Push Button Switch De-Bouncer

This circuit will remove the transient spikes and contact bounces from a non-latching push button switch.

Push Button Switch De-Bouncer Circuit Diagram :

Push Button Switch Debouncer Using a 555 Timer-Circuit-Diagram

Notes:

Using a 555 timer as a monostable circuit, it is easy to build a good switch debouncer circuit. There are many circuits for SPDT debouncing, but not many for a normally open, push-to-make press button switch (PBS). The 555 monostable gives an output pulse of around 20 msec with component values shown. The formula for determining the output pulse is:

tout = 1.1 R1 C1

With the values in my circuit this equates to:

tout = 1.1 x 1.8x106 x 10x10-9 = 0.0198 sec or 19.8ms

The 555 circuit can be re-triggered if the input is held low longer than the output pulse. To prevent this happening, I have included a further timing circuit comprised of the 1Meg resistor and 47n capacitor. Normally, the 47n capacitor is discharged via the 1 Meg resistor. When the switch is pressed the capacitor quickly charges and provides a brief negative pulse to the 555 input. When the capacitor is fully charged, the potential across the voltage divider formed by the 10k and 1Meg resistors is insufficient to retrigger the monostable. Releasing the switch quickly discharges the capacitor. The output of a 555 monostable is suitable for connecting to TTL and CMOS logic circuits.

 

Source : www.zen22142.zen.co.uk/Circuits/Switching/debounce.htm

Fast Electronic Fuse

A fast electronic fuse designed to operate on 230V AC with an adjustable trip current. 
  Fast Electronic Fuse Circuit Diagram :
 
 Note :
When the current through the load exceeds a level determined by the position of the wiper on the 1k wire-wound pot, this circuit cuts off the load immediately. If S1 is open, the range is approximately 300-650 mA, and 0.8-2A when it is closed.
The key variable in the operation of the fuse is the voltage drop across the power resistor(s) which are connected in series with the load. This voltage drop is directly proportional to the current the load draws. When this current is low, the voltage across the resistors is also small and cannot trigger T1. At the same time the gate of T2 is fed from a little power supply built around a negative voltage regulator. T2 is conducting and the load is on.
If the current through the load then gets too high, so that the voltage created across the resistor(s) can trigger the gate of T1 through the 330R resistor and the pot: T1 starts to conduct, swiftly taking away all the current from the gate of T2. The voltage drop across T1 (MT1-MT2) will then be only 0.7 V and T2 will be firmly off. T1 stays this way all until the momentary (normally closed, "push-to-break") Reset push-button is pressed: this causes the current through T1 to drop below the hold level and forces this triac to turn off. Releasing the Reset button re-enables the current flow to and through the gate of T2, switching it on.
T1 must be a TIC225M, as this particular type has a very low trigger current. T2 is a snubberless BTA12-600CW which cannot be replaced with a 'normal' triac, but maybe with a BTA12-600BW which also can be used without a snubber circuit, but has a little less sensitive gate (not a problem in this design). If the gates of snubberless triacs are DC-controlled and they are switching AC, the DC to control the gates MUST be negative - flowing from the gate, hence the negative voltage regulator. (Snubberless is in fact a registered trade mark belonging to ST.)
The output AC voltage will be 1-5 volts below the input level, depending on the load. Varistors are 250V AC. Pay attention to the "reverse" polarities of the electrolytic capacitor and the diodes before the 7906.
DANGER! This circuit connects directly to 220-230V AC which can be lethal! Please do not attempt to build any of the circuits/projects unless you have the expertise, skill and concentration that will help you avoid an injury. Please see Disclaimer on this site.

Energy-Efficient Backlight

The backlights used in some LCD panels are not exactly economical: typical current draws of 20 mA to 100 mA are common. Normally the current is determined by a series resistor, which leads to additional power losses. It is considerably more efficient, if a little more complex, to use a switching regulator IC. Alternatively, it is often the case that driving the LCD panel is a microcontroller, which we can press into service to provide regulation in software. Fortunately, the regulation does not need to be exceptionally precise.
At the heart of our circuit is T1, a p-channel MOSFET, which is driven by an inverted (active low) pulse-width modulated signal from the microcontroller. Component s D1, L1 and C1 form the remainder of the standard step-down switching regulator configuration. In the circuit diagram the LCD backlight is represented by two LEDs; the current flowing through these LEDs is measured by a shunt resistor, filtered, and finally amplified to a level suitable for input to the A/D converter in the microcontroller using an operational amplifier. R1 ensures that the transistor switches off completely when the microcontroller is reset (at which time all ports become inputs).
Energy-Efficient Backlight Circuit Diagram :
Energy-Efficient-Backlight-Circuit Diagram
The circuit can be used with any microcontroller that can generate an inverted PWM signal at a frequency in the range 10 kHz to 100 kHz. We have developed a demonstration program and code module for the Atmel ATmega32 using GNU C. The source code can be downloaded from the Elektor website at http://www.elektor.com or from the author’s site at http://reweb.fh-wein-garten.de/elektor.
The program generates the PWM sig-nal at 31.25 kHz (if the processor clock is 8 MHz) on the OC2 output (PD2) of the ATmega32 microcontroller. The pulse width can be adjusted in 256 steps. If the gain of the operational amplifier is 25. 5 a current of 100 mA through the LEDs will correspond to a voltage of 2.55 V at the input to the A/D converter. The internal reference voltage of the ATmega32 is nominally 2.56 V, and so an LED cur-rent of 100 mA will lead to a ten-bit conversion result of 03FFh. It is sufficient to monitor only the top eight bits of this value, and depending on the error from the desired value, increment or decrement the pulse width of the PWM signal. This forms an integral controller.
As it stands, this solution can-not compete on simplicity with a series resistor. However, we can make some simplifications to the circuit by eliminating the regulation feedback loop. Dispense with the operational amplifier and surrounding circuitry, and have the software output a fixed PWM signal. This loses the ability of the circuit to compensate for part-to-part variation in the components and for temperature, but in practice such compensation is rarely necessary. The software also supports the simplified version of the circuit.

Author : Rainer Reusch - Copyright : Elektor

Wireless LED Driver

There are times when you want to control a LED indicator light through the side of a plastic box, without wires and without drilling a hole in the box.  One example where this may be needed is in data collection systems.  These are often used out of doors in harsh environments and have to be hermetically sealed. Holes drilled in the side of the box for panel mounted LEDs or light pipes can often leak.

Wireless LED Driver Circuit Diagram :

Wireless-LED-Driver-Circuit Diagram

The circuit below solves this problem by sending power to the LED through the plastic, using a magnetic coupling technique. The circuit below can route power through plastic enclosures as thick as ¼ inch.  The circuit will not work through metal boxes.  An expensive inductor, driven by a series resonant mode 125KHz oscillator, forms the power transmitter.  A similar inductor, wired as a 125KHz parallel resonant circuit, forms the power receiver.  A voltage doubler circuit at the receiver efficiently converts the collected AC into DC.  The circuit will operate over a wide 3v to 6v supply range.

With a 5v supply, the circuit draws about 25ma of current.  However, by gating the oscillator on for a brief 20ms period, with a 0.5Hz rate, the average power can be reduced to about 250 microamps.  If you want to extend the range of operation out to ½ inch, try using a 74C14 (CD4069) with a 12v supply.  Using surface mounted components; the complete LED assembly can be encapsulated and glued to the outside surface of the box.  Tiny unshielded surface mounted inductors can be used to reduce the size of the transmitter and receiver.  However, smaller parts will reduce the power transfer range to perhaps only a 1/8 inch separation.
A very nice bright green LED, which works great for this circuit, is one from Kingbright, available from Digikey, part number 754-1089-1.
Source : discovercircuits.com

40 MHZ-Wide Band RF Amplifier

The sensitivity of receiver it is possible to increase itself considerably, if is interfered, between this and his aerial, a amplifier RF.

40 MHZ-Wide Band RF Amplifier Circuit Diagram :

40 MHZ-Wide-Band RF-Amplifier-Circuit Diagram

The amplifier of circuit, does not use in resonant circuits and for this is suitable, so much for mid waves, what for the low waves, up to the 40 MHZ. The gain his it is the order 20db and it consumes 7mA, when it is supplied with 12 until 15V dc. His entry and the exit are adapted with coaxial cable, complex resistance 75ohm.

Source : users.otenet.gr

Solar Cell Array Charger with Regulator

This simple circuit can be used to charge batteries from a solar cell array. The circuit consists of an oscillator, a DC-DC step-up or ‘boost’ converter and a regulator that pro-vides regulation of the output voltage.The oscillator is built around a hex Schmitt trigger inverter IC, the 40106B, one resistor, R1, inserted between the input and the output of one of the gates in the 40106 to supply charge to C3. Depending on the values of resistor R1 and capacitor C3 you’re using in the circuit, the oscillator will operate at different frequencies, but a frequency below 100 kHz is recommended. By consequence, the oscillator frequency should not exceed the maximum ripple frequency of capacitor C2 connected on the output. C2 should be an electrolytic capacitor with a DC working voltage larger than the desired output voltage. Besides, it should have a low ESR (equivalent series resistance).

Solar Cell Array Charger with Regulator Circuit Diagram :

Solar-Cell Array-Charger with Regulator-Circuit-Diagram

IC1A is used as a buffer, ensuring that the oscillator sees a light, fairly constant load and so guaranteeing that the output frequency remains stable (within limits, of course). VCC of the Schmitt trigger can be connected directly to the battery charged, provided the charged batter y voltage does not exceed the max. or min. limits of the Schmitt trigger’s supply voltage. This ensures the Schmitt trigger can operate even if little power is obtained from the solar cell array.

When transistor T2 is turned on, (output from oscillator buffer IC1A is high), a collector current flows through inductor L1 which stores the energy as a magnetic field and creates a negative voltage VL1. When transistor T2 is switched off, (output from oscillator buffer IC1A is low), the negative voltage VL1 switches polarity and adds to the voltage from the solar cell array. Consequently, current will now flow trough the inductor coil L1 via diode D1 to the load (capacitor C2 and possibly the battery), irrespective of the output voltage level.

Capacitor C2 and/or the battery will then be charged. So, in the steady state the out-put voltage is higher than the input voltage and the coil voltage VL1 is negative, which leads to a linear drop in the current flowing through the coil. In this phase, energy is again transferred from the coils to the out-put. Transistor T2 is turned on again and the process is repeated. A type BC337 (or 2N2222) is suggested for T2 as it achieves a high switching frequency. Inductor L1 should have a saturation current larger than the peak current; have a core material like ferrite (i.e. high-frequency) and low-resistance. Diode D1 should be able to sustain a forward current larger than the maxi-mum anticipated current from the source. It should also exhibit a small forward drop and a reverse voltage spec that’s higher than the output voltage. If you can find an equivalent Schottky diode in the junk box, do feel free to use it.

The most important function of the shunt regulator around T1 is to protect the batteries from taking damage due to overcharging. Besides, it allows the output voltage to be regulated. Low-value resistor R3 is switched in parallel with the solar cell array by T1 so that the current from the solar cell array flows through it. Zener diode D2 is of course essential in this circuit as its zener voltage limits the output voltage when T1 should be turned on, connecting the solar cell array to ground via R3. In this way, there is no input voltage to the boost converter and the battery cannot be overcharged.

Sealed lead-acid (SLA) batteries with a liquid electrolyte produce gas when over-charged, which can ultimately result in damage to the battery. So, it’s important to choose the right value for zener diode D2. Special lead-acid batteries for solar use are available, with improved charge-discharge cycle reliability and lower self-discharge than commercially-available automotive batteries.

Finally, never measure directly on the out-put without a load connected the ripple current can damage your voltmeter (unless it’s a 1948 AVO mk2).

 

Author : Lars Näs - Copyright : Elektor

Light Sensor with Twilight Detection

This is not the first light sensitive circuit to be published in Elektor magazine. This circuit however, distinguishes itself that in addition to light and dark it can also signal twilight (dusk). This lets you automatically turn on a light in the living room when it becomes dark and turn on a lamp in a dark hallway when dusk sets in.

Light Sensor with Twilight Detection Circuit Diagram :

Light Sensor-with Twilight Detection-Circuit Diagram

The circuit described here generates a logic signal on three separate out-puts for light, twilight and dark. The transition thresholds are set with two trimpots.The part of the circuit that is to the left of the dashed line can be located outside, on the roof, for example. This is possible because the LM258 can withstand frost, unlike the LM358, for instance. R1 and R2 together form a light dependent voltage divider, the voltage variations of which are damped by R3 and C1. This is desirable so that the circuit is less sensitive to birds that could cause the curtains to be closed when they fly across the sensor.

Opamp IC1a is wired as a buffer, so that the voltage that is seen by the remainder of the circuit does not deviate too much from the voltage ‘on the roof’. Any arbitrary LDR is suitable for R1, but do make sure that the voltage level at pin 3 of IC1a is at least 2 V below the power supply voltage when it is light. This is because that is the maximum voltage that IC1 and IC2 can tolerate at their inputs. Otherwise fit an additional resistor of, for example, 2.2 kΩ between R1 and the power supply. Two comparators (IC2a and IC2b) compare the incoming voltage with the threshold voltages set by P1 and P2. R4 and R6 (R5 and R7) prevent that that output of IC2a (IC2b) will jitter around the threshold. R8 and R9 have been added because IC2 has open-collector outputs.

It is actually already possible to determine whether it is light, dark or twilight by looking at the outputs of IC2a and IC2b, but the four gates of IC3 turn these into three separate signals. To make the adjustment easier, there are three LEDs of different colour connected to the outputs: green for light, yellow for twilight and red for dark. In the box is a description of the steps that are necessary to adjust the circuit.

It is best to do this towards the evening, that is when it is still light outside before the fall of dusk.To adjust the threshold values, P1 is intended for the transition from light to twilight and P2 for the transition from twilight to dark. With a correctly adjusted circuit, the voltage at the wiper of P1 has to be lower than the voltage at the wiper of P2.Because the outputs of the CMOS gates can-not drive heavy loads, low-current LEDs are essential. These have enough with only 2 mA, while ordinary LEDs will often need 20 mA. The power supply voltage can be from 9 VDC to 15 VDC.

Adjustment :

  1. First turn the wipers of both P1 and P2 to ground. If all is well only the green LED should be on.
  2. Wait until dusk falls.
  3. Now turn P1 just to the point where the green LED turns off and the yellow LED just turns on.
  4. Now wait until it is dark.
  5. Turn P2 just to the point where the yellow LED turns off and the red LED turns on. The adjustment is now complete.

 

Author : Heino Peters - Copyright : Elektor

Simple Electronic Water Alarm

Aburst water-supply hose of the washing machine, a bathroom tap that you forgot to close, or a broken aquarium wall may turn your house into a pond. You can avoid this mess by using an electronic water alarm that warns you of the water leakage as soon as possible.

The acoustic water alarm circuit presented here takes advantage of the fact that the tap water is always slightly contaminated (or has salts and minerals) and thus conducts electricity to a certain extent. It is built around IC LMC555 (IC1), which is a CMOS version of the bipolar 555 timer chip. IC1 is followed by a complementary pair of emitter followers (T1 and T2) to drive a standard 8-ohm speaker (LS1). Power is supplied by a compact 9V PP3 battery.

 Simple Electronic Water Alarm Circuit Diagram :

 
Power is applied when power switch S1 is closed. The reset input (pin 4) of IC1 is held low by resistor R1 (2.2-kilo-ohm). The astable oscillator wired around IC1 is in disabled mode. When probes P1 and P2 become wet, these conduct to reverse the state of IC1’s reset terminal. As a result, the astable multivibrator starts oscillating at a frequency determined by resistor R2 and capacitor C3. The output of IC1 drives the complementary pair of transistors T1 and T2.

Although this combination causes significant crossover distortion, it doesn’t have any adverse effect on the square-wave audio signal processing. A 10-kilo-ohm potentiometer (VRI) is inserted between output pin 3 of IC1 and the bases of transistors T1 and T2 for volume control. The probes can be made using two suitable copper needles or small pieces of circuit board with the copper surface coated with solder. Fit these at the lowest point where water will accumulate. After construction, place the alarm circuit well away from the point of possible leakage. Use a pair of thin twisted flexible wires to connect the probes to the circuit.

Capacitor C1 connected across IC1 input (pin 4 and GND) keeps the alarm circuit from responding to stray electrostatic fields. Similarly, twisting the wires together makes the relatively long connection between the probes and the circuit less sensitive to false alarms due to external electromagnetic interference. Finally, if you want to lower the probe sensitivity, reduce the value of grounding resistor R1.

Author : T.K. Hareendran - Copyright : EFY

Ultra Low Power LED Flasher

The efficiency of some newer LEDs is amazing.  Some of the latest green LEDs can launch blinding light with just one milliamp of current.  I take advantage of one of these newer devices in the circuit below.  The flashing circuit uses a classic multivibrator oscillator, made from a tiny National Semiconductor’s LMC7215 low power voltage comparator.

Ultra Low Power LED Flasher Circuit Diagram :

Ultra Low Power LED-Flasher-Circuit-Diagram

The circuit produces a short 10ms pulse every two seconds, drawing power from a 3v supply.  I suggest using a surface mounted green LED from Kingbright.  Although the LED peak current is restricted to just one milliamp, this part generates a very bright flash of light.  The circuit draws and average DC current of only 6 microamps from a 3v supply. 

When powered by a small lithium coin cell with a 100ma-hour rating, it will flash for about two years.  This flasher might be a great add-on circuit for a flashlight, a key chain or even attached to a cell phone, so the thing can be found in the dark.  Another application might be for a fake car alarm indicator.  The circuit might also be used in conjunction with some other battery powered product, to let the user know when it is in operation.  The small 6uA current drain would not tax even the most stingy power budget. Link

Experimental Hall Sensor

Hall sensors can of course be purchased but making them yourself is far more interesting (and satisfying)!
According to the theory the crucial thing is to use a touch layer that’s as thin as possible; the length and width are unimportant. An ‘obvious’ starting point for our trials would be copper, which in the form of printed circuit board material is easy to find and handle. Copper-clad board may be obvious but not ideal, because it has a very weak Hall constant.

Nevertheless we should be able to use it to demonstrate the Hall effect by using very powerful magnets in our sensor.To achieve detection we need the highest possible level of amplification. In the circuit shown here the voltage amplification is set by the relationship of the two feedback resistors of the first op-amp. With the values given (2.2 MΩ and 330 Ω) produce a gain of 6,667.

Experimental Hall Sensor Circuit Diagram :
Experimental Hall-Sensor-Circuit Diagram
This also creates a convenient bridge connection for taking measurements. The trimmer potentiometer allows fine adjustment. With zero set ting that’s accurate to within millivolts we could use this test point to measure Hall voltages of well below a microvolt. Finally in this way we could also measure the flux density of a magnet.

Copper has a Hall constant of AH= –5.3·10-11m3/C. The thickness of the copper layer is d = 35 µm. The Hall voltage then amounts to:VH= AH× I × B / d

When the field B= 1 T and current I= 1 A a Hall voltage of VH= 1.5 µV is produced. The6,667-fold gain then achieves a figure of 10 mV. The circuit thus has a sensitivity of 10 mV per Tesla. That said, adjusting the zero point with P1 is not particularly easy. The amplifier has a separate power supply in the form of a 9 V battery (BT1). To take measurements we connect a lab power supply with adjustable output current (BT2) to the Hall sensor (the copper surface) and set the current flowing through the sensor to exactly 1 A. Then the zero point must be adjusted afresh.

Next we place a strong Neodymium magnet below the sensor. The output voltage of the circuit should now vary effectively by several millivolts. Note that there are several effects that can influence the measurements we take. Every displacement of the magnet will pro-duce an induction voltage in the power feed wires that is significantly greater than the Hall voltage itself. Every time you move the magnet you must wait a while to give the measurements time to stabilise. With such small voltage measurements problems can also arise with thermal voltages due to temperature variations. It’s best not to move and inch — and to hold your breath as long as possible!

Author : Burkhard Kainka - Copyright : Elektor

Small Personal Alarm

Small, portable, anti-bag-snatching unit. Also suitable for doors and windows control

This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound.

The device can be reverse connected, i.e. the box can be placed in a pocket and the cord connected to the bag. This device can be very useful in signalling the opening of a door or window: place the box on the frame and the magnet on the movable part in a way that magnet and reed switch are very close when the door or window is closed.
Personal Alarm Circuit Diagram :
Mini Alarm Circuit Daigeram Personal Alarm Circuit Diagram
Circuit operation:
A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part count and 3V battery supply allow a very compact construction.
Parts:
R1____________330K   1/4W Resistor
R2____________100R 1/4W Resistor

C1_____________10nF 63V Polyester or Ceramic Capacitor
C2____________100µF 25V Electrolytic Capacitor

Q1____________BC547 45V 100mA NPN Transistor
Q2____________BC327 45V 800mA PNP Transistor

SW1____________Reed Switch and small magnet (See Notes)

SPKR___________8 Ohm Loudspeaker (See Notes)

B1_____________3V Battery (two A or AA cells wired in series etc.)
 
 Notes:
  • The loudspeaker can be any type, its dimensions are limited only by the box that will enclose it.
  • An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
  • Current consumption when the alarm is sounding is about 100mA.
  • If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. mono Jack plug having its internal leads shorted. The Jack plug will be connected to the tiny cord etc.
  • Do not supply this circuit at voltages exceeding 4.5V: it will not work and Q2 could be damaged. In any case a 3V supply is the best compromise.

Source : www.redcircuits.com

Reducing Relay Power Consumption

Relays are often used as electrically controlled switches. Unlike transistors, their switch contacts are electrically isolated from the control input. On the other hand, the power dissipation in a relay coil may be unattractive for battery-operated applications. Adding an analogue switch lowers the dissipation, allowing the relay to operate at a lower voltage. The circuit diagram shows the principle. Power consumed by the relay coil equals V2/RCOIL. The circuit lowers this dissipation (after actuation) by applying less than the normal operating voltage of 5 V. Note that the voltage required to turn a relay on (pickup voltage)is usually greater than that to keep it on (dropout voltage).

Reducing Relay Power Consumption Circuit Diagram : 
Reducing_Relay_Power_Consumption Circuit Diagram

In this respect the relay shown has specifications of 3.5 and 1.5 V respectively, yet the circuit allows it to operate from an intermediate supply voltage of 2.5 V. Table 1 compares the relay’s power dissipation with fixed operating voltages across it, and with the circuit shown here in place. The power savings are significant. When SW1 is closed, current flows through the relay coil, and C1 and C2 begin to charge. The relay remains inactive because the supply voltage is less than its pickup voltage. The RC time constants are such that C1 charges almost completely before the voltage across C2 reaches the logic threshold of the analogue switch inside the MAX4624 IC.

Table_Reducing_Relay_Power_Consumption_Circuit_Diagramq

When C2 reaches that threshold, the on-chip switch connects C1 in series with the 2.5 V supply and the relay coil. This action causes the relay to be turned on because its coil voltage is then raised to 5 V, i.e., twice the supply voltage. As C1 discharges through the coil, the coil voltage drops back to 2.5 V minus the drop across D1. However, the relay remains on because the resultant voltage is still above the dropout level (1.5 V). Component values for this circuit depend on the relay characteristics and the supply voltage. The value of R1, which protects the analogue switch from the initial current surge through C1, should be sufficiently small to allow C1 to charge rapidly, but large enough to prevent the surge current from exceeding the specified peak current for the analogue switch.

The switch’s peak current (U1) is 400 mA, and the peak surge current is IPEAK = (VIN – VD1) / R1 + RON) where RON is the on-resistance of the analogue switch (typically 1.2 Ω). The value of C1 will depend on the relay characteristics and on the difference between VIN and the pickup voltage. Relays that need more turn-on time requires larger values for C1. The values for R2 and C2 are selected to allow C1 to charge almost completely before C2’s voltage reaches the logic threshold of the analogue switch. In this case, the time constant R2C2 is about seven times C1(R1 + RON). Larger time constants increase the delay between switch closure and relay activation. The switches in the MAX4624 are described as ‘guaranteed break before make’. The opposite function, ‘make-before break’ is available from the MAX4625. The full datasheets of these interesting ICs may be found at http://pdfserv.maxim-ic.com/arpdf/MAX4624-MAX4625.pdf

Simple Fuse Monitor Indicator

The idea for this project may have come to me in a flash of inspiration, and its a very simple way to check if a fuse has blown without removing it from its holder.

Simple Fuse Monitor Indicator Circuit Diagram :
 

The simplicity of this circuit uses just two components, but with just one resistor and an LED this circuit gives visual indication of when a fuse has blown. LED1 is normally off, being "short circuited " by the fuse, F1. Should the inevitable "big-bang" happen in your workshop then LED1 will illuminate and led you know all about it! Please note that the LED will only illumininatet under fault conditions, i.e. with a short circuit or shunt on the load. In this case the current is reduced to a safe level by R1.


5-Zone Alarm System

Each zone uses a normally closed contact. These can be micro switches or standard alarm contacts (usually reed switches). Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 - 5 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1-C5. C7 and R14 also form a transient suppresser.

 5-Zone Alarm System Circuit Diagram : 


The key switch acts as the Set/Unset and Reset switch. For good security this should be the metal type with a key. At switch on, C6 will charge via R11, this acts as the exit delay and is set to around 30 seconds. This can be altered by varying either C6 or R11. Once the timing period has elapsed, LED6 will light, meaning the system is armed. LED6 may be mounted externally (at the bell box for example) and provides visual indication that the system has set.

Once set any contact that opens will trigger the alarm, including Zone 1. To prevent triggering the alarm on entry to the building, the concealed re-entry switch must be operated. This will discharge C6 and start the entry timer. The re-entry switch could be a concealed reed switch, located anywhere in a door frame, but invisible to the eye. The panic switch, when pressed, will trigger the alarm when set. Relay contacts RLA1 provide the latch, RLA2 operate the siren or buzzer.
 

Low Cost Garage Stop Light

A novel use of solar cells makes positioning your car in the garage rather easier than old tyres, a mirror, or a chalk mark.The six solar cells in figure 1 serve as power supply and as proximity sensor. They are commercially available at relative low cost. The voltage developed across potentiometer Pi is mainly dependent on the intensity of the light falling onto the cells. The circuit is only actuated when the main beam of one of the car's headlights shines direct onto the cells from a distance of about 200 mm (8 inches). The distance can be varied somewhat with P,
Low Cost Garage Stop Light Circuit Diagram :
Low Cost Garage Stop Light-Circuit Diagram
Under those conditions, the voltage developed across C1 is about 3 V, which is sufficient to trigger relaxation oscillator Ni. The BC547B is then switched on via buffer N2 so that D3 begins to lfash. Diodes Di and D2 provide an additional in-crease in the threshold of the circuit. The total voltage drop of 1.2 V across them ensures that the potential at pin I of the 4093 is always 1.2 V below the voltage developed by the solar cells. As the trip level of Ni lies at about 50 per cent of the supply voltage, the oscillator will only start when the supply voltage is higher than 2.4 V.
The circuit, including the solar cells, is best constructed on a small veroboard as shown in figure 3, and then fitted in a translucent or transparent manmade fibre case. The case is fitted onto the garage wall in a position where one of the car's headlights shines direct onto it. The LED is fitted onto the same wall, but a little higher so that it is in easy view of the driver of the car. When you drive into the garage, you must, of course, remember to switch on the main beam of your headlights!

Flashing Lights for Planes and Helicopters

There are two sorts of lights on aircraft: red or white flashing lights, which are called ‘anti-collision lights’, and steady lights, red on the tip of the left wing, green on the tip of the right wing, and white at the tail, called ‘position lights’, which enable an observer to see if the aircraft is approaching or going away. On the tip of each wing, in addition to the steady lights, there may also be flashing white strobe lights. The position light simulator given here takes a few liberties with the real position lights, making them flash (it’s more fun!) and using a little trick to simulate the strobe effect.

Flashing Lights for Planes and Helicopters Circuit Diagram :

Flashing-Lights for Planes-and Helicopters-Circuit Diagram

The well-known NE555 is found in its SMD version for the timebase, combined with a 4017 decade counter with ten decoded outputs, also in SMD version. Normally, each output is used independently. In this circuit, two out-puts are coupled with a one-output gap: Q0 and Q2 (front left, red LED), Q1 and Q3 (rear left, red LED), Q5 and Q7 (front right, green LED), Q6 and Q8 (rear right, green LED). To avoid the low output’s shorting the High output, a diode is used in series with each output. In this way, we get ‘double flashing’ of each LED, giving the strobe effect.

Output Q4 is used for the tail of the plane (white LED) or helicopter (red LED) with a single flash, without the strobe effect. Output Q9 is used for the reset.Only one LED is lit at any given moment, so the consumption is kept low so as not to reduce battery life in flight. The 150 Ω resistor limits the supply voltage/current to each LED. The circuit’s power rail (4.5 V) can be taken from an unused output on the model’s decoder. A sub-miniature switch could be fitted if necessary, but since a plane or helicopter is required to have its lights on at all times…

 

Author : Jean-Louis Roche - Copyright: Elektor

Simple Six-way Switch Using IC1 40106N

The 40106 is a versatile CMOS IC containing six Schmitt trigger inverters. It can be used to implement a set of alternating –action switches with hard ware contact bounce suppression.

Simple Six-way Switch Circuit Diagram :

Simple Six-way-Switch-Circuit Diagram

Aside from one gate of the IC, all you need for each switch is a pushbutton, a resistor and two capacitors. It works as follows. The 1 μF capacitor at the output is charged or dis-charged via a 1 MΩ resistor, depending on the output level of the inverter.

Pressing the button causes the input level of the gate to change, which in turn causes the output level to toggle. The 10 nF capacitor determines the output state after the supply voltage is switched on. You can connect it to the supply voltage rail or the ground rail as required. If you hold the button pressed, the output signal will be a square wave with a frequency determined by the RC time constant, which is approximately 1 second.

You may experiment with the component values if you wish.

 

Author : Kees van het Hoff - Copyright : Elektor

Water Level Detector

To monitor the filling of a bath, a water-tank, or a swimming pool, or to warn when a gully is overflowing, here’s a very simple water level detector built around a CD4011 CMOS quad NAND chip. Gates IC1.A and IC1.B are wired as an astable multivibrator. The oscillator frequency is determined by C1, R2 and preset P1.

Water Level Detector Circuit Diagram :

Water Level Detector-Circuit Diagram

When quiescent, resistor R1 pulls the input to gate IC1.A down to logic low, which there-fore by default blocks the operation of the oscillator in the absence of water. When water is present between the e+ an d e−electrodes, IC1. A is taken high, enabling the oscillator. The output signal from gate IC1.B is shaped by IC1.C to obtain a rectangular waveform. Gate IC1.D inverts the signal so that transistor T1 is held of f in the absence of water, which avoids current flowing in the primary of transformer TR1 when the system is at rest. TR1 is a 12 V 1.5 VA AC power transformer wired as a step-up trans-former i.e. with the low-volt age winding connected to T1. The transformer’s step up ratio affords ‘passive’ amplification of the signal present at the drain of T1. The trans-former’s high voltage winding is connected to piezo sounder BZ1 (e.g. Murata; the ‘28’indicates the diameter) which produces the audible warning.

In order to optimise the sound output of the unit, you’ll need to adjust P1 so as to set the oscillator frequency to the resonant frequency of the piezo transducer; this setting can be done by ear. The electronics and batteries can be housed into a salvaged case (for example, the kind of oval box found inside giant chocolate ‘surprise’ eggs). The electrodes, formed from simple rigid copper wires, pass out through the case; the join is made watertight using epoxy adhesive.

 

Author : André Thiriot  - Copyright : Elektor

Simple Long Range FM Transmitter

The power output of many transmitter circuits are very low because no power amplifier stages are incorporated. The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.

Simple Long Range FM Transmitter Circuit Diagram :

Simple Long Range FM Transmitter-Circuit diagram

The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signal power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet. Coil winding details are given below:

  • L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
  • L2 - 2 turns of 24 SWG wire near top end of L1.
    (Note: No core (i.e. air core) is used for the above coils)
  • L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.
  • L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a license is illegal in India.

WARNING: Transmitting on the UK Commercial FM band is also illegal in the UK, please see the general disclaimer. This circuit is shown for educational purposes only.

Dag-gerboard Position Detector

In sailing regattas it’s handy to have a dag-gerboard that can be raised and lowered vertically. As the winding handle or positioning motor needs to rotate the spindle of the lifting device some 100 to 150 times throughout its full range it would be extremely handy to have a quick idea of its current position. An electronic count of the number of revolutions would be ideal. Thank goodness most sailors now have a 12-V supply available!

To get this to work you need to apply white and black markings to the spindle, each covering half of the circumference. Next, mask off two electric eye devices (reflected light sensors) next to one another (approximately 10 mm apart). For secure detection both sensors should be positioned not more than 5 mm from the paint markings.

Dag-gerboard Position Detector Circuit Diagram :

Dag-gerboard Position-Detector-Circuit Diagram

The markings to be read by the sensor should be displaced laterally, so that the direction of rotation can be recognised in addition to the number of revolutions counted. At the heart of our circuit is a PIC16F628 from Microchip, which as usual can be bought ready programmed from Elektor or you can do this bit yourself by downloading free firmware (for details of both see [1]).

At pins 1 of the two reflected light sensors IC3 and IC4 we need to ‘see’ more than 2.0 V from the white segment and less than 0.8 V from the black mark (with an operating volt-age between 4.5 and 5.5 V). The two signals detected are taken to plug connector along with the operating voltage and ground. It’s convenient if you also provide a connector from the microcontroller as well, so that the sensor and the controller board can be linked by a test lead.

The multiplexing of the three seven-segment displays is programmed at a rate of 100 Hz.

Acceptable values for the revolution count are between 0 and 140. If the count exceeds or falls below these limits, then the counter is not incremented. The RESET key S2 sets the counter back to zero. Jumper K2 enables you to reverse the direction of counting. The count is retained if the operating voltage is removed and is loaded again when next pow-ered up.

The source code can also be downloaded from the website mentioned above, making it possible (for instance) to define alternative counter limit values (the maximum value is defined in the line #define max 140). For compiling the code you can use the CC5X compiler, of which there is a free version (www.bknd.com/cc5x).

Author : Hermann Sprenger - Copyright : Elektor

Simple Security Alarm

Thwart any attempt of burglary in your house using this alarm circuit. When someone opens the door of your room, it sounds an alarm intermittently and flashes light as well. The circuit can also be used as an audio/visual alarm in case of fire or other emergency by momentarily pressing switch S3.

The circuit (refer Fig.1) is built around transformer X1, a standard bar magnet, reed switch S2, timer IC NE555 (IC1), opto-coupler IC MOC3020 (IC2), TRIAC BT136 and a few discrete components. Timer IC1 is wired as an astable multi-vibrator whose reset pin 4 is controlled by the reed switch. The reed switch fitted in the door frame acts as the sensor. A magnet is fixed on the door panel close to the reed switch.

Fig.1: Simple Security Alarm Circuit Diagram : 

Security-alarm-circuit-Daigram

The reed switch consists of a pair of contacts on ferrous metal sealed in a glass envelope. The contacts may be normally-open (which close when a magnetic field is present) or normally closed type (which open when a magnetic field is applied). A normally open- type reed switch is used here.

When the door is closed, reed switch S2 is in open state. When the door is opened, the bar magnet moves away from reed switch S2. As a result, reset pin 4 of IC NE555 goes high. The high output at pin 3 of IC1 enables IC2. Pin 4 of IC2 is connected to the gate of TRIAC1. When the door is opened, bulb B1 flashes and the bell sounds (provided switch S4 is closed) indicating that the door has been opened. Flashing of the bulb and the alarm continue until the door is closed.

Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet. Connect the call bell at the back side and the bulb at the front side of the cabinet. Install the unit on the door of the room as shown in Fig.2.

Fig.2: Reed Switch Fitting in Door :

Reed-switch-fitting-in-door-d

The circuit is powered by mains supply.

Author :S.C. Dwivedi - Copyright : EFY

High and Low Voltage Cut-Off with Delay and Alarm

This straight forward circuit will protect electrical appliances from over voltage as well as under voltage. The circuit also produces an alarm when the power supply comes back. An ideal circuit for home to protect your valuable equipments from voltage fluctuations. The same circuit with some modifications can be used  to make a automatic voltage stabilizer.

High and Low Voltage Cut-Off with Delay and Alarm Circuit Diagram :

high-and-low-voltage-cut-of-circuit Diagram

When the mains voltage is in the normal level, the voltage at the negative terminal of zener diode D4 will be less than 5.6 Volts. At this condition transistor T1 will not conduct. The same time voltage at the negative terminal of zener diode D5 will be greater than 5.6 and so the transistor T2 will be conducting. The relay will be activated and the green LED will be glowing.

When the mains voltage is higher than the set limit the transistor T1 becomes conducting since the voltage at the negative terminal of  D4 is greater than 5.6 V. At the same time transistor T2 will be non conducting which results in the deactivation of relay to cut the mains supply from load. When the mains voltage is less than the set limit transistors T1 & T2 becomes non conducting  making the relay to de-activate and cut the load from mains.

The timer NE555 is wired as a monostable multivibrator with a pulse width of 10ms.When the power comes back after a cut off a negative voltage is obtained at the trigger pin which triggers the IC NE555. The transistor T3 gets forward biased and it drives the buzzer to produce a beep as an indication of power resumption. Also the transistor T1 is made on which in turn makes T2 off. As a result the relay will remain de- activate for 10ms and this provides the sufficient delay and the equipment  is protected from surge voltages.

Notes :

  • To calibrate the circuit a autotransformer is needed. Connect the output of autotransformer to the transformer primary.
  • Set the voltage to 260V and adjust  VR1 to make the relay deactivated.
  • Now set the autotransformer to 160V and adjust VR2  so that the relay is de-energized.
  • VR3 can be used to vary the volume of buzzer.

Ground-free DVM Module Supply from 5 V

The majority of hand-held digital volt meters use an LCD screen and are powered from a nine volt battery. Inside is most probably an ICL7106 chip (or something compatible). This takes care of measuring the input and driving the LCD. This IC is very popular and can be found in other laboratory and homebrew equipment where it offers a simple solution for both measuring current/voltage and driving the display. So far so good, there is how-ever one feature of this device which needs careful consideration. The power supply to the chip (both the positive and negative connection) must not have any direct connection to either of the two measuring input terminals. It requires floating supplies. This is not a problem for battery powered equipment but needs more thought when the ICL7106 is fitted into mains powered equipment.

Ground-free DVM Module Supply from 5 V Circuit Diagram :

Ground-free DVM Module Supply from 5 V-Circuit Daigram

The simplest, most expensive solution is to use two independent power supplies in the equipment. A battery could also be used as an isolated supply but in a mains powered device it would seem a bit out of place and inconvenient.

In this case the term ‘floating supplies’ means that it is possible to have two separate DC levels. This level of isolation can be achieved with capacitors to separate the two DC supplies. Back in 2003 we published a circuit in the July/August edition of Elektor (circuit number 75) which used a NE555 IC. Unfortunately this design required a supply voltage upwards of 10 V. If the DVM module is fitted to equipment which only uses a 5 V supply (as is often the case) the circuit will not be of much use.The author has solved the problem by modifying the original circuit, using a hex Schmitt trigger inverter type 74HC14N instead of the NE555. One of the inverters generates a square wave of about 75 KHz. The remaining five inverters are wired in parallel to pro-vide more output drive current for the voltage multiplier stage.

DC isolation is provided by capacitors C2 and C3. A classic voltage multiplier configuration is made up of capacitors and diodes. The circuit generates an output of around 8.5 V at a load current of 1 mA. This is sufficient to power the DVM chip. The 5 V supply for the circuit must be stabilised.

The values of the input voltage divider resistors R2 and R3 are independent of the chip’s power supply and must be selected according to the desired measurement range.

Author : Heinz Kutzer  - Copyright : Elektor

Voltage Controlled Oscillator

In most cases, the frequency of an oscillator is determined by the time constant RC. However, in cases or applications such as FM, tone generators, and frequency-shift keying (FSK), the frequency is to be controlled by means of an input voltage, called the control voltage. This can be achieved in a voltage-controlled oscillator (VCO). A VCO is a circuit that provides an oscillating output signal (typically of square-wave or triangular waveform) whose frequency can be adjusted over a range by a dc voltage.

Voltage Controlled Oscillator Block Diagram :

Voltage-controlled-oscillator-Block-Circuit Diagram

An example of a VCO is the 566 IC unit, that provides simultaneously the square-wave and triangular-wave outputs as a function of input voltage. The frequency of oscillation is set by an external resistor R1 and a capacitor C1 and the voltage Vc applied to the control terminals. Figure shows that the 566 IC unit contains current sources to charge and discharge an external capacitor Cv at a rate set by an external resistor R1 and the modulating dc input voltage.

A Schmitt trigger circuit is employed to switch the current sources between charging and discharging the capacitor, and the triangular voltage produced across the capacitor and square-wave from the Schmitt trigger are provided as outputs through buffer amplifiers. Both the output waveforms are buffered so that the output impedance of each is 50 f2. The typical magnitude of the triangular wave and the square wave are 2.4 Vpeak.to-peak and 5.4Vpeak.to.peak.

The frequency of the output waveforms is approximated by : fout = 2(V+ - Vc)/R1C1V+

Voltage Controlled Oscillator Circuit Diagram :

VCO-Circuit-Diagramw

Figure shows the pin connection of the 566 unit. The VCO can be programmed over a 10-to-l frequency range by proper selection of an external resistor and capacitor, and then modulated over a 10-to-l frequency range by a control voltage, Vc The voltage controlled oscillators (VCOs) are commonly used in converting low-frequency signals such as EEG (electro-encephalograms) or ECG (electro-cardiograms) into an audio­frequency (AF range).

Car Alarm Sound Booster

For car alarms, emphasis should be put on hearing the audible alert and identifying it as belonging to your ‘wheels’. Unfortunately, modern car alarm systems seem to have more or less the same alarm sound especially if they are from the same brand. Also, to comply with legal noise restrictions, the alarm sound is not always loud enough to be heard if the car is parked down the road.

The circuit shown here is designed to help boost the alarm sound by also activating the car’s horn(s) when the alarm goes off.Internally the car alarm system often provides a signal that activates the (optional) engine immobilizer and/or volume (ultrasound) sensors. This signal usually goes Low upon system triggering and high again when the alarm system is deactivated.

Car Alarm Sound Booster Circuit Diagram :

Car Alarm Sound Booster-Circuit Diagram

The alarm activation signal is fed to the circuit through D1. When in idle state, T1’s gate is High and consequently the FET conducts, keeping power FET T2 firmly switched of f. When the system gets an active low signal, T1 switches of f allowing timing capacitor C2 to charge via R2. About 15 seconds later, when the voltage across C2 is high enough, T2 starts to conduct and relay RE1 is energized. This, in turn, provides the required path for the ‘lights flashing’ signal to energize RE2 and feed battery power to the car’s horn(s).

When the alarm system is turned off the activation signal returns to High. T1 starts to con-duct and rapidly discharges C2 via R3. T2 is then cut off and RE1 is de-energized. Diode D2 suppresses back EMF from RE1.The circuit draws less than 2 mA when idling. When activated the circuit’s current consumption is virtually that of the RE1 coil.RE1 is any simple SPST or SPDT relay, capable of switching about 0.5 A (at 12 V). The coil rating is for 12 VDC and a current requirement as low as you can find. Fuse F1 should be a slow blow type and rated about twice RE1’s coil current.

The BS170 in position T2 can sink a continuous current of about 0.5 A. However, a value of 1.2 A pulsed is specified by Fairchild for their devices. To keep the FET’s d-s current due to C2 discharging within safe limits, R2 may be increased, C2 decreased and R3 increased, all proportionally. A factor of 2 will keep the FET out of harm’s way with maybe a slight change in the 15-second delay and the sensitivity of the circuit.C1 is used as a smoothing capacitor and F2 should be rated in accordance with the horn(s) maximum current draw.

Caution.The installation and use of this circuit may be subject to legal restrictions in your country, state or area.

Author : Hagay Ben-Elie - Copyright : Elektor

Simple Mini FM Receiver


This is the most simple fm radio receiver with good performances that works great even if the sensitivity is not too high. The working principle of this fm receiver may seem a little unusual. It is made of an oscillator (T2 and T3) that is synchronized with the received frequency of T1. This transistor works as a broadband preamplifier in VHF range.

Mini FM Receiver Circuit Diagram :


The oscillator is adjusted between 87 … 108 MHz with C5. Because of the synchronization, the oscillator output will have the same frequency deviation as the received signal from the fm antenna. This deviations are caused by the broadcasted audio informations. The frequency modulated signal show up on P1 + R5. Low pass filter R6/C6 extracts the audio signal and then is amplifier by T4 … T6 and transmitted at the output through C9 capacitor.

The coil details are presented in the fm receiver circuit diagram. The radio receiver is adjusted on different stations with the help of C5. P1 potentiometer is adjusted untill the best reception is obtained. If we attach an audio amplifier and a speaker then this fm receiver can be made very compact as a pocket radio.


Outdoor Lighting Controller

When you step out of your brightly-lit house  into the darkness, it takes a while for your  vision to adjust. A solution to this problem  is this outdoor light with automatic switch-off. As a bonus, it will also make it a little bit  easier to find the keyhole when returning  late at night. Often no mains neutral connection is avail-able at the point where the switch-off timer  is to be installed, which makes many circuit  arrangements impractical. However, the circuit here is designed to work in this situation. The design eschews bulky components such as transformers and the whole unit can  be built into a flush-mounted fitting. The circuit also features low quiescent current consumption.

Outdoor Lighting Controller Circuit Diagram :

Outdoor Lighting Controller-Circuit Diagram
The circuit is star ted by closing switch (or  pushbutton) S1. The lamp then immediately receives power via the bridge rectifier. The drop across diodes D5 to D10 is 4.2 V, which provides the power supply for the delay circuit itself, built around the CD4060 binary  counter.

When the switch is opened the lighting sup-ply current continues to flow through Tri1. The NPN optocoupler in the triac drive circuit detects when the triac is active, with antiparallel LED D1 keeping the drive sym-metrical. The NPN phototransistor inside the  coupler creates a reset pulse via T1, driving  pin 12 of the counter. This means that the  full time period will run even if the circuit is retriggered. The CD4060 counts at the AC grid frequency.  Pin 3 goes high after 213clocks, which corresponds to about 2.5 minutes. If this is not long  enough, a further CD4060 counter can be cascaded. T2 then turns on and shorts the internal LED of opto-triac IC2; this causes Tri1 to  be deprived of its trigger current and the light  goes out. The circuit remains without power until next triggered.

The circuit is only suitable for use with resistive loads. With the components shown (in particular in the bridge rectifier and D5 to  D10) the maximum total power of the connected bulb(s) is 200 watts. As is well known, the filament of the bulb is most likely to fail at the moment power is applied. There is little risk to Tri1 at this point as it is bridged by  the switch. The most likely consequence of overload is that one of diodes D1 to D6 will  fail. In the prototype no fuse was used, as it would not in any case have been easy to change. However, that is not necessarily recommended practice!

Circuits at AC line potential should only be constructed by suitably experienced persons and all relevant safety precautions and  applicable regulations must be observed during construction and installation.
Author : Harald Schad - Copyright : Elektor

Small DC Motor Control Using PWM

Small DC motors are efficiently controlled using pulse-width modulation (PWM) method. The circuit described here is built around an LM324 low-power quad-operational amplifier. Of the four op-amps (operational amplifiers) available in this IC, two are used for triangular wave generator and one for comparator. Op-amp N2 generates a 1.6kHz square wave, while op-amp N1 is configured as an integrator. The square wave output of N2 at its pin 14 is fed to the inverting input (pin 2) of N1 through resistor R1. As N1 is configured as an integrator, it outputs a triangular wave of the same frequency as the square wave. The triangular wave is fed to pin 5 of op-amp N3, which is configured as a comparator.

Small DC Motor Control Circuit Diagram :

Small DC Motor Control Cirucit diagram

The reference voltage at pin 6 of the comparator is fixed through the potential divider arrangement formed by potmeter VR1 and resistors R4 and R5. It can be set from –6V (lowermost position of VR1) to +6V (uppermost position of VR1).
The triangular wave applied at pin 5 of N3 is compared with the reference voltage at its pin 6. The output at pin 7 is about +12V when the voltage at pin 5 is greater than the voltage at pin 6. Similarly, the output at pin 7 is about -12V when the voltage at pin 5 is lower than the voltage at pin 6.

The output from comparator N3 is the gate voltage for n-channel MOSFET (T1). T1 switches on when the gate voltage is positive and switches off when the gate voltage is negative. Setting of the reference voltage therefore controls the pulse-width of the motor. When T1 is switched on for a longer period, the pulse width will be wider, which means more average DC component and faster speed of the motor. Speed will be low when the pulse width is small. Thus potmeter VR1 controls the speed of the motor.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The circuit requires ±12V power supply for its working. It can also be modified to control the speed of a 6V or 24V DC motor.

Author : Stutee Saxena - Copyright: EFY

Simple Security Monitor

A remote listening circuit. The area to be monitored is connected via a cable and allows remote audio listening.

Starting from the right hand side, the power supply. I have used 12V as a standard power supply voltage, or a 12V car battery may be used. The circuit is in two halves, a remote microphone preamp, and an audio amplifier based around the National Semiconductor LM386 audio amplifier.

Security Monitor Circuit Diagram :

Security Monitor-Circuit Diagram

The remote preamp uses an ECM microphone to monitor sound. A direct coupled 2 stage amplifier built around Q1 and Q2 amplify the weak microphone signal. Preset resistor R2 acts as a gain control, and C1 provides some high frequency roll off to the overall audio response. Q1 is run at a low collector current for a high signal to noise ratio, whilst Q2 collector is biased to around half the supply voltage for maximum dynamic range. The power supply for this preamp is fed via R10 and R6 from the 12V supply. C4 ensures that the preamp power supply is decoupled and no ac voltages are present on the power lines. The amplified audio output from Q2 collector is fed onto the supply lines via C6 a 220u capacitor. The output impedance of Q2 is low, hence the relatively high value of C6. C6 also has a second purpose of letting the output audio signals pass, whilst blocking the dc voltage of the power supply.

At the opposite end, C7 a 10u capacitor, brings home the amplified audio to the listening location. The signal is first further amplifier by a x10 voltage gain amplified using the TL071. C8, a 22p capacitor again rolls off some high frequency response above 100kHz. This is necessary as long wires may pick up a little radio interference. After amplification by the op-amp, the audio is finally passed to the LM386 audio amplifier. R14 acts as volume control. R13 and C12 prevent possible instability in the LM386 and are recommended by the manufacturer. Audio output is around 1 watt into an 8 ohm loudspeaker, distortion about 0.2%. If preferred headphones could be used, although I'd recommend a series resistor of the same value impedance as the headphones.

Notes:

You can use this in your garden and listen for any unusual sounds, or maybe just wildlife noises. If you have a car parked in a remote location, the microphone will also pick up any sounds od activity in this area. The cable may be visible or hidden, screened cable is not necessary and you can use bellwire or speaker cable if desired.

Petrol/Diesel Level Sensor

This sensor is particularly suitable for use in small spaces, such as the petrol tank of a  motorbike. It has the advantage of not having any moving parts, unlike a conventional sensor with a float and float arm that make it difficult to fit in a tank.

The sensor circuit is made from standard, inexpensive components and can be put together for little money.

Petrol/Diesel Level Sensor Circuit Diagram :

Petrol Diesel Level Sensor-Circuit Diagram

The operating principle is  based on  measuring  the forward volt-ages of two identical diodes (check this  first by measuring  them).  The forward voltage of a diode decreases with increasing junction temperature. lf a resistor is placed close to one of the two diodes, it will be heated slightly if it extends above the surface of the  petrol. For best results,the other diode (used for reference) should be located at the same level. lf the diodes are covered by the petrol in the tank, the heating resistor will not have any effect because it will be cooled by the petrol. An opamp compares the voltage across the two diodes, with a slightly smaller current passing through the reference diode. When the petrol level drops, the output of the opamp goes high and the output transistor switches on. This causes a sense resistor to be connected in parallel with the sensor output. Several sensor circuits can be used together, each with its own switched sense resistor connected in parallel with the output, and the resulting output  signal can be used to drive a meter or the like.

Using this approach, the author built a petrol tank' sensors trip' tank consisting of five PCBs, each fitted with two sensor circuits. With this sensor strip installed at an angle in the tank, a resolution of approximately 1.5 litre per sensor is possible. Many tanks have an electrical fitting near the bottom for connection to a lamp on the instrument panel that indicates the reserve level. The sensor strip can be used in its place.

You will have to experiment a bit with the values of the sense resistors, but do not use values lower than around'100 O. It is also important to fit the diodes and heater resistor in a little tube with a small opening at the bottom so that splashing petrol does not cool the heater resistor, since this would result in false readings.

The circuit should be powered from a regulated supply voltage of 5 to 6 V to prevent the heating resistors from becoming too hot. After testing everything to be sure that it works properly, it's a good idea to coat the circuit board with epoxy glue to provide better protection against the petrol.

Tip: you can use the well-known 1M3914 to build a LED display with ten LEDs, which can serve as a level indicator. Several examples of suitable circuits can be found in back issues of Elektor.

Note: this sensor circuit is not suitable for use in conductive liquids.

Author : Paul de Ruijter - Copyright: Elektor