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.

PARTS LIST

Resistors (all ¼-watt, ± 5% Carbon)

R₁, R₃, R₆, R₇, R₉ = 10 KΩ

R₂, R₄, R₅, R₈ = 220 KΩ

Semiconductors

T₁ – T₅ = BC557B

D₁ = 1N4001

Miscellaneous

RL₁ = 12V/200Ω relay

Touch plate

..Read more http://electronicsproject.org/low-cost-touch-sensitive-switch/