Low-cost Touch Sensitive Switch
While experiment with a high gain transistor it may be noticed the transistor gets saturated by just touching its base. Here is a single, low-cost touch switch based on this idea. The 50Hz hum present in our body is the key of this circuit.
BEL BC557B pnp transistor has been chosen for this circuit. All the transistors used in this circuit are of pnp type. The circuit is basically a RS flip-flop formed by T3 and T4. Set and reset inputs are buffered by T1 and T2. Set and Reset inputs are buffered by T1 and T2. On switching the power supply on the bases of T3 and T4 become positive simultaneously. But due to slight difference in characteristics of T3 and T4 (since it is not possible to make perfectly matched transistor) one of the transistor become unsaturated. Transistor T5 is used as relay driver transistor.
Resistors (all ¼-watt, ± 5% Carbon)
R1, R3, R6, R7, R9 = 10 KΩ
R2, R4, R5, R8 = 220 KΩ
T1 – T5 = BC557B
D1 = 1N4001
RL1 = 12V/200Ω relay