Low-Voltage Remote Mains Switch

This circuit allows a 240V mains appliance to be controlled remotely via low-voltage cabling and a pushbutton switch. The mains appliance (in this case, a light bulb) is switched with a suitably-rated relay. All of the electronics is housed in an ABS box located in proximity to the appliance. The pushbutton switch and plugpack are located remotely and can be wired up with 3-core alarm cable or similar. Cable lengths of 20m or more are feasible with this arrangement. When the switch (S1) is pressed, the input (pin 8) of IC1c is briefly pulled low via the 10mF capacitor, which is initially discharged.

Circuit diagram:

Low-Voltage Remote Mains Switch Circuit Diagram

The output (pin 10) immediately goes high and this is inverted and fed back to the second input (pin 9) via another gate in the quad NAND package (IC1d). In conjunction with the 1MW resistor and 470nF capacitor, IC1d eliminates the effects of contact "bounce" by ensuring that IC1c’s output remains high for a predetermined period. The output from IC1c drives the clock input of a 4013 D-type flip-flop (IC2). The flipflop is wired for a "toggle" function by virtue of the Q-bar connection back to the D input. A 2.2MW resistor and 100nF capacitor improve circuit noise immunity. Each time the switch is pressed, the flipflop output (pin 13) toggles, switching the transistor (Q1) and relay on or off. Note that all mains wiring must be properly installed and completely insulated so that there is no possibility of it contacting the low-voltage side of the circuit.

Author: Bob Hammond - Copyright: Silicon Chip Elecronics

Source : www.extremecircuits.net

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Clap Switch Circuit Diagram

Here’s a clap switch free from false triggering. To turn on/off any appliance, you just have to clap twice. The circuit changes its output state only when you clap twice within the set time period. Here, you’ve to clap within 3 seconds. The clap sound sensed by condenser microphone is amplified by transistor T1. The amplified signal provides negative pulse to pin 2 of IC1 and IC2, triggering both the ICs. IC1, commonly used as a timer, is wired here as a monostable multivibrator. Trigging of IC1 causes pin 3 to go high and it remains high for a certain time period depending on the selected values of R7 and C3. This ‘on’ time (T) of IC1 can be calculated using the following relationship: T=1.1R7.C3 seconds where R7 is in ohms and C3 in microfarads. On first clap, output pin 3 of IC1 goes high and remains in this standby position for the preset time.Also, LED1 glows for this period. The output of IC1 provides supply voltage to IC2 at its pins 8 and 4.

Circuit diagram :

Clap Switch Circuit Diagram

Now IC2 is ready to receive the triggering signal. Resistor R10 and capacitor C7 connected to pin 4 of IC2 prevent false triggering when IC1 provides the supply voltage to IC2 at first clap. On second clap, a negative pulse triggers IC2 and its output pin 3 goes high for a time period depending on R9 and C5. This provides a positive pulse at clock pin 14 of decade counter IC 4017 (IC3). Decade counter IC3 is wired here as a bistable. Each pulse applied at clock pin 14 changes the output state at pin 2 (Q1) of IC3 because Q2 is connected to reset pin 15. The high output at pin 2 drives transistor T2 and also energizes relay RL1. LED2 indicates activation of relay RL1 and on/off status of the appliance. A free-wheeling diode (D1) prevents damage of T2 when relay de-energizes.

Author : Mohammad Usman Qureshi - Copyright : Electronics For You May 2003

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Heat Sensitive Switch Circuit Diagram

At the heart of this heat-sensitive switch is IC LM35 (IC1), which is a linear temperature sensor and linear temperature-to-voltage converter circuit. The converter provides accurately linear and directly proportional output signal in millivolts over the temperature range of 0°C to 155°C. It develops an output voltage of 10 mV per degree centigrade change in the ambient temperature. Therefore the output voltage varies from 0mV at 0°C to 1V at 100°C and any voltage measurement circuit connected across the output pins can read the temperature directly. The input and ground pins of this heat-to-voltage converter IC are connected across the regulated power supply rails and decoupled by R1 and C1. Its temperature-tracking output is applied to the non-inverting input (pin 3) of the comparator built around IC2. The inverting input (pin 2) of IC2 is connected across the positive supply rails via a voltage divider network formed by potentiometer VR1. Since the wiper of potentiometer VR1 is connected to the inverting input of IC2, the voltage presented to this pin is linearly variable.

Circuit diagram :

 Heat Sensitive Switch  Circuit Diagram

This voltage is used as the reference level for the comparator against the output supplied by IC1. So if the non-inverting input of IC2 receives a voltage lower than the set level, its output goes low (approximately 650 mV). This low level is applied to the input of the load-relay driver comprising npn transistors T1 and T2. The low level presented at the base of transistor T1 keeps it nonconductive. Since T2 receives the forward bias voltage via the emitter of T1, it is also kept non-conductive. Hence, relay RL1 is in de-energised state, keeping mains supply to the load ‘off’ as long as the temperature at the sensor is low. Conversely, if the non-inverting input receives a voltage higher than the set level, its output goes high (approximately 2200mV) and the load is turned ‘on.’ This happens when IC1 is at a higher temperature and its output voltage is also higher than the set level at the inverting input of IC2. So the load is turned on as soon as the ambient temperature rises above the set level.

Capacitor C3 at this pin helps iron out any ripple that passes through the positive supply rail to avoid errors in the circuit operation. By adjusting potentiometer VR1 and thereby varying the reference voltage level at the inverting input pin of IC1, the temperature threshold at which energisation of the relay is required can be set. As this setting is linear, the knob of potentiometer VR1 can be provided with a linear dial calibrated in degrees centigrade. Therefore any temperature level can be selected and constantly monitored for external actions like turning on a room heater in winter or a room cooler in summer. The circuit can also be used to activate emergency fire extinguishers, if positioned at the probable fire accident site. The circuit can be modified to operate any electrical appliance. In that case, relay RL1 must be a heavy-duty type with appropriately rated contacts to match the power demands of the load to be operated.

Author : M.K. Chandra Mouleeswaran & Miss Kalan Priya - Copyright :  EFY

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Auto Power Off

We are surrounded by battery operated equipment of all kinds, and this array is growing still. Manufacturers and designers lean over backwards to make sure that their equipment draws a small current and can thus be operated by a battery. This has its flip side, too. because even if the equipment in question draws only a small current, when it is not switched off, the battery is flat after a few days or weeks. The circuit presented here can prevent this happening. It may be added to all kinds of equipment operating from a 9 V battery and switches this off automatically one minute after a preset time has elapsed. The peak switching current is 20 mA, which is more than enough for most applications.
Circuit diagram:

Automatic Power Off Circuit Diagram

The switch is formed by a p-n-p darlington, T1, which is actuated by push-button switch S1. The very high amplification of the darlington enables it to be kept on fairly long with the aid of a relatively small-value capacitor, C1 (= 100 µF). Resistor R3 limits the charging current of C1 to ensure a long life of S1. Resistors R1 and R2, in conjunction with C1, determine the switch-on time. When this time has elapsed, R1 ensures that T1 is switched off. Since the darlington can handle a UBE of –10 V, a polarity protection diode is not needed.

Author:H. Bonekamp - Copyright: Elektor ELectronics

www.extremecircuits.net

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Speaker-Headphone Switch For PCs

If you need to use a headset with your PC, then you will know how frustrating it is continuously swapping over speaker and microphone cables. This is even worse if the PC is parked in a dark corner and the hard-to-read writing on the sound card sockets is covered in dust. This simple switch box eliminates all these problems. It sits on top of the desk and connects to the PC with stereo one-to-one cables. On the rear of the box are sockets for the PC speaker and microphone connections and the existing speakers. On the front of the box are the sockets for the headset microphone and headphones, an input for an external microphone and two switches. One switch is used to direct the sound card output from the PC to either the existing speakers or the headphones.

Circuit diagram:

Speaker-Headphone Switch Circuit Diagram For Computers

The second switch connects either the headset microphone or the external microphone to the input socket of the PC sound card. The switches used were 3 position 4 pole rotary switches with the last pole unused and adjusted for 2-position operation. All sockets were stereo 3.5mm types. This multiple switching arrangement is very flexible and is especially handy if you want to use an external microphone while monitoring with headphones. The ground wire as well as the left and right wires are all switched to prevent noise that could otherwise be induced into the microphone input through joining separate earths. For the same reason, a plastic case is used so that the earths of the sockets are not shorted together as would happen with a metal case. You will require two additional short stereo extension cables to connect the box to the PC.

Author: Leon Williams - Copyright: Silicon Chip Electronics

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RC (Remote Control) Switch

It is sometimes necessary for an RC (remote control) model to contain some kind of switching functionality. Some things that come to mind are lights on a model boat, or the folding away of the undercarriage of an aeroplane, etc. A standard solution employs a servo, which then actually operates the switch. Separate modules are also available, which may or may not contain a relay. A device with such functionality is eminently suitable for building yourself. The schematic shows that it can be easily realised with a few standard components.
Picture of the project:
 

RC Switch Circuit

The servo signal, which consists of pulses from 1 to 2 ms duration, depending on the desired position, enters the circuit via pin 1 of connector K1. Two buffers from IC2 provide the necessary buffering after which the signal is differentiated by C2. This has the effect that at each rising edge a negative start signal is presented to pin 2 of IC1. D1 and R4 make sure that at the falling edge the voltage at pin 2 of IC2 does not become too high. IC1 (TLC555) is an old faithful in a CMOS version. A standard version (such as the NE555) works just as well, but this IC draws an unnecessarily high current, while we strive to keep the current consumption as low as possible in the model. The aforementioned 555 is configured as a one-shot. The pulse-duration depends on the combination of R2/C1. Lowering the voltage on pin 5 also affects the time. This results in reducing the length of the pulse. In this circuit the pulse at the output of IC will last just over 1.5 ms when T1 does not conduct.

Circuit diagram:

RC Switch Circuit Diagram

When T1 does conduct, the duration will be a little shorter than 1.5 ms. We will explain the purpose of this a little later on. Via IC2.C, the fixed-length pulse is, presented to the clock input of a D-flip-flop. As a consequence, the flip-flip will remember the state of the input (servo signal). The result is that when the servo-pulse is longer than the pulse form the 555, output Q will be high, otherwise the output will be low. It is possible, in practice, that the servo signal is nearly the same length as the output from the 555. A small amount of variation in the servo signal could therefore easily cause the output to ‘chatter’, that is, the output could be high at one time and low the next. To prevent this chatter there is feedback in the form of R1, R3 and T1. This circuit makes sure that when the flip-flip has decided that the servo-pulse is longer than the 555’s pulse (and signals this by making output Q high), the pulse duration from the 555 is made a little shorter. The length of the servo-signal will now have to be reduced by a reasonable amount before the servo-pulse becomes shorter than the 555’s pulse.

Parts and PCB layout:

Parts and PCB layout Of RC Switch

The moment this happens, T1 will stop conducting and the mono-stable time will become a little longer. The servo-pulse will now have to be longer by a reasonable amount before the flip-flip changes back again. This principle is called hysteresis. Jumper JP1 lets you choose between the normal or inverted output signals. Buffers IC2.D through to IC2.F together with R5 drive output transistor T2, which in turn drives the output. Note that the load may draw a maximum current of 100 mA. Diode D2 has been added so that inductive loads can be switched as well (for example, electrically operated pneu-matic valves).

COMPONENTS LIST:
Resistors:
R1 = 470k
R2 = 150k
R3 = 47k
R4 = 100k
R5 = 4k7
Capacitors:
C1 = 10nF
C2 = 1nF
C3,C4 = 100nF
Semiconductors:
D1 = BAT85 or similar Schottky diode
D2 = 1N4148
IC1 = CMOS 555 (e.g., TLC555 or ICM7555)
IC2 = 4049
IC3 = 4013
T1,T2 = BC547B
Miscellaneous:
JP1 = jumper with 3-way pinheader
K1 = servo cable
K2 = 2-way pinheader or 2 solder pins

Author: Paul Goossens - Copyright: Elektor Electronics

Source : www.extremecircuits.net

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Mains Slave Switcher Circuit

There are many situations where two or more pieces of equipment are used together and to avoid having to switch each item on separately or risk the possibility of leaving one of them on when switching the rest off, a slave switch is often used. Applications which spring to mind are a computer/printer/scanner etc or audio amplifier/record deck/tuner combinations or perhaps closest to every electronics enthusiast’s heart, the work bench where a bench power supply/oscilloscope/soldering iron etc are often required simultaneously. The last is perhaps a particularly good example as the soldering iron, often having no power indicator, is invariably left on after all the other items have been switched off. Obviously the simplest solution is to plug all of the items into one extension socket and switch this on and off at the mains socket but this is not always very convenient as the switch may be difficult to reach often being behind or under the work bench. Slave switches normally sense the current drawn from the mains supply when the master unit is switched on by detecting the resulting voltage across a series resistor and switching on a relay to power the slave unit(s).

This means that the Live or Neutral feed must be broken to allow the resistor to be inserted. This circuit, which is intended for switching power to a work bench when the bench light is switched on, avoids resistors or any modifications to the lamp or slave appliances by sensing the electric field around the lamp cable when this is switched on. The lamp then also functions as a ‘power on’ indicator (albeit a very large one that cannot be ignored) that shows when all of the equipment on the bench is switched on. The field, which appears around the lamp cable when the mains is connected, can be sensed by a short piece of insulated wire simply wrapped around it and this is amplified by the three stage amplifier which can be regarded as a single super-transistor with a very high gain. The extremely small a.c. base current results in an appreciable collector current which after smoothing (by C3) is used to switch on a relay to power the other sockets. Power for the relay is obtained from a capacitor ‘mains dropper’ that generates no heat and provides a d.c. supply of around 15 volts when the relay is off.

Circuit diagram:
 

Mains Slave Switcher Circuit Diagram

The output current of this supply is limited so that the voltage drops substantially when the relay pulls in but since relays require more current to operate them than they do to remain energized, this is not a problem. Since the transistor emitter is referenced to mains Neutral, it is the field around the mains Live which will be detected. Consequently, for correct operation the Live wire to the lamp must be switched and this will no doubt be the case in all lamps where the switch is factory fitted. In case of uncertainty, a double-pole switch to interrupt both the Live and Neutral should be used. The sensitivity of the circuit can be increased or decreased as required by altering the value of the T2 emitter resistor. The sensing wire must of course be wrapped around a section of the lamp lead after the switch otherwise the relay will remain energized even when the lamp has been switched off. The drawing shows the general idea with the circuit built into the extension socket although, depending on the space available an auxiliary plastic box may need to be used.

Warning:

The circuit itself is not isolated from the mains supply so that great care should be taken in its construction and testing. The sensor wire must also be adequately insulated and the circuit enclosed in a box to make it inaccessible to fingers etc. when it is in use.

Source :  www.extremecircuits.net

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Mains Slave Switcher II

As a guide, a one-inch reed switch with 40 turns reliably switched on with the current flowing through a 150-watt lamp (approx. 625 mA) but larger reeds may require more turns. If the master appliance draws less current (which is unlikely with power tools) more turns will be required. The reed switch is used to switch on transistor T1 which in turn switches the relay RE1 and powers the slave appliance. Since reed switches have a low mechanical inertia, they have little difficulty in following the fluctuations of the magnetic field due to the alternating current in the coil and this means that they will switch on and off at 100 Hz.

Circuit diagram:

Mains Slave Switcher II Circuit Diagram

C3 is therefore fitted to slow down the transistor response and keep the relay energised during the mains zero crossings when the current drawn by the appliance falls to zero and the reed switch opens. C1 drops the mains voltage to about 15 V (determined by zener diode D1) and this is rectified and smoothed by D2 and C2 to provide a d.c. supply for the circuit. The relay contacts should be rated to switch the intended appliance (vacuum cleaner) and the coil should have a minimum coil resistance of 400 R as the simple d.c. supply can only provide a limited current. C1 drops virtually the full mains voltage and should therefore be a n X2-class component with a voltage rating of at least 250V a.c.

Warning:

The circuit is by its nature connected directly to the mains supply. Great care should therefore be taken in its construction and the circuit should be enclosed in a plastic or earthed metal box with mains sockets fitted for the master and slave appliances.

Author: Elektor - Copyright: Elektor Electronics Magazine

Source : www.extremecircuits.net

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Hard Disk Switch Circuit

In these times with viruses and other threats from the Internet it would be nice to have reassurance that the PC cannot be infected. That is why this circuit was designed. It makes it possible to install multiple hard disks inside the case of a PC, which are separated in such a way that viruses cannot move from one disk to another. In this case there are three drives installed, one for use of the Internet via ADSL, one for working with email and one for other applications. If data from the Internet never arrives on the third disk, it is effectively protected against viruses. The solution outlined here has been in satisfactory use for a couple of years. There is an additional benefit: if there are ever any problems with the operation of the computer, then it is very easy to change to another hard disk to check if the problem manifests itself there as well. In this case, fault finding can be made much easier. The circuit operates by only switching over the power supply voltages (5 V and 12 V) of the hard disks. The hard disk is out of service without a power supply. This works without a problem with S-ATA disks.

Circuit diagram:

Hard Disk Switch Circuit Diagram

With IDE disks this only works with modern drives. There may only be a combination of hard disks on the relevant port and no CD-ROM, DVD-drive, CD-burner or something similar. The selection of the desired hard disk is done with a rotary switch. This has to be set to the correct position before the computer is switched on. When the power supply is turned on, one of three relays is driven via diode D1, D2 or D3. The relays are provided with a hold circuit via a second diode (D4, D5 and D6). In this way the selected relay remains energised as long as the power supply voltage is present. After switching on, electrolytic capacitor C1 is charged via R1, so that the common contact of the rotary switch is quickly at 0 V. This prevents an accidental change of hard disk while the computer is in operation. The ADSL modem is powered from the PC. This power supply voltage is only present if hard disk number 2 is selected. This prevents the use of the Internet if one of the other disks is selected.

Author: Uwe Kardel - Copyright: Elektor Electronics Magazine

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Remote Control Mains Switch

As the only electronics engineer in my family and circle of friends, it is sometimes 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/off 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? 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 (carefully) open the remote control and solder wires to the push buttons for ‘on’ and ‘off’.

Circuit diagram:

Remote Control Mains Switch Circuit Diagram

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-energize 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.

Jaap van der Graaff - Elektor Electronics 2008

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On And Off Button Circuit

In this simple circuit we give the chip a little more attention than usual. It is astonishing what can be built with a 555. We are always infatuated with simple circuits using this IC, such as the one shown here. The 555 is used here so that a single push-button can operate a relay. If you press the button once, the relay is energized. When you press it again the relay turns off. In addition, it is possible to define the initial state of the relay when the power supply is switched on. The design is, as previously mentioned, very simple. Using R1 and R2, the threshold and trigger inputs are held at half the power supply voltage.

Circuit diagram:

On/Off Button Circuit Diagram

When the voltage at the threshold pin becomes greater that 2/3 of the power supply voltage, the output will go low. The output goes high when the voltage at the trigger input is less than 1/3 of the power supply voltage. Because C2, via R3, will eventually have the same level as the output, the output will toggle whenever the push-button is pressed. If, for example, the output is low, the level of the trigger input will also become low and the output will go high! C1 defines the initial state of the relay when the power is applied. If the free end of C1 is connected to Vcc, then the output is high after power up; the output is low when C1 is connected to ground.

Author: Ger Langezaal - Copyright: Elektor Electronics

Source : www.extremecircuits.net

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Automatic Switch For Voltage Converters

New applications for DC voltage converters, such as the ‘workhorse’ LT1070, arise every day. These converters can be adapted to nearly every imaginable ratio of input and output voltages. However, all of these circuits and devices have the same shortcoming, which is that they lack an on/off switch. Especially when they are used as a source of 6-V / 12-V power for a car radio, this is highly impractical. The circuit described here adds automatic load detection to the converter. For use in a car, the additional circuitry must be small and fit into a compact enclosure together with the converter. Since the battery voltage and ambient temperature vary over wide ranges, a simple form of load detection must be used. Besides this, the voltage drop across the load sensing circuitry must naturally be as small as possible. This can be achieved by using ‘ultra-modern’ SiGe technology.

The 6 V from the battery and the 12 V from the converter are combined in the MB R2545 dual diode. Consequently, a voltage of at least 6 V is always applied to the radio (for memory retention). If the radio is switched on, it draws a current from the 6-V battery, which may be around 100 mA.This current produces a voltage across R1. If this voltage is 75 mV or greater, the AC128 germanium transistor starts conducting and charges electrolytic capacitor C1, which is connected to the gate of the BUZ10. The MOSFET energises RE1 and thus connects the supply voltage to the converter. As a result, 12-V power is connected to the radio. The resulting increased current causes the voltage drop across R1 to increase, which is undesirable, so a 10-A Schottky diode is connected in parallel. The total voltage drop is thus approximately 0.6 V. The RC network connected to the BUZ10 ensures that the transistor always remains switched on for at least several seconds, to prevent the circuit from ‘chattering’ with varying current consumption.

Circuit diagram :

Automatic Switch For Voltage Converters

If the load is switched off, the AC128 cuts off, the electrolytic capacitor discharges and the relay again disconnects the voltage converter. The residual current consumption is so small that the circuit can also be connected ahead of the ignition switch. The Schottky diodes need only be rated for the necessary voltages and currents, and above all, they should have the lowest possible saturation voltage. The exact type is not critical. Two separate diodes can also be used. A small heat sink for the MBR diode won’t hurt, but this is normally not essential. Practically any type of PNP germanium transistor that is still available or on hand can be used (AC125, AC126 and AC128 work perfectly).

It may be necessary to modify the value of R1. In combination with the germanium transistor, R1 determines which level of current will be ignored (for memory retention) and which level of current will cause the converter to be switched on. With the component values shown in Figure 1, this level is between 10 mA and 25 mA. It is recommended to measure the quiescent current (at 6 V) and switch-on current of the load and then simulate the switching process using dummy load resistors. When selecting the 6-V relay, ensure that its contacts have an adequate current rating. The actual value can be significantly greater than the nominal output current. With a load of 5 A at 12 V and a converter efficiency of 70 percent, the current through the relay contacts rises to 14.3 A.

Author: C. Wolff - Copyright: Elektor Electronics

Source : www.extremecircuits.net

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Clap-Controlled Switch

This circuit can switch two or more devices on and off in response to a series of rapid handclaps. The claps are picked up by an electret microphone and amplified by a 741 op amp (IC1). IC1 is configured as an inverting amplifier, with its gain and hence the sensitivity of the circuit adjustable via trimpot VR1. Its output is then fed into a pulse-shaping stage based on a 555 timer (IC2). The 555 is configured as a monostable multivibrator, with its trigger input (pin 2) normally biased to about 0.4VCC by the 150kW and 100kW resistors. When a loud enough sound is detected, pin 2 will be pulled below 1/3VCC, triggering the monostable. The output (pin 3) will immediately swing high, causing transistor Q1 to conduct.

The result is a 250ms low-going pulse at the collector of Q1. The output from the pulse-shaper stage is fed into a missing-pulse detector based on a second 555 timer (IC3), which is also configured as a monostable multivibrator. However, this monostable circuit differs from the first because it includes a "retrigger" function. Retriggering is made possible by transistor Q2, which acts to rapidly discharge the 10mF timing capacitor should a pulse arrive when the timer is already running. This means that once triggered, IC3’s output will remain high as long as additional trigger pulses are received within its set timing period. Pulses from Q1’s collector are also applied to the clock (CP0) input of a 4017 decade counter (IC4).

Circuit diagram:

Clap-Controlled Switch Circuit Diagram

In the initial state, output 0 (pin 3) of the counter is high, illuminating LED1. The first pulse advances the count and lights LED2, indicating that the circuit is active and ready to receive a handclap "command". Each time another pulse (clap) is received before IC3’s timing period expires, the process repeats, incrementing the counter by one. When IC3’s timing period is allowed to expire (ie, no claps have been detected for 750ms), the output (pin 3) will go low, turning off transistors Q3 & Q4. The rising voltage on the collector of transistor Q3 clocks two J-K flipflops (IC5a & IC5b), with the result at their Q outputs dependent on the state of the O2-O5 counter outputs. Considering all the possible logic states of a J-K flipflop, the "commands" will therefore be:

2) - claps: turn Device 1 on
3) - claps: turn Device 1 off
4) - claps: turn Device 2 on
5) - claps: turn Device 2 off

Finally, the rising voltage on the collector of Q4 resets the counter, ready for the next clap sequence. Note the addition of a 10nF capacitor between the reset input (pin 15) and ground, which in conjunction with the 10kW resistor adds a short delay to the reset signal. This ensures that the counter is not reset until after the J-K flipflops have been clocked. If desired, the circuit could easily be expanded by adding more flip-flops and counters.

Author: Li-Wen Yip - Copyright: Silicon Chip Electronics

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Push Off Push On

The ubiquitous 555 has yet another airing with this bistable using a simple push-button to provide a push-on, push-off action. It uses the same principle of the stored charge in a capacitor taking a Schmitt trigger through its dead-band. Whereas the Schmitt trigger in that reference was made from discrete components, the in-built dead-band arising from the two comparators, resistor chain and bistable within the 555 is used instead. The circuit demonstrates a stand-by switch, the state of which is indicated by illumination of either an orange or red LED, exclusively driven by the bipolar output of pin 3. Open-collector output (pin 7) pulls-in a 100-mA relay to drive the application circuit; obviously if an ON status LED is provided elsewhere, then the relay, two LEDs and two resistors can be omitted, with pin 3 being used to drive the application circuit, either directly or via a transistor.

Circuit diagram:

Push Off Push On Circuit Diagram

The original NE555 (non-CMOS) can source or sink 200 mA from / into pin 3. Component values are not critical; the ‘dead-band’ at input pins 2 and 6 is between 1/3 and 2/3 of the supply voltage. When the pushbutton is open-circuit, the input is clamped within this zone (at half the supply voltage) by two equal-value resistors, Rb. To prevent the circuit powering-up into an unknown condition, a power-up reset may be applied with a resistor from supply to pin 4 and capacitor to ground. A capacitor and high-value resistor (Rt) provide a memory of the output state just prior to pushing the button and creates a dead time, during which button contact bounce will not cause any further change. When the button is pressed, the stored charge is sufficient to flip the output to the opposite state before the charge is dissipated and clamped back into the neutral zone by resistors Rb. A minimum of 0.1 µF will work, but it is safer to allow for button contact-bounce or hand tremble; 10 µF with 220 k gives approximately a 2-second response.

Author: Trevor Skeggs - Copyright: Elektor July-August 2004

Source : www.extremecircuits.net

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Master/Slave Switch Circuit

In this age of enlightenment any sort of relationship that could be described as master/slave would be questionable but for the purposes of this circuit it gives a good idea of how it functions. The circuit senses mains current supplied to a ‘master’ device and switches ‘slave’ equipment on or off. This feature is useful in a typical hi-fi or home computer environment where several peripheral devices can all be switched on or off together. A solid-state relay from Sharp is an ideal switching element in this application; a built-in zero crossing detector ensures that switching only occurs when the mains voltage passes through zero and any resultant interference is kept to an absolute minimum.

Circuit diagram:

Master/Slave Switch Circuit Diagram

All of the triac drive circuitry (including optical coupling) is integrated on-chip so there are very few external components and no additional power supply necessary. This makes the finished design very compact. Diodes D1, D2, D3 and D4 perform the current sensing function and produce a voltage on C2 when the master equipment is switched on. A Schottky diode is used for D5 to reduce forward voltage losses to a minimum. The circuit is quite sensitive and will successfully switch the slave even when the master equipment draws very little mains current. The RC network formed by R1 and C1 provides some protection for the solid-state relay against mains-borne voltage transients.

Warning:

This circuit is connected to the mains. it is important to be aware that the chip has lethal voltages on its pins and all appropriate safety guidelines must be adhered to! This includes the LED, for safety it must be fitted behind a transparent plexiglass shield.

Author: Karl Köckeis - Copyright: Elektor July-August 200

Source : www.extremecircuits.net

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Smart Foot Switch Circuit

Such jobs as jewel cutting and polishing require the workers to switch on/ off two electrical appliances one after another repeatedly for two different services on the same workpiece. This is cumbersome as they need to fully concentrate on delicate handwork on precious jewels. Switching in such situations cannot be done by hand, and doing it by foot using ordinary switches is too tedious. This is mainly because of the difficulty in sensing and controlling the switch position by foot. Ordinary pushbutton switches make or break a contact momentarily, and they cannot hold the keypress status. You need a bistable multivibrator with two independent trigger inputs to solve this problem.

Here’s a smart foot switch based on dual negative-edge triggered master slave JK flip-flop IC 74LS76 (IC1). J1 and J2 inputs are conneted to 5V through resistors R2 and R5 (each 10k), respectively.K1 and K2 inputs are grounded. Preset pins 2 and 7 are shorted and connected to 5V via resistor R7 (10k). Push-to-on switch S3 connected to the preset inputs is also grounded. Clock and clear inputs of the two flip-flops are cross-connected, i.e. CLK1 (pin 1) is conneted to CLR2 (pin 8) and CLR1 (pin 3) is connected to CLK2 ( pin 6). Clock input pins 1 and 6 are pulled up high through resistors R1 and R4 (each 4.7k), respectively.

Circuit Diagram:

Smrat tFoot Switch Circuit diagram

Push-to-on switches S1 and S2 are connected between clock and ground of the flip-flops. Switch S1 activates device 1, while switch S2 activates device 2. Switch S3 activates both device 1 and device 2 simultaneously. Device status is indicated by LED1 and LED2. Glowing of LED1 and LED2 indicates that device 1 and device 2, respectively, are in on condition. The LEDs are connected from +5V to Q1 (pin 14) and Q2 (pin 10) of IC1 through resistors R3 and R6, respectively.

Initially when the power supply is switched on, Q1 and Q2 outputs of the JK flip-flops are at low level (logic 0). When switch S1 is pressed for the first time, the high level (logic 1) present at J1 input is transferred to Q1 output on the trailing edge of clock (CLK1). The high level (logic 1) at Q1 activates relay RL1 through pin 16 of IC ULN2003 (IC2), turning on device 1 via its normally-opened (N/O) contacts. Clock CLK1 of flip-flop IC1(A) is also connected to clear input CLR2 of flip-flop IC1(B) so as to clear it asynchronously. Switch debounces don’t affect the circuit as the same J1 state is being transferred to Q1 output on succeeding trailing edges. At the same time, device 2 is switched off.

When switch S2 is pressed, flip-flop IC1(A) gets cleared via CLR1 and the high state of J2 input of flip-flop IC1(B) is transferred to its Q2 output on the trailing edge of clock (CLK2). This high level (logic1) activates relay RL2 through pin 15 of IC2, turning on device 2 via its N/O contacts. At the same time, device 1 is switched off.

Now if you want to turn on both the devices simultaniously, press switch S3 momentarily. Switch S3 provides ground to preset inputs PRE1 and PRE2 of flipflops IC1(A) and IC1(B), making their Q1 and Q2 outputs high, which energises both the relays turning on the two devices. LEDs glow to indicate that both the devices are ‘on.’

Place all the three switches (S1 through S3) where you can easily press them by foot when required. The LEDs can also be mounted at a convenient location to know whether the devices are turned on.

Author : JAYAN A.R. Copyright : Electronics For You Mach 2004

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Petrol Gas Switch For A Pajero

My current vehicle, a Pajero, was modified for dual fuel - ie, petrol and gas. However, it's necessary to run the vehicle on petrol at regular intervals to stop the injectors from clogging up. This simple circuit allows the vehicle to be started using petrol and then automatically switches it to gas when the speed exceeds 45km/h and the brake pedal is pressed. Alternatively, the vehicle may be run on petrol simply by switching the existing petrol/gas switch to petrol. You can also start the vehicle on gas by pressing the brake pedal while starting the vehicle. The circuit is based on an LM324 dual op amp, with both op amps wired as comparators. It works like this: IC1a buffers the signal from the vehicle's speed sensor and drives an output filter network (D1, a 560kO resistor and a 10µF capacitor) to produce a DC voltage that's proportional to the vehicle's speed.

Circuit diagram:

This voltage is then applied to pin 5 of IC1b and compared with the voltage set by trimpot VR1. When pin 7 of IC1b goes high, transistor Q1 turns on. This also turns on transistor Q2 when the brake pedal is pressed (pressing the brake pedal applies +12V from the brake light circuit to Q2's emitter). And when Q2 turns on, relay 1 turns on and its contacts switch to the gas position. Trimpot VR1 must be adjusted so that IC1b's pin 7 output switches high when the desired trigger speed is reached (ie, 45km/h). In effect, the speed signal is AND'ed with the brake light signal to turn on the relay. The vehicle has been running this circuit for several years now and is still running well, with no further injector cleans required.

Author: J. Malnar - Copyright: Silicon Chip Electronics

Source : www.extremecircuits.net

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Computer Off Switch

How often does it happen that you close down Windows and then forget to turn off the computer? This circuit does that automatically. After Windows is shut down there is a ‘click’ a second later and the PC is disconnected from the mains. Surprisingly enough, this switch fits in some older computer cases. If the circuit doesn’t fit then it will have to be housed in a separate enclosure. That is why a supply voltage of 5 V was selected. This voltage can be obtained from a USB port when the circuit has to be on the outside of the PC case. It is best to solder the mains wires straight onto the switch and to insulate them with heat shrink sleeving. C8 is charged via D1. This is how the power supply voltage for IC1 is obtained. A square wave oscillator is built around IC1a, R1 and C9, which drives inverters IC1c to f.

The frequency is about 50 kHz. The four inverters in parallel power the voltage multiplier, which has a multiplication of 3, and is built from C1 to C3 and D2 to D5. This is used to charge C5 to C7 to a voltage of about 9 V. The generated voltage is clearly lower than the theoretical 3x4.8=14.4 V, because some voltage is lost across the PN-junctions of the diodes. C5 to C7 form the buffer that powers the coil of the switch when switching off. The capacitors charge up in about two seconds after switching on. The circuit is now ready for use. When Windows is closed down, the 5-V power supply voltage disappears. C4 is discharged via R2 and this results in a ‘0’ at the input of inverter IC1b. The output then becomes a ‘1’, which causes T1 to turn on.

Circuit diagram:

Computer Off Switch Circuit Diagram

A voltage is now applied to the coil in the mains switch and the power supply of the PC is turned off. T1 is a type BSS295 because the resistance of the coil is only 24R. When the PC is switched on, the circuit draws a peak current of about 200 mA, after which the current consumption drops to about 300 µA. The current when switching on could be higher because this is strongly dependent on the characteristics of the 5-V power supply and the supply rails in the PC. There isn’t much to say about the construction of the circuit itself. The only things to take care with are the mains wires to the switch. The mains voltage may not appear at the connections to the coil. That is why there has to be a distance of at least 6 mm between the conductors that are connected to the mains and the conductors that are connected to the low-voltage part of the circuit.

Author: Uwe Kardel - Copyright: Elektor Electronics Magazine

Source : www.extremecircuits.net

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220V AC Lamp Toggle Switch

Compact, transformer less circuitry No relays employed

Due to the low current drawing, the circuit can be supplied from 230Vac mains without a transformer. Supply voltage is reduced to 12Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. IC1A, IC1B, R2, R3 and C3 form a reliable bounce-free toggle switch operated by P1. R4 and C4, wired to pin #6 of IC1B reset the circuit (lamp off) when power supply is applied. IC1C and IC1D wired in parallel act as a buffer, driving the Gate of the Triac through R5.

Circuit diagram:

 220V AC Lamp Toggle Switch Circuit Diagram

Parts:
R1 = 470R
R2 = 10K
R3 = 100K
R4 = 100K
R5 = 1K
C1 = 330nF-400V
C2 = 100uF-25V
C3 = 100nF-63V
C4 = 10uF-25V
D1 = 1N4007
D2 = 1N4007
D3 = BZX79C12
D4 = TIC206M
IC1 = 4011 NAND Gate

Notes:

• The circuit can be wired permanently to the mains supply as current drain is negligible.
• Due to transformerless design there is no heat generation.
• Low Gate-current Triacs are recommended.
• Obviously, other appliances can be powered in place of a lamp, provided their power dissipation does not exceed about 400W @ 230V
• 110-120Vac operation is easily obtained by simply changing C1 value to 680nF 250V. No further changes are necessary.
• In some cases, e.g. when the controlled device is far from the toggle switch, a pilot LED could be necessary for monitoring purposes. If so, disconnect pin #10 of IC1C from pin #11 of IC1D and wire a LED and its 1K series current limiting resistor across pin #10 of IC1C and negative supply.
• Warning! The circuit is connected to 230Vac mains, so some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged in and enclose it in a plastic box.
• P1 will SPST Pushbutton

Source :www.extremecircuits.net

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Electronic Touch Switch

Mechanical contacts have the disadvantage that they wear out. That is why it is practical to use an electronic ‘touch switch’ in some situations. With such a touch switch the resistance of the human skin is used for the switching action. The schematic shows the design of a circuit that senses the resistance of the skin and converts it into a useful switching signal. The touch switch contacts can be made from two small metal plates, rivets, nails, etcetera, which are placed close together on a non-conducting surface. In this circuit a comparator of the type LM393 has been used. In the idle state there is, via R1, a voltage equal to the power supply voltage on the non-inverting input of IC1a. Because the inverting input of IC1a is set with R2 and D3 to D5 at the supply voltage minus 1.8 V, the open-collector output of IC1.a is, via R3, equal to the power supply voltage. This voltage is inverted by IC1.b. The voltage at the non-inverting input of IC1.b amounts to half the power supply voltage (through voltage divider R4 and R5) and is lower than the voltage on the inverting input.

Circuit diagram:

Electronic Touch Switch Circuit Diagram

The output of IC1.b is therefore a ‘0’. If the two touch contacts are bridged with a finger, the voltage at the non-inverting input will become low enough to cause the comparator to toggle state. The moistness of the skin results in a resistance of 1 to 10 MR. If this circuit is used in the vicinity of equipment that’s connected to the mains, then it can be sufficient to touch only the upper contact to operate the switch, provided that the circuit has been earthed. The body then acts as an antenna which receives the 50 Hz (or 60 Hz) from the mains. This is enough to toggle IC1.a at the same 50 Hz. C1/R3 prevent this 50 Hz from reaching the input of IC1b and provide a useable ‘pulse’ of about 10 s at the output of IC1.b. Note that a fly walking across the touch switch conducts enough to generate a switching signal. So do not operate important things with this circuit (such as the heating system or the garage door). Do not make the wires between the touch contacts and the circuit too long to prevent picking up interference. The power supply voltage for the circuit is not very critical. Any regulated DC voltage in the range from 6 to 20 V can be used.

Author: Heino Peters - Copyright: Elektor Electronics Magazine

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