Tuesday, April 9, 2013

Touch switches

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.
Touch switches Circuits Diagram 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.
Touch switch 19 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.
Touch switch with manual reset parts list :
Quantity Part Description
1 R1 1-ku, 1/4-W resistor
2 C1, C2 0.5-gF, 25-V capacitor (disk)
1 D1 Red LED
1 U1 555 timer IC
1 S1 Sense-plate copper circuit board
Touch switch parts list:
Quantity Part Description
1 R1 100-kS2, /4-W resistor
1 C1 4.7-10-µF, 25-V electrolytic capacitor
1 C2 0.1-.tF, 25-V capacitor
1 C4 0.05- tF, 25-V capacitor (disk)
1 C3 50-pF, 25-V electrolytic capacitor
1 D1 1N4001 silicon diode
1 Q1 2N2222 pnp transistor
1 U1 555 IC timer
1 Ry-1 6-V SPST relay
1 S1 Sense-plate copper circuit board
Author: Thomas Petruzzellis
Copyright:McGraw-Hill - Alarm,Sensor & Security Circuit  Cookbook