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Quite often, we forget to turn off the soldering iron. This results in not only a smoking oxidised iron but also waste of electricity. To solve this problem, here’s a circuit that automatically switches off the soldering iron after a predetermined time. The circuit draws no power when it is inactive. The circuit can also be used for controlling the electric iron, kitchen timer or other appliances.

At the heart of the circuit is a monostable multivibrator built around timer IC 555. When the circuit is in sleep mode, to switch on the soldering iron, you should push switch S1 momentarily. The multivibrator gets triggered and its output pin 3 goes high for around 18 minutes to keep relay RL1 energised via transistor T1. At the same time, capacitor C3 charges and AC supply is provided to switch on the soldering iron via normally opened (N/O) contacts of relay RL1.

The soldering iron remains ‘on’ for the time period predetermined by resistor R1 and capacitor C2. Here, this time is set for 18 minutes. Flashing of LED1 indicates the heating progress of the soldering iron. When the predetermined time is over, relay RL1 de-energises to turn off the soldering iron and the buzzer sounds until capacitor C3 gets discharged.

For switching on the circuit, use either a bell push switch or a similar switch with appropriate current carrying capacity.

This is a simple Fog Lamp Switch Circuit Diagram. This circuit recommended to have a rear fog light on a trailer with the additional requirement that, when the trailer is coupled to the car, the rear fog light of the towing car has to be off. The circuit shown here is eminently suitable for this application. The circuit is placed near the rear fog light of the car. The 12-V connection to the lamp has to be interrupted and is instead connected to relay contacts 30 and 87A (K1, K3). When the rear fog light is turned on it will continue to operate normally.

If a trailer with fog light is now connected to the trailer connector (7- or 13-way, K2), a current will flow through L1. L1 is a coil with about 8 turns, wound around reed contact S1. S1 will close because of the current through L1, which in turn energizes relay Re1 and the rear fog light of the car is switched off. The fog light of the trailer is on, obviously. The size of L1 depends on reed contact S1. The fog lamp is 21 W, so at 12 V there is a current of 1.75 A. L1 is sized for a current between 1.0 and 1.5 A, so that it is certain that the contact closes. The wire size has to be about 0.8 mm. The relay Re1 is an automotive relay that is capable of switching the lamp current. The voltage drop across L1 is negligible. 

 

 

Author : J. Geene Copyright :Elektor Electronics 2008

This inexpensive, fully transistorised switch is very sensitive to sound signals and turns on a lamp when you clap within 1.5 metres of the switch. One of its interesting applications is in discotheques, where lights could be turned on or off in sync with the music beats or clapping.

The condenser microphone senses the sound and converts it into electrical variations. The electrical signals are amplified by the two-stage direct-coupled (DC) amplifier formed by transistors T1 and T2 and fed to the switching circuit. The switching circuit comprises transistors T3, T4 and T5, which conduct only when the circuit senses sound signals. Transistor T5 supplies sufficient gate voltage to the triac to drive the 230V lamp.

The regulated 12V DC power supply for the circuit is derived from AC mains by using resistor R14, diode D1 and zener diode ZD1. The circuit can be assembled on any general-purpose PCB.

Soursed By: EFY: Author Pradeep G.

Certain industrial controls require accurate switching operations. For example, in case of a foot-switch for precise drilling work, even a small error in switching may cause considerable loss. This low-cost but accurate foot-operated switch can prevent that.

IC NE555 is wired in one-shot mode. Its output pin 3 goes high only when both switches S1 and S2 are pressed simultaneously. You can release any one of the switches without changing the output state. When you release both the switches, the output goes low.

Simple Accurate Foot-Switch Circuit Diagram

The switches are placed under a foot paddle as shown in Fig. 2. LED1 is used as a warning indicator. If either S1 or S2 gets pressed erroneously, LED1 blinks to warn the operator. The operator can then withdraw his foot in case of a mistake or depress the other switch also to trigger the circuit. LED1 is to be mounted on the operator’s desk.

The circuit operation is simple. Resistors R2, R3 and R4 form a voltage divider. IC NE555 has two comparators, a flip-flop and power output section built into it. Pressing either S1 or S2 puts the input voltage between the upper comparator (2/3Vcc) and the lower comparator (1/3Vcc). Thus, it has no effect on the state of the internal flip-flop of IC NE555. Pressing the two switches simultaneously sets the flip-flop and the output of NE555 goes high. Transistor T2 energises relay RL1 for driving the load.


Releasing any of the switches brings the comparator voltage back to the initial level inside NE555 and it has no effect on the state of the flip-flop. Releasing both the switches brings the input level with respect to ground below the low trigger level, and thus it resets the output.

Use of the voltage divider results in stable operation over the entire permissible supply voltage range. The RC circuit at pin 4 provides power-on reset.

When only S1 is pressed, R3 (1 kilo-ohm) is less than R5 (1.5 kilo-ohms) and IC1 is not triggered. However, transistor T1 (BC548) gets forward biased and LED1 glows. When both S1 and S2 are pressed, the effective resistance between +Vcc and pin 2 of IC1 is about 500 ohms, which is less than R5 (1.5 kilo-ohms), and IC NE555 gets triggered.

Sometimes we forget to switch off the bathroom light and it remains on unnoticed for long periods. This circuit solves the problem of electricity wastage by switching off the lamp automatically after 30 minutes once it is switched on. The back-up LED lamp provided in the circuit turns on for three minutes when mains fails. This is helpful especially when you are taking a shower at night.

The circuit is built around binary counter CD4060 (IC2), which has a built-in oscillator and 14 cascaded bistable multivibrators. The oscillator generates clock pulses based on the values of resistors R3 and R4 and capacitor C3.

Automatic Bathroom Light with Back-up Lamp Circuit Diagram

 

For the given values, Q11 output of IC2 goes high after 30 minutes of power-on. Resistor R2 resets the IC for proper operation. The output of IC2 is fed to the gate of the SCR via resistor R6 and LED2, which function as a voltage dropper as well as output status indicator.

When the SCR gets gate drive, it fires to energise relay RL1. The latching function of the SCR keeps the relay energised until the power to the circuit is switched off using switch S1. When the relay energises, its normally closed (N/C) contacts break and light turns off. LED1 indicates that the oscillator is working.

The back-up white-LED lamp comprising LED3 and LED4 gives ample light in the event of mains failure. It is powered by a 9V rechargeable battery, which is charged at around 200mA current via diode D6 and resistor R7 when the circuit is switched on.

The back-up lamp circuit is built around timer NE555 (IC3) designed as a monostable. The output of IC3 goes high for three minutes based on the values of preset VR1 and capacitor C9. When the circuit is switched on, IC3 gets power supply via diode D6 and its trigger pin 2 remains high due to resistor R8. As a result, its output remains low as long as mains is present.

When power fails, pin 2 of IC3 get striggered via capacitor C8 and the monostable output goes high to switch on the white LEDs (LED3 and LED4). Resistor R9 limits the current through the LEDs to a safe level. Diode D7 is forward biased to give full voltage to the monostable when power fails.

The power supply for the circuit is derived from a 15V AC, 250mA transformer. The secondary output is rectified by a full-wave rectifier comprising diodes D1 through D4. Capacitor C1 smoothes the resulting DC. Regulator IC 7812 (IC1) and capacitors C4 and C5 provide stabilised 12V for the circuit.

Assemble the circuit on a Vero board and enclose it in a watertight plastic case. Connect the bathroom lamp (either 25-watt bulb or 11-watt CFL tube) to the circuit via N/C contacts of the relay, so that it turns on when switch S1 is pressed. For easy access, fix switch S1 along with the neon indicator outside the bathroom.

This is a Simple Sixteen Way Clap-Operated Switch Circuit Diagram. Control your home appliances without getting out of your bed. You just have to clap in the vicinity of the microphone used in this circuit, which you can keep by the bedside. You can switch on/off up to four different electrical equipment (TV, fan, light, etc) in 16 different ways.This circuit is built around timer IC 555 (IC1), CMOS IC 74LS93 (IC2) and five BC547 npn transistors (T1, T2, T3, T4 and T5). Transistor T1 is used as the preamplifier and the rest are used for driving the relays.

A small condenser microphone is connected at the base of transistor T1, which is biased from resistor R1 (10 kilo-ohms). The clapping sound is converted into electrical energy by the microphone and amplified by transistor T1. The transistor output is fed to the monostable circuit wired around IC 555. Output pin 3 of the timer is connected to the clock input of divide-by-16 IC 74LS93.The outputs of IC2 are fed to npn transistors T2, T3, T4 and T5 via 100ohm resistors to drive relays RL1, RL2, RL3 and RL4 connected to appliances 1 though 4, respectively. Freewheeling diodes D1 through D4 connected across the relays protect the transistors from the back electromagnetic field (e.m.f.) produced by the relays.The output states of IC 74LS93 (Q0 through Q3) for different numbers of claps are shown in the table.

The circuit is powered from regulated 5V DC. For testing the circuit, disconnect the resistors from the outputs of IC2 and connect four LEDs in series with 220-ohm resistors between the outputs and ground. Now switch on the power supply and clap near the microphone. You can see the four LEDs glowing in the manner shown in the table. A reset push switch is provided to switch off all the ‘on’ devices. Now you can connect the desired appliances to the relays and control them with your claps.

Sourced and Thanx By: Raj K. Gorkhali

This is a Simple Complementary metal–oxide–semiconductor connect switch circuit diagram, CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. This connect switch does not rely on mains hum for switching-It-can be used with battery powered circuits.

Schmitt trigger IC1 forms a 100 kHz oscillator and IC2a which is biased into the linear region, amplifies the output and charges CI via the diode. IC2b acts as a level detector. When the sensor is touched, the oscillator signal is severely attenuated which causes CI to discharge and IC2b to change state.Sourced by : Circuitsstream

This is a simple Audio Controlled Mains Switch Circuit Diagram. Simple but very forkful project for audio liker circuit. This circuit will switch off the line supply to audio or video equipment if there has been no input signal for about 2 seconds. SI provides manual operation and S2 acts as a reset. This circuit allows for time to change a tape or compact disc.About 50 mV of audio signal is necessary. Read:  Solid-State Switch For Dc-Operated Gadgets

This is a simple power on switching circuit. This type of reset pulse is ideally provided by this circuit. Because of the high input impedance of the Schmidt trigger, long reset pulse times may be achieved without the excess dissipation that results when both output devices are on simultaneously, as in an ordinary gate device (B). A reset pulse is often required at power-on in a digital system. See circuit diagram below.

Simple Remote on-off Switch Circuit Diagram. This circuit provides power control without running line-voltage switch leads. The primary of a 6-volt filament transformer is connected between the gate and one of the main terminals of a triac. 

The secondary is connected to the remote switch through ordinary low-voltage line. With switch open, transformer blocks gate current, prevents the triac from firing and applying power to the equipment. Closingthe switch short-circuits the secondary, causing the transformer to saturate and trigger the triac.

This is the simple and Fast Breaker Circuit Diagram. This 115 Vac, electronic circuit breaker uses the low drive power, low on resistance and fast turn off of the TMOS MTM15N50. The trip point is adjustable, LED fault indication is provided and battery power provides complete circuit isolation. The two `circuit breaker` terminals are across one leg of a full wave diode bridge consisting of D1-D4. Normally, Q1 is turned ON so that the circuit breaker looks like a very low resistance. 

One input to comparator Ul is a fraction of the internal battery voltage and the other input is the drop across zeners D6 and D7 and the voltage drop across R1. If excessive current is drawn, the voltage drop across Rl increases beyond the comparator threshold (determined by the setting of R6), U1 output goes low, Q1 turns OFF, and the circuit breaker `opens.` When this occurs, the LED fault indicator is illuminated.

This is a Simple Touch Sensitive Switch Circuit Diagram. This circuit A high impedance input is provided by Ql, a general purpose field effect transistor 741 op amp is used as a sensitive voltage level switch which in turn operates the current Q2, a medium current PNP bipolar transistor, thereby energizing the relay which can be used to control equipment, alarms, etc.

 The UCC27518 and UCC27519 single-channel, high-speed, low-side gate driver device is capable of effectively driving MOSFET and IGBT power switches. Using a design that inherently minimizes shoot-through current, UCC27518 and UCC27519 are capable of sourcing and sinking high, peak-current pulses into capacitive loads offering rail-to-rail drive capability and extremely small propagation delay typically 17 ns.

 The UCC27518 and UCC27519 provide 4-A source, 4-A sink (symmetrical drive) peak-drive current capability at VDD = 12 V. The UCC27518 and UCC27519 are designed to operate over a wide VDD range of 4.5 V to 18 V and wide temperature range of -40°C to 140°C. Internal Under Voltage Lockout (UVLO) circuitry on VDD pin holds output low outside VDD operating range.

Here is another simple circuit to mute the volume of Audio devices through simple touch. It exploits the action of the flip-flops in the timer IC 555 to reduce the volume of the Audio amplifier. IC NE555 is designed in the toggle mode. Its lower and upper comparator inputs are connected to the touch plates which can be membrane switches or two pieces of conducting plates. The inputs of comparators are stabilized through R1 and R2 to avoid floating.

When the touch plate connected to pin 2 is touched momentarily, output of IC1 goes high and T1 conducts. The centre tap of the volume control is connected to the collector of T1. So when T1 conducts current going to the amplifier drains through T1. This reduces the volume.IC1 remains latched in this position with LED on. When the touch plate connected to pin 6 is touched momentarily, output of IC1 goes low and T1 turns off. This restores the volume.

Here we have three choices, with which we can make electronic switches that use our touch or pressing (push button). We thus exploit the very big resistance of entry, that present the gates CMOS. In the fig.1 we have two gates NAND or NOR (IC1), connected as R-S flip-flop. Just as we press the switch S1, the exit 3 it becomes [H], even it is maintained in this situation.

To change the situation, it should we press switch S2. Now exit 3, takes price (L), reversely exit 4 becomes (H). In order to we maintain the situation that we want, we can connect at parallel with the corresponding switch, a capacitor C=100nF. This entry will always drive the corresponding exit to logic (L), immediately afterwards the benefit of supply to the circuit.

Switch ON-OFF Touch or with Push Button Schematic

In the fig. 2, we have a circuit of inverter CMOS, in the entry of which is applied logic situation (H), from the resistance R, which the other end of, is in the supply. Exit 2 has situation (L).

When we press switch S2, in the entry of 3 IC2, we have situation (L), this it goes to the ground, the exit now becomes (H). This situations are maintained as long as we keep pressed switch S2 and they change immediately hardly the touch. If we want opposite logic operation then it will be supposed we connect the resistance R, in the ground and switch S2, in the supply. The same logic we will have if we replace gate IC2, with a gate NAND or NOR, as it appears in the fig. 3, the result is the himself.

Because the situation in the case of fig.1 and 3, does not remain constant and change when we pull our finger , in order to him we retain, it should we connect a J-K or D flip-flop as T, after the IC2 and IC3. Thus the flip-flop, will change situation, each time where we will touch the switch or will touch the contacts and him it will retain.

All the switches can be replaced with contacts, it is enough we replace also resistances R with the price of 10MΩ. The Resistances R when we use pressing switches can are, from 100KΩ until 1MΩ. Because when we use contacts instead of switches, the noise can turn on the gates of fig. 2 and 3, then can place a capacitor 100nF, parallel with the contacts.[via]