Per request the circuit today we have relay circuit. It is worth noting again that the diagram provides a time delay of about 0.5 seconds for every microfarad in the value of capacitor C1.
Automatic Turn off Relay Circuit Diagram
This permits delays of up to several minutes. If desired, the delay periods can be made variable by replacing resistor R2 with a fixed and variable resistor in series whose nominal values are approximately equal of the total value of R2 (680K).
AC switches are silicon devices that control AC loads directly connected to the AC mains. This means that the driving reference terminal of the AC switch can be connected to the Line potential. This circuit explains the need of an insulation layer for the control unit and the way to implement it for an AC switch device. It was thought in the past that connecting an MCU to the Line should be avoided as it will lead to poor appliance immunity. But it has been demonstrated over the years that such topology provides good immunity. Connecting an MCU supply to a stable non-floating reference is even better for immunity.
Safety insulation should be provided between accessible parts and high-voltage circuits to protect end users against electric shocks. It’s not required to ensure safety insulation by insulating low-voltage control circuits (like MCU) from high-voltage parts (like AC switches). In fact, the insulation could be implemented elsewhere—for example, on the keyboard to which the end user has access—leaving the MCU connected to the Line. This could be cost-effective as a non-insulated power supply and non-insulated drivers would then be sufficient.
Fig. 1: Circuit of AC switch control with opto-triac
Operational insulation is required when the control circuit reference is not the same as the AC switch reference. This is the case with new appliances using an inverter for 3-phase motor control, where the MCU is connected to the DC rail and the AC switch is referenced to Line. A level-shifter is used to allow communication between the MCU and the power switch. A usual way to implement this is to use an opto-triac but such a device will not work properly for all AC switches.
Among AC switches available today, different technologies and designs are used. The main known families are the standard triacs, the snubberless triacs and ACS devices. To switch-on a triac or an ACS device, a gate current must be applied between the gate (G) and terminal A1 for the triac, or between gate and terminal COM for the ACS device.
Fig. 2: Pin configuration of ACS108
For the triac, the gate current could be positive or negative, but the silicon structure of an ACS device is different from a triac. Here the gate is the emitter of an npn bipolar transistor. So there is only one p-n junction. The gate current can then only be sunk from the gate, not sourced to it. As ACS de-vices can be triggered only by a negative current, an opto-triac will drive the ACS device only when the Line voltage is negative. This will lead to half-cycle conduction, which is inconvenient for most applications. However, there are new applications where such an operation is requested—for example, pumps used in coffee machines that feature an internal diode, and electromagnets used for door-lock function in washing machines.
As shown in Fig. 1, the circuit is built around ACS108 (Triac 1), opto-triac IC MOC3020 (IC1) and a few discrete components. Working of the circuit is simple. When you press switch S1, the load is switched on. When you release switch S1, the load turns off. Once the switch is pressed, the opto-triac (IC1) conducts to charge capacitor C1 up to VGT (about 0.7 volt). COM-G junction forward-biases, triggering the ACS device by a negative gate current. The ACS device will remain ‘on’ up to the next zero-current crossing point. G-COM voltage is down to –0.7V due to ACS device conduction and the capacitor remains charged. As the current through the ACS device increases, VG-COM increases and there-fore a negative current is applied by C1 which triggers the AC switch for the next cycle.
In this solution, the ACS device is ‘off’ at the beginning of each time cycle required to recharge capacitor C1. The ACS device turns ‘on’ when the voltage across its terminals equals approximately 10V. This behaviour doesn’t result in high conducted noise as the Line current is still approximately sinusoidal due to the charge current flowing through capacitor C1 at zero-voltage crossing point.
Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The pin configuration of ACS108 is shown in Fig. 2. Fix switch S1 on the front panel, and two terminals for the load on the rear side of the cabinet.
Author : STMicroelectronics - Copyright: EFY
This circuit uses an MC2830 to form a voice activated switch ( VOX ). A traditional VOX circuit is unable to distinguish between voice and noise in the incoming signal. In a noisy environment, the switch is often triggered by noise, or the activation sensitivity must be turned down.
Voice Activated Switch Circuit Diagram:
This circuit overcomes this weakness. The switch is activated by voice level above the noise and not activated by background noise. This is done by utilizing the differences in voice and noise waveforms. Voice waveforms generally have a wide range of variation in amplitude, whereas noise waveforms are more stable. The sensitivity of the voice activation depends on the value of R6. The voice activation sensitivity is reduced from 3.0dB to 8.0dB above the noise if R6 changes from 14k to 7.0k .
Tiny Timer Light Switch presented here is a simple transistorised electronic timer which drives a high efficiency white LED for a finite time out. This circuit is very useful for in-car reading etc. The circuit works off 12 volt dc supply. After construction, fit the unit at a suitable location inside your car and power the circuit from the in-dash standard cigar lighter socket. The timer light switch is ultra simple, economic, straight forward and self explanatory.
Timer light switch Circuit Diagram :
the timer light switch works?
Normally T1 is turned off by P1 and R2. When the trigger switch S1 is pressed the base of T1 is connected to the +12V supply via R2. Now T1 turns on and this action turns on the next transistor T2 which in turn energises the white LED (D2). Resistor R4 limits the operating current of white LED (D2).
When the switch is pressed is current also flows into capacitor C1 (through R1) and charges it. So when the switch S1 is released the charge in the capacitor C1 keeps T1 turned on until the charge has decayed away through R2 and P1. You can easily increase the ouput on time by increasing the resistance of the potentiometer P1. link
This circuit will remove the transient spikes and contact bounces from a non-latching push button switch.
Push Button Switch De-Bouncer 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
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 :
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
This circuit can control any one out of 16 devices with the help of two push-to-on switches. An up/down counter acts as a master-controller for the system. A visual indication in the form of LEDs is also available. IC1 (74LS193) is a presettable up/ down counter. IC2 and IC3 (74LS154) (1of 16 decoder/demultiplexer) perform different functions, i.e. IC2 is used to indicate the channel number while IC3 switches on the selected channel.
Digital Switching System Circuit Diagram :
Before using the circuit, press switch S1 to reset the circuit. Now the circuit is ready to receive the input clock. By pressing pressing switch S2 once, the counter advances by one count. Thus, each pressing of switch S2 enables the counter to advance by one count. Likewise, by pressing switch S3 the counter counts downwards.
The counter provides BCD output. This BCD output is used as address input for IC2 and IC3 to switch one (desired channel) out of sixteen channels by turning on the appropriate triac and the corresponding LED to indicate the selected channel. The outputs of IC3 are passed through inverter gates (IC4 through IC6) because IC3 provides negative going pulses while for driving the triacs we need positive-going pulses. The high output of inverter gates turn on the npn transistors to drive the triacs. Diodes connected in series with triac gates serve to provide unidirectional current for the gate-drive.
Author : Rajesh K.P - Copyright : EFY
Get the circuit instead of a standard on-off switch. Switching is very gentle. If we don’t use the PCB, connect unused input pins to an appropriate logic level (‘+’ or ‘-’). Unused output pins *NEED* be left open! On the Print Circuit Board this has completed already . One step ’push’ activates the relay, another ‘push’ de-activates the relay. IC1 (the 4069) is a regular Hex-inverter type and is constructed with MOS P-channel and N-channel enhancement mode devices in a single monolithic structure.
Alternating On-Off Switch Circuit Diagram :
Parts List :
R1 = 10K
R2 = 100K
R3 = 10K
R4 = 220 Ohm (optional)
C1 = 0.1µF, Ceramic (100nF)
C2 = 1µF/16V, Electrolytic
D1 = 1N4001
Led1 = Led, 3mm, red (optional)
Q1 = 2N4401 (see text) IC1 = 4069, CMOS, Hex Inverter (MC14069UB), or equivalent
S1 = Momentary on-switch
Ry1 = Relay )
It is going to operate on voltages from 3 to 18 volts, but most applications are in the 5-15 volts. Although the IC1 4069 contains protection circuitry against damage from ESD , use common sense when handling this device. Depending on your application you may want to use an IC-socket with IC1. It makes replacement easy if the IC ever fails. The IC is CMOS so watch for static discharge! You can use any type of 1/4 watt resistors including the metal-film type.
The type for D1 in not critical, even a 1N4148 will work. But, depending on your application I would suggest a 1N4001 as a minimum if your relay type is 0.5A or more. Any one in the 1N400x series diodes will work. Any proper replacement for Q1 will work, including the european TUN’s. Since Q1 is just a driver to switch the relay coil, almost any type for the transistor will do. PN100, NTE123AP, BC547, 2N3904, 2N2222, 2N4013, etc. will all work for the relays mentioned here. For heavier relays you may need to change Q1 for the appropriate type.
For C2, if you find the relay acts not fast enough, you can change it to a lower value. It is there as a spark-arrestor together with diode D1. For the relay I used an 8 volt type with the above circuit and a 9 volt battery. Depending on your application, if the current-draw is little, you can use a cheap 5V reed-relay type. Use a 8V or 9V relay type if your supply voltage is 12V. Or re-calculate resistor R3 for a higher value.
The circuit and 9V will work fine and will pull the relay between 7 and 9 volt, the only thing to watch for is the working voltage of C2; increase that to 50V if you use a 12V supply. The pcb was designed for an Aromat/Omron relay, 12V/5A, #HB1-DC12V. You can easily re-design the relay pads on the PCB for the relay of your choice. If you wish to use something you already have, and you don’t want to re-design the PCB, you can glue the relay up-side-down on the pcb and wire the relay contacts manually to the pcb-holes or directly to your application. Use a 2N2222 transistor for Q1 if your supply voltage is higher than 9V and/or your relay is heavy duty, or doesn’t want to pull-in for any other reason.
Again, the pcb drawing is not to scale. Use ‘page-setup’ to put the scale to 103% for a single pcb, vertically, and your scale should be correct. I use a laser printer and so I don’t know if this scale of 103% is for all printers. To check, print a copy onto regular paper and see if the IC pins fit the print. If so, your copy is correct. If not, change the scale up of down until a hardcopy fits the IC perfectly.
The Led is nice for a visual circuit indication of being ‘on’. For use with 12V supply try making make R4 about 330 ohms. The LED and R4 are of course optional and can be omitted. Your application may already have some sort of indicator and so the LED and R4 are not needed.
This solid state DC switch can be assembled using just three transistors and some passive components. It can be used to switch on one gadget while switching off the second gadget with momentary operation of switch. To reverse the operation, you just have to momentarily depress another switch.
The circuit operates over 6V-15V DC supply voltage. It uses positive feedback from transistor T2 to transistor T1 to keep this transistor pair in latched state (on/ off), while the state of the third transistor stage is the complement of transistor T2’s conduction state.
Initially when switch S3 is closed, both transistors T1 and T2 are off, as no forward bias is available to these, while the base of transistor T3 is effectively grounded via resistors R8 and R6 (shunted by the load of the first gadget). As a result, transistor T3 is forward biased and gadget 2 gets the supply. This is indicated by glowing of LED2.
Circuit diagram :
Solid-State Switch For Dc-Operated Gadgets Circuit Diagram
When switch S1 is momentarily depressed, T1 gets the base drive and it grounds the base of transistor T2 via resistor R4. Hence transistor T2 (pnp) also conducts. The positive voltage available at the collector of transistor T2 is fed back to the base of transistor T1 via resistor R3. Hence a latch is formed and transistor T2 (as also transistor T1) continues to conduct, which activates gadget 1 and LED1 glows.
Conduction of transistor T2 causes its collector to be pulled towards positive rail. Since the collector of T2 is connected to the base of pnp transistor T3, it causes transistor T3 to cut off, switching off the supply to gadget 2) as well as extinguishing LED2. This status is maintained until switch S2 is momentarily pressed. Depression of switch S2 effectively grounds the base of transistor T1, which cuts off and thus virtually opens the base-emitter circuit of transistor T2 and thus cutting it off. This is the same condition as was obtained initially. This condition can be reversed by momentarily pressing switch S1 as explained earlier.
EFY lab note. During testing, it was noticed that for proper operation of the circuit, gadget 1 must draw a current of more than 100 mA (i.e. the resistance of gadget 1 must be less than 220 ohms) to sustain the latched ‘on’ state. But this stipulation is not applicable for gadget 2. A maximum current of 275 mA could be drawn by any gadget.
Author : Praveen Shanker - Copyright : EFY
This circuit acts like a two-position switch but is operated using a pushbutton. After power has been applied, the circuit is in the following initial state: the bases of T1 and T2 are at the positive supply potential and the base of T3 is at ground potential. All transistors are cut off. The other contact of the pushbutton is at ground potential. No current flows through the relay coil and the status LED is off.
Circuit Diagram :
Pushbutton Switch Circuit diagram
If the pushbutton is pressed, T2 and (after a slight delay due the RC network) T3 switch on. The collector of T3 is now nearly at ground potential, so current flows through the relay coil and the function LED is illuminated. T1 can also switch on. This situation is stable, since ground potential can reach the base of T2 via R1, so nothing changes when the pushbutton is released. C1 is charged via R3 to cause a positive potential to be present at the pushbutton. If the pushbutton is now pressed again, it connects a positive potential to the base of T2 instead of the ground potential. This causes everything to toggle back into the initial state.
Similar operation can be obtained using a thyristor circuit, and in fact T2 and T3 form a sort of thyristor. However, the circuit shown here is largely independent of the voltage and current demands of the connected load. The relay coil should be suit-able for the supply voltage (5–12 V) and should not draw more than 250 mA, since otherwise T3 will go up in smoke. With our lab prototype, we measured a current consumption of 70 mA in the ‘on’ state and less than 0.1 mA in the ‘off’ state.
Author : K. Lorenz - Copyright : Elektor
Anyone experimenting or developing USB ported peripheral hardware soon be comes irritated by the need to disconnect and connect the plug in order to reestablish communication with the PC. This process is necessary for example each time the peripheral equipment is reset or a new version of the firmware is installed. As well as tiresome it eventually leads to excessive contact wear in the USB connector. The answer is to build this electronic isolator which disconnects the peripheral device at the touch of a button. This is guaranteed to reduce any physical wear and tear and restore calm once again to the workplace.
Circuit image :
USB Switch Schematic Circuit Image
The circuit uses a quad analogue switch type 74HC4066. Two of the switches in the package are used to isolate the data path. The remaining two are used in a classic bistable flip-flop configuration which is normally built using transistors. A power MOSFET switches the power supply current to the USB device. Capacitor C2 ensures that the flip flop always powers-up in a defined state when plugged into the USB socket (‘B’ in the diagram). The peripheral device connected to USB socket ‘A’ will therefore always be ‘not connected’ until pushbutton S2 is pressed. This flips the bistable, turning on both analogue gates in the data lines and switching the MOSFET on. The PC now recognises the USB device. Pressing S1 disconnects the device.
Circuit diagram :
USB Switch Schematic Circuit Diagram
The circuit does not sequence the connections as a physical USB connector does; the power supply connection strips are slightly longer than the two inner data carrying strips to ensure the peripheral receives power before the data signals are connected. The electronic switch does not suffer from the same contact problems as the physical connector so these measures are not required in the circuit. The simple circuit can quite easily be constructed on a small square of perforated strip-board. The design uses the 74HC(T)4066 type analogue switch, these have better characteristics compared to the standard 4066 device. The USB switch is suitable for both low-speed (1.5 MBit/ s) and full-speed (12 MBit/s) USB ports applications but the proper ties of the analogue switches and perf-board construction will not support hi-speed (480 MBit/s) USB operation.
The IRFD9024 MOSFET can pass a current of up to 500 mA to the peripheral device with-out any problem.
Author : Rainer Reusch - Copyright : Elektor
This AC mains-operated bistable switch turns on or turns off a device using a miniature neon lamp and a few discrete components. This switch can be used for control pan-els, appliances and lighting controls. A push-to-on switch is used to light up the neon lamp. The light emit-ted by the neon lamp, in turn, enables the switching action of the circuit. Use of a 555 timer wired for bistable operation makes the circuit act as a bistable switch.
Circuit diagram :
AC Mains Bistable Switch Circuit Diagram
The neon lamp (NL1) and the push-to-on switch (S1) are directly connected to 230V AC mains. The 12V DC supply for timer 555 (IC1) is derived from 230V AC mains through capacitive dropper C1, resistor R1 and a 12V zener diode. IC1 works as a flip-flop circuit, with the signal at its output pin 3 toggling every time it receives a pulse at its pins 2 and 6.
The operation of the circuit is simple. When you press switch S1 momentarily, the neon lamp glows, making phototransistor T1 conduct to provide a pulse at pins 2 and 6 of IC1. When switch S1 is pressed, the output of IC1 goes high and LED1 glows. Pressing S1 again makes the output of IC1 low and LED1 stops glowing.
In place of LED1, you can use an optodiac or suitable relay (not shown in the circuit) along with a suitable driver circuit to drive AC loads. Assemble the circuit on a general-purpose PCB with the neon lamp and the phototransistor housed in a small black tube isolated from the external light source, and enclose in a suitable cabinet. Fix switch S1 on the front panel of the cabinet, and mains power cord at the rear. At the rear, also fix a 3-pin socket to connect the AC load.
Caution. Take care when operating this circuit as it is directly connected to 230V AC mains. Better still, don’t attempt this circuit if you have no experience in handling high-voltage circuits.
Author : T.A. Babu- Copyright : EFY
According to the Energy Saving Trust, if you add up all the current drawn in standby mode by items such as stereos, TVs, VCRs and DVDs over a year in the UK alone, it amounts to 3.1 million tonnes of CO2 released into the atmosphere.This is without factoring in the current drawn by all the PCs,laptops and their associated peripherals left in standby mode.
Circuit diagram :
Green USB switch Circuit Diagram
It is not necessary to spend a great deal of money or time to make a difference on a personal level. The circuit described here is designed for use by laptop or notebook computers. It will automatically switch off all mains powered peripheral equipment including monitor, printer, scanner, TV tuner and USB hub etc when it detects that the notebook is switched off. The circuit is quite straightforward; in addition to an optocoupler it requires a 12 V double-pole relay with mains rated contacts and a small power supply for the optocoupler. When the laptop is switched on 5 V appears at the USB socket, activating the relay and switching through the mains supply on K3 and K4. The notebook’s USB socket is still available to be used as normal but it’s worth remembering that the optocoupler takes a few milliamps from the USB supply and this may cause a problem if a high-current device is plugged into the USB socket. In the case where the laptop has more than enough USB sockets it may be worthwhile us-ing one of them solely for this circuit, the extension USB connector K2 would then not be required.
The circuit is mounted into a mains plug enclosure which provides a socket where the mains extension strip will be plugged into. With any luck there will be sufficient space to fit the entire circuit into the mains extension strip enclosure and save the need for a separate enclosure. The slow-blow 6.3-A fuse (F1) protects the equipment plugged into the strip.
In addition to the optocoupler and relay the circuit also has a ‘freewheel’ diode D1 and a relay driver formed by T1 and its base bias voltage divider network R2/ R4. The two ‘snubber’ networks C1/R3 and C2/R5 reduce the possibility of arcing which can occur when the relay contacts open (especially with inductive loads). Capacitors C1 and C2 must be class X2 types which can handle mains voltage plus any spikes. The power supply consists of a small mains trans-former (12 V, 50 mA), bridge rectifier and smoothing capacitor C3.
The laptop’s mains adaptor itself can also be switched by this circuit when the laptop is fitted with its rechargeable battery which allows the computer to boot up without a mains supply. The en-tire USB switch circuit draws cur-rent even when it is off but this value is tiny compared to the combined standby current of all the peripherals.
Note that parts of this circuit are connected to the (potentially lethal) mains supply voltage; it is essential to provide protection to ensure that nothing can accidentally make contact with these parts of the circuit. It is also important to observe correct separation between parts of the circuit carrying low voltage and those carrying the high volt-age. Please observe the electrical Electrical Safety guide-lines which are reprinted in Elektor Electronics several times a year.
The circuit is less suitable for use with desktop PCs be-cause the majority of these machines supply 5 V over the USB socket even though they have been shut down via soft-ware. The only way to turn off in this case is to reach around the back of the machine and switch off at the main switch.
Author : Wolfram Winfera - Copyright : Elektor
Only simple materials and a little bit of skill are needed to build an RS232 switch. All that you need are two 9-way sub-D plugs with solder pins, a small piece of sheet aluminium, two sets of screw retainer posts, a 4-pole double-throw switch, a strain relief sleeve and a suitable plastic connector shell for a 25-way sub-D connector, with both in-line and right-angle cable entries (such as Conrad Electronics #711322). What is important is that the side cable entry together with its associated strain relief leaves enough room for the switch. If necessary, you may have to cut away a few square millimetres of the sidewall or a few ribs of the plastic shell.
Project image :
The switch is operated via the in-line cable opening, as can be seen from the photo. A suitable switch with an overall length of 29 mm can be found in the Conrad catalogue under order number 708232. The only modification that must be made to the connector shell is to drill two holes for the retaining screws for the switch (M2.6 screws) at a spacing of 24 mm.
Circuit diagram :
Mini RS232 Data Switch Circuit Diagram
Connect the two sub-D connectors together using the piece of aluminium and the screw retainer posts. Then solder the cable to the connectors and the switch as indicated. The two connectors are wired somewhat differently. While the upper sub-D plug is connected 1:1 with the input cable (with the switch in the appropriate position), the DCD, DTR, DSR and RI pins of the lower connector are left open. This is because RTS and CTS are fully sufficient for handshaking, as long as DTR and DSR are connected to each other. The only leads that are switched are RXD, RTS, TSD and CTS. The ground potential is fed from the cable to both connectors. After everything has been properly soldered together, you can fit everything into the cable shell as shown.
Author : H. J. Bohling - Copyright : Elektor
The author is the happy owner of a television set with built-in Ambilight lighting in the living room. Unfortunately, the television set in the bedroom lacks this feature. To make up for this, the author attached a small lamp to the wall to provide background lighting, This makes watching television a good deal more enjoyable, but it ’s not the ideal solution. Although the TV set can be switched off with the remote control, you still have to get out of bed to switch off the lamp.
Circuit diagram :
Automatic TV Lighting Switch Circuit Diagram
Consequently, the author devised this automatic lighting switch that switches the background light on and off along with the T V set. The entire circuit is fitted in series with the mains cable of the TV set, so there’s no need to tinker with the set. It works as follows: R1 senses the current drawn by the TV set. It has a maximum value of 50 mA in standby mode, rising to around 500 m A when the set is operating. The voltage across R1 is limited by D5 during negative half- cycles and by D1– D4 during positive half-cycles. T he voltage across these four diodes charges capacitor C1 via D6 during positive half-cycles. This voltage drives the internal LED of solid-state switch TRI1 via R2, which causes the internal triac to conduct and pass the mains voltage to the lamp. Diode D7 is not absolutely necessary, but it is recommended because the LED in the solid-state switch is not especially robust and cannot handle reverse polarisation. Fuse F1 protects the solid-state switch against overloads. T he value of use d here (10 Ω) for resistor R1 works nicely with an 82-cm (32 inch) LCD screen.
With smaller sets having lower power consumption, the value of R1 can be increased to 22 or 33 Ω, in which case you should use a 3-watt type. Avoid using an excessively high resistance, as otherwise TRI1 will switch on when the TV set is in standby mode. Some TV sets have a half-wave rectifier in the power supply, which places an unbalanced load on the AC power outlet. If the set only draws current on negative half-cycles, the cir-cuit won’t work properly. In countries with reversible AC power plugs you can correct the problem by simply reversing the plug. Compared with normal triacs, optically cou-pled solid-state relays have poor resistance to high switch-on currents (inrush currents).
For this reason, you should be careful with older-model TV sets with picture tubes (due to demagnetisation circuits). If the relay fails, it usually fails shorted, with the result that the TV background light remains on all the time. If you build this circuit on a piece of perf-board, you must remove all the copper next to conductors and components carrying mains voltage. Use PCB terminal blocks with a spacing of 7.5 mm. This way the separation between the connections on the solder side will also be 3 mm. If you fit the entire arrangement as a Class II device, all parts of the circuit at mains potential must have a separation of at least 6 mm from any metal enclosure or electrically conductive exterior parts that can be touched.
Author :Piet Germing - Copyright : Elektor
This cell-phone-controlled audio/ video mute switch is highly useful in automobiles. The circuit automatically disconnects power supply to the audio/video system whenever the mobile handset is lifted off the holder for making or receiving a call. You can use any readily available cell-phone holder with some mi-nor alterations or fabricate it yourself as shown in Fig. 1.
Fig. 1: Proposed cell-phone holder
The circuit is wired around IC LM555 (IC1), the CMOS version of timer NE555, as shown in Fig. 2. IC1 is used as a medium current line driver with either an inverting or non-inverting output. It can sink (or source) current of up to 50 mA only, so take care while handling it. The audio/video system is connected to the circuit via normally opened (N/O) contacts of the relay.
Fig. 2: The circuit of the cell phone-controlled audio/video mute switch
When the cell phone is in its holder, LDR1 does not receive any light from white LED1 and its resistance is high. As a result, the voltage at pin 2 of IC1 re-mains high to provide a low output at pin 3. The low output of IC1 activates relay RL1 and the audio/video system gets power supply via its N/O contacts. LED3 glows to indicate that the audio/video system is ‘on.’ When the handset is taken off the holder, light rays from LED1 fall on LDR1 and its resistance decreases. As a result, the voltage at pin 2 of IC1 de-creases to provide a high output at its pin 3. The high output of IC1 deactivates relay RL1 and the audio/video system does not get power supply. LED2 glows to indicate that the audio/video system is ‘off.’ Preset VR1 is used to control the sensitivity of the circuit. Zener diode ZD1 is used for protecting white LED1 from the higher voltage. The circuit works off a 12V car battery. Switch S1 can be used to manually switch on/off the audio/video system.
Author : T.K. Hareendran - Copyright: Electronics For You
Nowadays a lot of intercom units are equipped with video cameras so that you can see as well as hear who is at the door. Unfortunately, the camera lens is perfectly placed to serve as a sort of support point for people during the conversation, with the result that there’s hardly anything left see in the video imagery. One way to solve this problem is to install two cameras on the street side instead only one, preferably some distance apart. If you display the imagery from the two cameras alternately, then at least half of the time you will be able to see what is happening in front of the door. Thanks to the video switch module described here, which should be installed on the street side not too far away from the two cameras, you need only one monitor inside the house and you don’t need to install any additional video cables.
Circuit diagram :
Video Switch for Intercom System Circuit Diagram
Along with a video switch, the circuit includes a video amplifier that has been used with good results in many other Elektor projects, which allows the brightness and the contrast to be adjusted separately. This amplifier is included because the distance between the street and the house may be rather large, so it is helpful to be able to compensate for cable attenuation in this manner. The switch stage is built around the well known 4060 IC, in which switches IC2a and IC2d alternately pass one of the two signals to the output. They are driven by switches IC2b and IC2c, which generate control signals that are 180 degrees out of phase. The switching rate for the video signals is determined by a clock signal from an ‘old standby’ 555 IC, which causes the signals to swap every 2 seconds with the specified com ponent values.
Naturally, this circuit can also used in many other situations, such as where two cameras are needed for surveillance but only one video cable is available.
Author :Jacob Gestman Geradts - Copyright : Elektor
Circuit diagram :
Dual High Side Switch Controller Circuit Diagram
One of the most frequent uses of n-channel MOSFET’s is as a voltage controlled switch. To ensure that the MOSFET delivers the full supply voltage to the load it is necessary for the gate voltage to be a few volts above the supply voltage level. This can be a problem if no other suitable higher volt-age sources are available for use elsewhere in the circuit. The LTC 1982 dual high-side switch controller from Lin-ear Technology (www.linear-tech.com) solves this problem by incorporating a voltage tripler circuit in the gate driver stage. The gate voltage is limited to +7.5 V which is 2.0 V above the IC’s maximum operating voltage. It can directly drive the gate of logic-level MOSFET with a VGS(th) from 1.0 V to 2.0 V. A suitable n-channel logic level MOSFET would be the BSP 295. This device can switch up to 1.5 A and is available in an SOT 233 SMD package.
It is often useful for audio or video equipment to be switched off automatically after there has been no input signal for a while. The function of the on-off switch in such equipment is then taken over by switch S2 in the accompanying diagram. It remains, however, possible to switch off manually by means of Si. Automatic switch-off occurs after there has been no input signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the relevant machine.
The audio input to the proposed circuit may be taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at + 6 V with respect to the circuit earth by potential divider Ri-R2-R3-R4. The two 741s function as comparators: the output of ICi goes high when the in- put signal is greater than + 50 mV, whereas the out- put of IC2 goes high when the input signal becomes more negative than -50 mV. Resistors R6, R7, and R8 form an OR gate that drives transistor Ti. If the output of either ICi or IC2 is logic 1, Ti conducts.
Circuit diagram :
Audio Controlled Mains Switch Circuit Diagram
The 555 operates as a retrigger able monostable, whose period is determined by Rio and Ci. The device is triggered when its pin 2 is earthed by the closing of S2. Its output, pin 3, then remains high for 1 to 2 minutes, depending on the leakage cur- rent of the 555. The monostable resets itself as soon as the potential across Ci exceeds a certain value. As long as there is an input signal to the circuit, Ti conducts and Ci remains uncharged. As soon as the audio signal ceases, Ti switches off, and Ci charges until the potential across it is sufficient to reset the 555. The monostable may also be reset by closing Si, which connects pin 6 of the 555 to + 12 V.
When IC3 is reset, Ci is discharged via its pin 7. Resistor Rrn serves as protection, because without it Ti could short-circuit the supply lines. When the output of IC3 goes high, T2 conducts, the relay is energized, and the relay contacts switch on the mains voltage as appropriate. To counter the induced potential when the relay contacts close, which could damage T2, diode Di has been connected in parallel with the relay coil.