

Author : T.Hareendran - Copyright : Elektor

12-V Glow Plug Converter

Most small internal-combustion engines commonly used in the model-building world use glow plugs for starting. Unfortunately, glow plugs have an operating voltage of 1.5 V, while fuel pumps, starter motors, chargers and the like generally run on 12 V. This means that a separate battery is always needed to power the glow plug. The standard solution is to use an additional 2-V lead storage battery, with a power diode in series to reduce the voltage by approximately 0.5 V. However, this has the annoying consequence that more than 30 percent of the energy is dissipated in the diode. Naturally, this is far from being efficient.

Circuit diagram :

12-V Glow Plug Converter-Circuit Diagram

12-V Glow Plug Converter Circuit Diagram

The converter presented here allows glow plugs to be powered from the 12-V storage battery that is usually used for fuelling, charging, starting and so on. A car battery can also be used as a power source. Furthermore, this circuit is con-siderably more efficient than the approach of using a 2-V battery with a series power diode.

The heart of the DC/DC converter is IC1, a MAX 1627. The converter works according to the well-known step-down principle, using a coil and an electrolytic capacitor. Here the switching stage is not integrated into the IC, so we are free to select a FET according to the desired current level. In this case, we have selected a 2SJ349 (T1), but any other type of logic-level FET with a low value of RDSonwould also be satisfactory. Of course, the FET must be able to handle the required high currents.

Diode D1 is a fast Schottky diode, which must be rated to handle the charging currents for C2 and C3. This diode must also be a fairly hefty type. The internal resistances of coil L1 and capacitors C2 and C3 must be as low as possible. This ensures efficient conversion and prevents the components from becoming too warm.

The resistor network R2/R3 causes 87 percent of the output voltage to be applied to the FB pin of IC1. This means that an output voltage of 1.5 V will cause a voltage of approximately 1.3 V to be present at the FB pin. The IC always tries to drive the switching stage such that it ‘sees’ a voltage of 1.3 V on the FB input. If desired, a different output voltage can be provided by modifying the values of R2 and R3.

When assembling the circuit, ensure that C5 and C1 are placed as close as possible to IC1, and use sufficiently heavy wiring between the 12-V input and the 1-5-V output, since large cur-rents flow in this part of the circuit. A glow plug can easily draw around 5 A, and the charging current flowing through the coil and into C2 and C3 is a lot higher than this!

Author : P. Goossens - Copyright : Elektor

Animal Friendiy Mousetrap

Animal Friendiy Mousetrap-Image This mousetrap is built around a PlC12F683 and uses an infrared transmissive optical sensor that is modulated at a frequency of 38 kHz, so that it isn't affected by the ambient light. The modulation is carried out by the PlC, which generates a 38 kHz signal at port GP2, which is connected to the lR LED. The lR receiver is a type that is usually found for use with remote controls. lt reacts only to 38 kHz signals. lt reports the presence of an lR signal to the PIC via port GP1.

When the lR lightbeam is broken the PIC turns of the relay via port GP4 and FET T1 , which: causes the door of the mousetrap to close. The transmissive optical sensor is housed inside a small wooden box. A small amount of food is placed inside this box.

Circuit diagram :

Animal Friendiy Mousetrap-Circuit diagram

Animal Friendiy Mousetrap Circuit Diagram

When a mouse walks through the light beam on its way to the food it causes the door to shut behind it and an LED starts flashing. The door is normally kept open by the coil of a relay that has been taken apart. When the coil is no longer powered the tin door is pushed shut by means of a spring. A piece of glass or transparent plastic should be put on top of the box, so that the mouse doesn't have to enter a dark space. When a mouse has been caught it can be let free again somewhere outside, some distance away from the house.

The reset button has to be pressed to ready the trap for its next victim. The author has managed to catch a few dozen mice with this device. The program is written in PICBASIC Pro and can be freely downloaded from the Elektor website, it is found in archive file # 100308-11.zip.

Author : Kees Reedijk Copyright :Elektor

Guitar and Microphone Amplifier

This simple audio power amplifier works with guitars and microphones, delivering an output power of around 20W over a load of 4 ohms.  The amplifier circuit is built around IC TDA2005M and a few discrete components. It uses only one potentiometer (VR1), which is quite advantageous as potentiometers take up a lot of space on the board and are difficult to mount and change if damaged. Also, if mounted improperly, potentiometers capture a lot of parasitic signals, especially if there is a strong magnetic field nearby. TDA2005M in bridged configuration is a low-cost IC that provides enough power and quality for portable audio amplifiers. The gain is set to 50 dB and can be reduced if needed. The bandwidth at -3 dB is 40 Hz to more than 20 kHz. The total harmonic distortion (THD) at up to 15W is maximum 1 per cent.

Circuit diagram :

Guitar and Microphone Amplifier-Circuit Diagram

Guitar and Microphone Amplifier Circuit Diagram

The amplifier has two inputs: one for microphone (MIC1) and the other for guitar pickup (CON1). When switch S1 is closed, a condenser microphone is used as the input. The microphone is dynamic. The distance between the speaker and the microphone determines the volume and the tone. The gain of the stage with T1 can go up to around 120. To lower it, reduce the value of R4 or increase the value of VR1.

Connector CON1 is a high-impedance input for guitar pick-up. Input resistor R7 has a value of around 1 mega-ohm, which can be lowered to 500 kilo-ohms—a value good enough for most passive pickups. The cable between the guitar with passive pickups and CON1 should be 1.5 to 1.8 metres long. CON1 should ground the inputs when not in use. In an environment full of electromagnetic noise, this could be very useful.  Transistor T1 should have a noise rating of 4 dB or lower, a high DC gain of 420 to 800, and a collector current of around 0.5 mA. Suitable transistors are BC109C, BC547, BC549C and BC550C. PN2222A can also be used.

The amplifier works off 12V-18V regulated/ unregulated power supply or a 12V-18V, 3A adaptor, depending on the required output power and selected load resistance. In all cases, the maximum power supply limit of the IC should be adhered to. The parameters of the amplifier are tested at 14.4V. The quiescent current from a 14.4V power supply is typically 75 mA and maximum 150 mA (without input signals, i.e., all inputs shorted to the ground).

Mount IC TDA2005M using a heat-sink with thermal resistance lower than or equal to 4°C/W. Alternatively, use a minimum 2mm thick aluminum metal plate with a surface area of at least 200 cm2. The electrolytic capacitors and the transistor should not be placed too close to the heat-sink.Connect the amplifier output to a 4-ohm, 20W speaker. The circuit handles well load impedance in the range of 4 to 16 ohms but at higher impedances the output power will be lower. So there is no need to use special speakers made for guitar amplifiers, which are expensive. A 20W-50W speaker will suffice for 50 Hz to 9 kHz. Four 8W, 4-ohm speakers connected as a 32W, 4-ohm circuit will also serve the purpose.

The total harmonic distortion (THD) at 20W is around 10 per cent, which is good enough for such a simple and low-cost amplifier. The noise is not noticeable. The amplifier is versatile and can be adapted to a variety of signal sources and applications.

 

Author : Petre Tzv Petrov - Copyright : EFY

Simple Flashing Lights Schematic

This is a simple flashing lights circuit can be used as beacon. The assembly consists basically of two blinking steps that commands two light bulbs. With the help of P1 you can adjust the flashing frequency between some limits. There are 2 parts for the circuit, the second one works the same way as the other but with the help of a wire bridge or a switch you can choose different operating modes.A bridge between M and 3 means: 2 independent blinks.

Circuit diagram :

Flashing Lights Schematic-Circuit Diagram

Flashing Lights Schematic Circuit Diagram

If there is a bridge between M and 2, then the lamps lights alternatively with a frequency that can be adjusted with P1. And finally there is one more possibility for M and 1, where the lamps blinks at the same time. The flashing lights circuit works with voltages between 3V and 15V. The lamps voltage must be 2/3 of working voltage. R5 and R10 are chosen so that the lamps are about to light.

Automatic Light Switch with Photo Transistor

This is an automatic light switch circuit that turns ON a light bulb in absence of light. The IC 4060 works as an oscillator and generates a signal that is applied to the base of T4 transistor. Phototransistor T3 BPW40 is in conduction when light is present and it keeps the T5 transistor’s base at ground potential.

Circuit diagram :

Automatic Light Switch with PhotoTransistor Circuit diagram

Automatic Light Switch with PhotoTransistor Circuit Diagram


When light goes out and it doesn’t reaches the phototransistor, it gets in insulation. T3 reaches positive potential through 22kohm. T4 and T5 both gets in conductance. The oscillator signal reaches the optic coupler and then turns on the lamp.

12-volt Cellar Drain Pump

This circuit lets you control a pump, to keep the level of water in a cellar below a certain threshold, for example. Power  is supplied to the pump by a battery that is recharged auto matically when the AC power line voltage is present.

Circuit Diagram

12-volt Cellar Drain Pump-Circuit Diagram

12-volt Cellar Drain Pump Circuit Diagram

lf the water  level  rises, the electrodes touch the liquid and a current begins to flow. The transistor then conducts and the pump runs. The pump stops when the water level has dropped sufficiently for the electrodes to no longer be in contact with it but not straight away, as the voltage on the transistor gate is maintained for a few seconds more by the 470 ytF capacitor. This makes it possible to ensure  the electrodes are completely clear of the water.

The battery is constantly tested by the comparator circuit around the T1071 lC.  lts output drives the  gate of the triac  in the transformer primary circuit via the optoisolator. The transformer secondary charges the battery via the rectifier,  using as little power as possible, and in this way keeps the battery at 13.2 V.

 

Author : Gustave Bolkaerts   - Copyright : Elektor

Stereo Power Amplifier Circuit based on BA5417

BA5417 is a stereo amplifier IC with a lot of good features like thermal shut down, standby function, soft clipping, wide operating voltage range etc. The IC can deliver 5W per channel into 4 ohm loud speakers at 12V DC supply voltage. The BA5417 has excellent sound quality and low THD (total harmonic distortion) around 0.1% at F=1kHz; Pout=0.5W.

Circuit Diagram :

stereo-amplifier-circuit BA5417-

Stereo Power Amplifier  Circuit Diagram

Setup and working of this stereo power amplifier circuit is somewhat similar to the BA5406 based stereo amplifier circuit published previously. C10 and C11 are DC decoupling capacitors which block any DC level present in the input signals. C2 and C6 couples the amplifiers left and right power outputs to the corresponding loud speakers. C1 and C5 are bootstrap capacitors. Bootstrapping is a method in which a portion of the amplifiers is taken and applied to the input. The prime objective of bootstrapping is to improve the input impedance. Networks R1,C3 and R2,C7 are meant for improving the high frequency stability of the circuit. C4 is the power supply filter capacitor. S1 is the standby switch. C8 is a filter capacitor. R3 and R4 sets the gain of the left and right channels of the amplifier in conjunction with the 39K internal feedback resistors.

Notes.
  • Supply voltage range of BA5417 is from 6 to 15V DC.
  • The recommended supply voltage for this circuit is 12V DC.
  • The power supply must be well regulated and filtered.
  • BA5417 requires a heatsink.
  • The circuit can be assembled on a perf board without much degradation in performance.

 

Source : circuitstoday.com

AC Power Indicator

The AC power line indicator presented here has a complete galvanic isolation from the grid. The indicator is an LED that lights up when a current flows, although the current can be measured more accurately with an AC voltmeter set to its mV range. The detector is a transformer taken from an old  mobile phone charger. The value of the secondary isn't important because we only make use of the primary 230 V (115 V) winding. The (extension) cable through which the current has to be detected should have  an  as  short  as possible  section  of its outer insulation removed. The wires should then be moved apart.

The blue wire should be placed on top of the transformer and the brown wire underneath, or the other way round. The brown and blue isolation shouldn't be removed, so there is no danger of the AC line voltage becoming exposed.lf there is a green/yellow wire as well, this can be placed on either side of the transformer. The brown and blue wires should be in parallel with the windings on the transformer. The secondary winding(s) should be left open circuit so that they don't attenuate the measured signal. 

Circuit diagram :

AC Power Indicator-Circuit Diagram

AC Power Indicator Circuit Diagram

In our prototype we found that an alternating 50 Hz voltage of about 2 mV was induced when a 30 watt soldering iron  was connected to the extension lead. With higher-powered devices the measured voltage rises proportionally. Since it  is unlikely that the iron core of the transformer will ever become saturated, the relationship between the measured  voltage and the currentflo should be fairly  linear.

The transformer output signal is amplified bv a differential amplifier built around T1 and T2. ; you wish, you can connect  an AC voltmeter across the collectors ofTl  and  T2 to get an 'ndication  of the  size of the current. The rest of the circuit takes care of lighting up the LED ',',,hen a current flows  through the (extension) cable. The measured signal is amplified again by T3 and then T4 is used to drive the LED with a 50Hz square wave. A 9 V battery is suitable for the power  supply.

When a capacitor is connected  in parallel with the primary winding of the transformer it can make the circuit less  sensitive to frequencies other than 50 Hz. ldeally, the circuit should resonate at exactly 50 Hz. This will make the circuit most sensitive. The capacitor should be chosen such that the measured signal across the collectors of T1 and T2 is at a maximum for a certain current flow. However, the capacitor isn't vital and the circuit stillworks well when just the transformer is used. When a low-currenttype is used forthe LED, Rl3 can be increased to  1.2 ka  (= 5 mn max. for D1).

 

Author : Jacob Gestman Geradts - Copyright : Elektor

Stereo Power Amplifier Using IC 7905

79xx is a widely known series of low-cost, fixed-negative-voltage regulators. These integrated circuits are available with output current of 100-150 mA (L series), 0.4-0.5A (M series), up to 1A (standard series), etc. They can be used in many applications other than regulators, audio power amplifier being one of them.  As shown in the circuit diagram, a simple stereo audio amplifier is built around two 7905 negative-voltage regulators (IC1 and IC2) and a few discrete components. The 7905 IC (a -5V regulator) used here is readily available. However, the circuit will also work with other 79XX regulators if appropriate power supply is used. Both channels shown in the diagram are identical. Hence the description below is only for the first channel. The quality of the output signal is within acceptable limits.

Circuit diagram :

Stereo-Power-Amplifier Circuit Daigram

Stereo Power Amplifier Circuit Diagram

Regulator IC 7905 works as an amplifier for the voltages applied to common pin2 (Ground or GND). The minimal voltage drop over the standard 7905 is around 2V and it depends on the output current. Feedback resistors in the IC set the gain of the channel internally. The amplifier is a class-A audio amplifier. The regulator IC produces the negative output signal.

Resistor R3 provides the positive signal. It limits the maximum output current of the regulator during the negative half period of the amplified sinusoidal signal. The minimal applicable value of R3 for the regulator 7905 is 8.2 to 10 ohms per 5W.  Optimisation of the value of R3 depends on the output voltage of the regulator, negative power supply (–5V) and load resistance of loudspeaker (LS1). If the required output current for LS1 is below 100 mA, the value of resistor R3 can be 33 to 51 ohms per watt.

Normally, the load resistance of the loudspeaker should be higher than of R3 in order to obtain a large peak-to-peak amplitude. But this can be neglected in order to obtain lower power dissipation on R3 and the IC. The circuit works with any load resistance (R3 in parallel with LS1 as the load) under the condition that the regulator is not overloaded with current and power dissipation. However, it is preferable to use a loudspeaker with a high resistance (8 ohms, 16 ohms or more). The amplifier works well with low-impedance headphones having a resistance of 24 to 32 ohms. The voltage difference between the ground pin of 7905 and the output pin is fixed internally.

The input resistance of the amplifier is relatively low and depends on potentiometer VR1 and input resistance of the ground pin. Practically, any stereo output capable of driving 24- or 32-ohm headphones and loudspeakers can drive the input of the stereo amplifier with 7905. If VR1 is removed, the amplifier will still work but there will be more distortion. Therefore potentiometer VR1 is used to provide sufficient variable audio signal.  The values of output capacitors C10 and C11 are usually between 0.1 µF and 1 µF. A small resistance can be connected in series with them if needed. S2 is the on/off switch. Switch S1 is for mono/stereo selection. When switch S1 is closed, the amplifier works as a two-way mono amplifier. If S1 is open, the amplifier works as a stereo amplifier.

The circuit is powered by a 12V battery. The positive terminal of the battery is the common node. The negative terminal is connected to pin 2 of IC1, which is the –12V supply line. The maximum operating voltage can be up to –35V. If no input signal is applied, the DC voltage on the output of the regulator 7905 should be around –5V, which depends to some extent on the value of VR1. The maximum output current of 7905 can be up to 1A and the maximum power dissipation is up to 15W. IC 7905 has internal thermal protection.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix the stereo female jack on the front panel and speaker to the rear side of the cabinet, and the 12V battery inside the cabinet. Fix LED1 and switches S1 and S2 too on the front panel of the cabinet. Mount the regulator IC 7905 on a heat-sink with thermal resistance below 15°C/W. The metallic part on the case is internally connected with the input pin of the regulator.

 

Author : Petre tzv. Petrov - Copyright : EFY

VGA Background Lighting

More and more people are using a PC (conventional or notebook) to view films. The VGA output can be used to provide a matching ‘Ambilight’ effect for this. If you restrict your-self to a single RGB LED, you can also draw the power for this circuit from the VGA connector, along with the RGB signals.

The following pins of the 15-way VGA connector (three rows of five pins) are used for  this circuit:

Pin 1:  Red video signal
Pin 2:  Green video signal
Pin 3:  Blue video signal
Pin 5:  GND
Pin 9:  +5 V

The video signals for the red, green and  blue channels are available at the RGB out-puts. These signals have an amplitude of 1 to 1.35 V, and they output the screen imagery at the rate of dozens of frames per second. This produces the visible image on the screen. The circuit described here drives an RGB LED according to the average values of each of these three signals. Of course, this is not a full-fledged ‘Ambilight’ system, but the RGB LED will produce a nice green light during a  football match or an orange hue if a sunset is shown on the screen.

VGA Background Lighting-Circuit Daigram

A sawtooth generator is built around IC1 and T1. It supplies a nice sawtooth signal to opamp IC2a via R6. The frequency of the sawtooth signal is approximately 850 Hz, and its amplitude ranges from 1.6 to 3.4 V. IC2A subtracts approximately 1.6 V from this due to voltage divider R4/R5. After this, voltage  divider R10/R11 reduces the peak value of the sawtooth to around 1.35 V. The resulting sawtooth signal is buffered by IC2b and  used to drive the three comparators in IC3. The level of the red video signal is averaged  by the R12/C2 network. IC3a constantly com-pares the previously generated sawtooth signal with the average value of the red video  signal. If the image has a high red content, the output of IC3a will be logic Low a good deal of the time, while with a low red content  it will be Low less often. This comparator circuit thus implements a PWM driver for the red LED. The same arrangement is used for the green and blue channels.

Note that with a notebook computer you always have to enable the VGA first, usually by pressing Fn-F5. If you use a desktop or tower PC, you can tap off the video signals from an adapter connected between the video cable and the monitor.

You can also use several LEDs or a LED strip (available from Ikea and other sources) in place of a single RGB LED. In this case you will need an external power supply for the LEDs, but the control circuit can still be powered from the PC. If you use multiple LEDs or a LED strip, connect the cathodes (negative leads) of the LEDs to the comparator outputs of IC3 as shown on the schematic diagram, and connect all the anodes (positive leads) to the external power supply. Resistors R15–R17 are often already integrated in the LED strip. There’s no harm in using an external supply with a higher working voltage, such as 12 V. Remember to connect the ground terminal of the external supply to the ground of the control circuit.

IC3 can handle a current of 15 mA on each  output. If this is not enough, swap the connections to the inverting and non-inverting inputs of the three comparators in IC3 and  connect their outputs to the bases of three  BC547 transistors. Connect a 10-kΩ resistor between each base and the positive supply  line (+5 V). Connect the emitter of each transistor to ground, and connect the collector  to the LED strip. A BC547 can switch up to  100 mA with this arrangement, and a BC517  can handle up to 500 mA.

Author : Heino Peters - Copyright : Elektor

Solid-State Switch For Dc-Operated Gadgets

This solid state DC switch can be assembled using just three transistors and some passive components. It can be used to switch on one gadget while switching off the second gadget with momentary operation of switch. To reverse the operation, you just have to momentarily depress another switch.

The circuit operates over 6V-15V DC supply voltage. It uses positive feedback from transistor T2 to transistor T1 to keep this transistor pair in latched state (on/ off), while the state of the third transistor stage is the complement of transistor T2’s conduction state.

Initially when switch S3 is closed, both transistors T1 and T2 are off, as no forward bias is available to these, while the base of transistor T3 is effectively grounded via resistors R8 and R6 (shunted by the load of the first gadget). As a result, transistor T3 is forward biased and gadget 2 gets the supply. This is indicated by glowing of LED2.

Circuit diagram :

Solid-State Switch For Dc-Operated Gadgets-Circuit Diagram

Solid-State Switch For Dc-Operated Gadgets Circuit Diagram

When switch S1 is momentarily depressed, T1 gets the base drive and it grounds the base of transistor T2 via resistor R4. Hence transistor T2 (pnp) also conducts. The positive voltage available at the collector of transistor T2 is fed back to the base of transistor T1 via resistor R3. Hence a latch is formed and transistor T2 (as also transistor T1) continues to conduct, which activates gadget 1 and LED1 glows.

Conduction of transistor T2 causes its collector to be pulled towards positive rail. Since the collector of T2 is connected to the base of pnp transistor T3, it causes transistor T3 to cut off, switching off the supply to gadget 2) as well as extinguishing LED2. This status is maintained until switch S2 is momentarily pressed. Depression of switch S2 effectively grounds the base of transistor T1, which cuts off and thus virtually opens the base-emitter circuit of transistor T2 and thus cutting it off. This is the same condition as was obtained initially. This condition can be reversed by momentarily pressing switch S1 as explained earlier.

EFY lab note. During testing, it was noticed that for proper operation of the circuit, gadget 1 must draw a current of more than 100 mA (i.e. the resistance of gadget 1 must be less than 220 ohms) to sustain the latched ‘on’ state. But this stipulation is not applicable for gadget 2. A maximum current of 275 mA could be drawn by any gadget.

Author : Praveen Shanker - Copyright : EFY

Vocal Adaptor for Bass Guitar Amp

These days, music is a major hobby for the young and not-so-young. Lots of people enjoy making music, and more and  more dream of showing off their talents on  stage. But one of the major problems often encountered is the cost of musical equipment. How many amateur music groups sing through an amp borrowed from a guitarist or bass player?

This is where the technical problems arise not in terms of the .25” (6.3 mm)  jack, but in terms of the sound quality (the words are barely understandable) and volume (the amp seems to produce fewer decibels than for a guitar). What’s more, unpredictable feedback may cause damage to the speakers and is very unpleasant on the ear. This cheap little easy-to-build project can help solve these technical problems.

Vocal Adaptor for Bass Guitar Amp-Circuit Diagram

A guitar (or bass guitar) amplifier is designed first and foremost to reproduce the sound of the guitar or bass as faithfully as possible. The frequency response of the amp doesn’t need to be as wide or as flat as in hi-fi (particularly at the  high end), and so this sort of amplifier won’t  permit faithful reproduction of the voice. If you build an adaptor to compensate for the amp’s limited frequency response by amplifying in advance the frequencies that are then attenuated by the amp, it’s possible to improve the quality of the vocal sound. That’s just what this circuit attempts to do.

The adaptor is built around the TL072CN low-noise dual FET op-amp, which offers good value for money. The NE5532 can be used with almost the same sound quality, but at (slightly) higher cost. The circuit breaks  down into two stages. The first stage is used to match the input impedance and amplify the microphone signal. For a small 15 W guitar or bass amplifier, the achievable gain is about 100 (gain = P1/R1). For more powerful amplifiers, the gain can be reduced to  around 50 by adjusting P1. The second stage  amplifies the band of frequencies (adjustable using  P2 and P3) that are attenuated by the guitar amp, so as to be able to reproduce the (lead) singer ’s voice as clearly, distinctly, and  accurately as possible. To refine the adaptor and tailor it to your amplifier and speaker, don’t be afraid to  experiment with the component values and the type of capacitors.

The circuit can readily be powered using a 9 V battery, thanks to the voltage  divider R4/R5 which converts it into a symmetrical  ±4.5 V supply.

Author :Jérémie Hinterreiter - Copyright : Elektor

Schematic Security System Switcher

An audio signal can be used as a form of input to control any security system. For example, an automatic security camera can be configured to respond to a knock on the door. The circuit described here allows the security system to automatic in on state. It uses a transducer to detect intruders and a 5V regulated DC power supply provides power to the circuit.

As shown in Fig. 1, a condenser microphone is connected to the input of small signal Pre- amplifier built around transistor T1. Biasing resistor R1 determines to a large extent the microphone sensitivity. A microphone usually has an internal FET which requires a bias voltage to operate. The sound picked up by the microphone is amplified and fed to input pin 2 of IC1 (LMC555) wired in monostable configuration.

Circuit diagram :

Schematic Security system switcher-Circuit Diagram

Fig. 1: Schematic Security system switcher Circuit Diagram

IC2 (CD4538B) is a dual, precision monostable multivibrator with independent trigger and reset controls. The output of IC1 is connected to the first trigger input pin 4 of IC2(A) through switch S1. If an intruder opens or breaks the door, IC1 is triggered by sound signals; the timer output pin 3 of IC1 goes high and enables first monostable multivibrator IC2(A). IC2(A) provides a time period of around 5 to 125 seconds, which is adjusted with preset VR1.

Another monostable multivibrator IC2(B) also provides a time period of around 25 to 600 seconds, which is adjusted with preset VR2. The output of IC2(B) is used to energise relay RL1. Indicator LED1 is provided to display the relay activity. Any AC/DC operated security gadget is activated or deactivated through a security switch. Thus, the security switch of the gadget is connected in the n/o contacts of the relay.You can also operate high power beacons, sirens or hooters in place of the security switch for any AC/DC operated security gadget.

Proposed cabinet

Fig. 2: Proposed cabinet

Assemble the circuit on a general-purpose PCB and enclose it in a cabinet as shown in Fig. 2 along with 5V adaptor for powering the circuit. Connect the security switch according to the circuit diagram and use appropriate AC/DC power supply required to operate the security gadget.

Warning! All relevant electrical safety precautions should be taken when connecting mains power supply to the relay contacts. With the help of single pole double throw (SPDT) switch S1, internal or external trigger input (active high signal) can be selected.

Electronic Telephone Ringer

This circuit produces a ringing sound similar to that made by more recent telephones. It consists of three almost identical oscillators connected in a chain, each generating a squarewave signal. The frequency of each oscillator depends on the RC combination: R4 and C1 around IC1.A, R8 and C2 around IC1.B and R12 and C3 around IC3.C. The pairs of 100 kΩresistors divide the asym-metric power supply voltage (between 5 V and 30 V) so that, in conjunction with the 100 kΩfeedback resistors (R3, R7 and R11) either one third or two thirds of the supply voltage will be present at the non-inverting inputs to the opamps. The voltage across the capacitor therefore oscillates in a triangle wave between these two values.

Circuit diagram :

Electronic Telephone Ringer-Circuit Diagram

Electronic Telephone Ringer Circuit Diagram

The first oscillator is free-running at a frequency of approximately 1/3 Hz. Only when its output is high, and D1 stops conducting, can the second oscillator run. The frequency of the second oscillator is about 13 Hz, and optional LED D3 flashes when it is running. When the output of the second oscillator is low, the third is allowed to run. The frequency of the third oscillator is around 1 kHz, and this is the tone that is produced. The second oscillator is not absolutely necessary:

its function is just to add a little modulation to the 1 kHz tone. A piezo sounder is connected to the output of the third oscillator to convert the electrical signal into an acoustic one. The current consumption of the circuit is just under 1 mA with a 5V power supply, rising to about 1.65 mA with a supply volt-age of 15 V.

 

Author : L.Libertin - Copyright : Elektor

Intruder Radio Alert System

Consider a situation where a burglar has entered your house and snapped the telephone wires, leaving you with no means of communication with the outside world. In such an emergency, you will find this intruder alarm to be very handy. It transmits a prerecorded emergency message repeatedly for reception by an FM receiver.

The message containing address, geographical location, name, etc is recorded onto a chip. The prerecorded message can then be transmitted repeatedly with the help of an FM transmitter, in the hope that some noble soul will hear it and inform the police about the incident.

Intruder Radio Alert System qwe 

Fig. 1: Block diagram of the intruder radio alert system

The circuit comprises a sound recording-and-playback chip (UM5506BH). This chip consists of a 96kbit SRAM and can record up to six seconds of audio. (For details, refer ‘Mini Voice Processor’ circuit published in April 2000 issue of EFY.) After the required message has been recorded, it is passed to a low-power, VHF FM transmitter wired around BC547 and 2N2369 transistors. The range of this transmitter is 60 to 100 metres using a 40-70cm long wire as an antenna.

Intruder Radio Alert System-Circuit Diagram

Fig. 2: Circuit diagram of intruder radio alert system

The major advantage of this circuit is its low power consumption. The author operated it on 3V button cells (Maxell CR 2032, CR 2025, etc used in digital diaries). To transmit the prerecorded message, the play button is pressed repeatedly. The transmitted message can be heard over the FM receiver.

A possible modification, though it has legal complications, is to vary the coil inductance such that the transmission is on police band, thus alerting the police for quick help. Even the need of repeatedly pressing play button can be obviated by configuring an astable multiviberator (using IC 555 timer) to trigger IC UM5506BH every six seconds so that the message is played repeatedly.

 

Author : David Nash Pious – Copyright : EFY

Remote Control Mains Switch

As the only electronics engineer in my  =family and circle of friends, it is some-times not possible to evade an appeal for help. This time the request came from a friendly elderly lady in a retirement home. In her room the light switch by the door  and the pull cord above the bed operate the light fitting on the ceiling in the middle of the room. However, she would prefer that her standing lamp was operated  by these switches instead, since she does not actually have a light fitting mounted  on the ceiling. This standing lamp has an  on/of f switch in the power cord and is  plugged into a power point. However, it  stands rather far from the bed so that she  always has to find her way in the dark. A  wireless operated power point is not really  a consideration, because it is just a matter of time before the remote is lost. Or maybe not?

Circuit Diagram :

Remote Control Mains Switch-Circuit Diagram

Remote Control Mains Switch Circuit Diagram

Behold a feasible circuit. Buy a wireless power point and an enclosure that is big enough for the remote control and a small piece of prototyping board. On the proto-typing board build the circuit according to the accompanying schematic and (care-fully) open the remote control and solder wires to the push buttons for ‘on’ and ‘off’.  Measure if these are polarised and if that is  the case connect them to the 4N25 opto-couplers as shown in the schematic, where  pin 5 has a higher voltage than pin 4.

The operation is as follows. The lady operates the pull cord or light switch to turn the light on. This causes the mains voltage to be applied to the transformer. The relay is activated which charges C1. While C1 charges, a small current flows through optocoupler 1. The result is that the ‘on’ button on the remote control is pressed.  The remote control switches the corresponding power point on and to which the  standing lamp is connected. The standing  lamp will therefore now turn on. Capacitor C2 is charged at the same time. If the lady pulls the cord again, or if she operates the  switch near the door, the relay will de-energise and C2 discharges across optocoupler  #2. This operates the ‘off’ contact of the  remote control and the light goes out.

The remote control continuous to operate from its normal battery and the white enclosure is attached to the ceiling in place of the light fitting. Diode D1 ensures that C1 is discharged when the relay de-energises. D2 ensures that C2 cannot discharge across the relay, but only across optocoupler 2.

 

Author : Jaap van der Graaff - Copyright :Elektor

8 Stage LED VU Meter

The circuit below uses two quad voltage comparators (LM339) to illuminate a series of 8 LEDs indicating volume level. Each of the 8 comparators is biased at increasing voltages set by the voltage divider so that the lower right LED comes on first when the input is about 400 millivolts or about 22 milliwatts peak in an 8 ohm system.

Circuit diagram :

8 Stage LED VU Meter-Circuit Daigram

8 Stage LED VU Meter Circuit Diagram

The divider voltages are set so that each LED represents about twice the power level as the one before so the scale extends from 22 milliwatts to about 2.5 watts when all LEDs are lit. The sensitivity can be decreased with the input control to read higher levels. I have not built or tested this circuit, so please let me know if you have problems getting it working. The power levels should be as follows:

  • 1 LED = 22mW
  • 2 LEDs = 42mW
  • 3 LEDs = 90mW
  • 4 LEDs = 175mW
  • 5 LEDs = 320mW
  • 6 LEDs = 650mW
  • 7 LEDs = 1.2 Watts
  • 8 LEDs = 2.5 watts

Car Anti-Theft Wireless Alarm

This FM radio-controlled anti-theft alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in the market at reasonable rate, is kept inside.

Receiver is tuned to the transmitter's frequency. When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transistor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energised.

Car Anti-Theft Wireless Alarm Circuit

When an intruder tries to drive the car and takes it a few metres away from the car porch, the radio link between the car (transmitter) and alarm (receiver) is broken. As a result FM radio module gene-rates hissing noise. Hissing AC signals are coupled to relay switching circuit via audio transformer. These AC signals are rectified and filtered by diode D1 and capacitor C8, and the resulting positive DC voltage provides a forward bias to transistor T2.

Thus transistor T2 conducts, and it pulls the base of relay driver transistor T3 to ground level. The relay thus gets de-activated and the alarm connected via N/C contacts of relay is switched on. If, by chance, the intruder finds out about the wireless alarm and disconnects the transmitter from battery, still remote alarm remains activated because in the absence of signal, the receiver continues to produce hissing noise at its output. So the burglar alarm is fool-proof and highly reliable. (Ed: You may have some problem catching the thief, though, if he decides to run away with your vehicle_in spite of the alarm!)

Copyright : EFY

Wide-band Receiver for Spark Transmissions

In the early years of radio technology spark transmissions ruled the (air) waves. They  occupied a relatively wide bandwidth, in what  came to be known as the long waveband. The receivers used had a corresponding band-width, as ‘wide open’ as the proverbial ‘barn door’. Most were simple detectors without an amplifier stage.

Circuit diagram :

Wide-band Receiver for Spark Transmissions-Circuit Daigram

Wide-band Receiver for Spark Transmissions Circuit Diagram

Today when you operate an electric light switch you produce a wideband spark that’s  audible on some radios as a crackle from long through to short waves. The same occurs with intermittent breaks in cables, high voltage strikes, defective transformers, poorly sup-pressed electric motors and all kinds of contacts that open and close. With a suitable receiver it’s possible to trackdown the source of these problems. Tests  using normal radios are largely unsuccessful, simply because they display restricted bandwidth and are too effective at suppressing short interference pulses. After some research the best results were obtained with a wideband Audion receiver.

The requirements for this kind of receiver are totally different from normal radio reception: the receiver must have bandwidth as wide as possible, with maximum sensitivity in the long wave region. A further special request: since the wave packets of a single spark are often extremely short, the receiver should integrate them into a longer pulse whose spectrum should lie well inside the audible range.

As for the circuit, the audion stage in the collector circuitry detunes the input circuit. To prevent self-oscillation we need to add a 10 kΩ resistor. Using an oscilloscope you can see extremely short pulses on the emitter of the BC557 being broadened. The amplitude is frequently sufficient to drive the final amplifier into limiting. A1 μs long input pulse  results in a circa 1 ms long audio pulse in the loudspeaker.

 

Author :  Burkhard Kainka  Copyright  : Elektor

Car-Reversing Horn With Flasher

Here is a simple circuit that starts playing the car horn whenever your car is in reverse gear. The circuit (refer Fig. 1) employs dual timer NE556 to generate the sound. One of the timers is wired as an astable multivibrator to generate the tone and the other is wired as a monostable multivibrator.

Circuit diagram :

Car reverse horn Circuit diagram

Fig. 1: Car Reverse Horn Circuit Diagram

Working of the circuit is simple. When the car is in reverse gear, reverse-gear switch S1 of the car gets shorted and the monostable timer triggers to give a high output. As a result, the junction of diodes D1 and D2 goes high for a few seconds depending on the time period developed through resistor R4 and capacitor C4. At this point, the astable multivibrator is enabled to start oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker, in turn, produces sound until the output of the monostable is high.

When the junction of diodes D1 and D2 is low, the astable multivibrator is disabled to stop oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker, in turn, does not produce sound.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect the circuit to the car reverse switch through two wires such that S1 shorts when the car gear is reversed and is open otherwise. To power the circuit, use the car battery.

The flasher circuit (shown in Fig. 2) is built around timer NE555, which is wired as an astable multivibrator that outputs square wave at its pin 3. A 10W auto bulb is used for flasher. The flashing rate of the bulb is decided by preset VR1.

Circuit diagram :

Flasher circuit

Fig. 2: Flasher Circuit Diagram

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The flasher bulb can be mounted at the car's rear side in a reflector or a narrow painted suitable enclosure.

EFY note. A higher-wattage bulb may reduce the intensity of the headlight. You can enclose both the car-reversing horn and flasher circuits together or separately in a cabinet in your car.

 

Author : Ashok K. Doctor  Copyright : EFY