How to Design Schematic Circuit of Touch switch

 Small appliances, lamps and motors can be controlled by a simple touchwith

this experimental circuit. Loads are ON during the time adiusted by Pl for a

few seconds to more than 10 minutes. conversely, by using the NC (normally

closed) contacts you can also turn otf an appliance after the adjusted time.

The sensor is formed by two small metal plates, and shouldn't be placed far

from the circuit. The metal plates are finger-touched at the same momentThe circuit can be powered from a 6- or 12-volt power supply, depending on

the relay used. Do not use transformeless power supplies with this proiect'o

avoid shocks or dangerous shorts. Transformerless power supplies are not

isolated from the Ac-power line and represent a dangerous shock hazard.

K1 is a relay with 6- or 12-volt coil and is energized by current up to 50 mA'

With relays in this range you can power the touch switch from AA cells (6

volts) or small Power supplies.

A schematic of theTouch Switch is shown tn Figurc l.The circuit uses a 555

lC timer as its heart, and time delay is determined by adjusting P1'

circuit mounting, using a small homemade printed-circuit board, is shown in

Figure2. lf you:d like to etch your own board use this parts placement diagram as guide.

ln Figure3we show the sensor that can be made with two small metal plates

on a-plastic or wood base. Wires to the sensor shouldn't be long. A maximum

lenght of 10 feet is recommended to avoid erratic operation.

Pos16n of the polarized components, such as the electrolytic capacitors and

diodes, should be observed. Load is connected to terminals A and B. Remember that the relay acts as a switch.

Don't power this circuit from transformerless power supplies to avoid dangerous shocks and shorts.

A sensitivity control can be added to this circuit in a simple manner. use a

1,000,000 ohm potentiometer for this task.

Parts List -Touch Switch

lC1 - 555 integrated circuit, timer

Q1, 2 - 8C548 general-purpose NPN transistors

D1 - 1N914 silicon diode

Kl - relay - see text

Rl - 100,000 ohm, 114W,5% resistor

R2 - 47,OOO ohm, 114W,5% resistor

R3 - 10,000 ohm, 114W,5% resistor

R4 - 1,000 ohm, 1l4W,5olo resistor

P1 - 1,000,000 ohm, 114W,5% resistor

C1 - 10 to 470 uF, 12 WVDC electrolyticapacitor - see text

C2 - 100 uF, 16 WVDC electrolyticapacitor

S1 - SPST toggle or slide switch

81 - 6 or 12V - M cells, battery or power supply