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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]

A touch switch is a useful circuit that can be used to detect humans or protect small objects, such as antiques. It can be used to turn on a lamp or as an annunciator to sound a buzzer when someone comes near a door or table. The touch switch, or capac-ity switch, can also be used to start a moving display sign. A touch switch is shown in Fig. 1-8, and it can be activated by  touching a small metal plate connected to pin 2 of the 555 timer chip. Once triggered, the load remains on until reset. A low logic level applied to pin 4 resets the circuit. The output is on pin 3,  which is used to drive an LED.

Fig. 1-8 Touch switch-manual reset.

Another variation of the touch switch is depicted in Fig. 1-9. This touch switch also uses the ubiquitous 555 chip. The circuit is configured as a monostable multivibrator. The load remains on for a time period determined by the R1/C1 combination. After the time period elapses, the circuit turns off until triggered again. The sense plate is connected to a capacitor placed in series with pin 2 of the IC timer to increase the charge accumulation.

Fig. 1-9 Touch switch.

The touch switch relies on the "stray capacitance effect" of a human body from the sense plate to a lower potential, i.e., ground. By completing a path to ground through the human body, the switch magically appears to turn on a light or external load. Always power your touch switch either with batteries or with a power supply that uses a transformer to ensure you are not in the direct path to a 110-V.. line.

Author: Thomas Petruzzellis

Copyright:McGraw-Hill - Alarm,Sensor & Security Circuit  Cookbook