Touch operated Band switch for Radio Set
Touch switch are now in vogue for stereo decks and other audio and video sets. This small circuit is meant for electronics enthusiasts to help them covert the manual bandswitch of their radio set to an electronics touch switch with band indicators using LEDs.
The circuit incorporates a 555 IC which operates in the biastable mode. Due to high input impedance presented by the threshold terminal (pin 6 of the IC) and the trigger terminal (pin 2), the timer can be set and reset with the touch of a finger.
Touching touch plate TP2 causes the output (pin 3) to go high. Hence relay RL2 gets energized while relay RL1 remains inactive. On touching TP1, relay RL1 conduct while RL2 is switch off.
If the relay are not switch on and off properly, the supply voltage may be increased to 7.5 volts. If it still doesn’t work, reduce the value of R3 and R4.
Resistors (all ¼-watt, ± 5% Carbon)
R1, R2 = 3.3 MΩ
R3, R4 = 470 Ω
R5, R6 = 150 Ω
C1 = 0.1 µF
IC1 = NE555
D1, D2, D3 = 1N4001
LED1, LED2 = different color LED
TP1, TP2 = Touch plate
RL1, RL2 = 6V, 100 Ω Relay