When triggered into its on state, it will remain in that state for a preset period before switching off again. Some circuits of this type use two transistors (npn or pnp types) configured so that in the stable state one transistor is on while the other remains off. Following a trigger pulse, both transistors change state. A disadvantage of this circuit is that during the off state, one of the transistors is always turned on, and so consuming power. An alternative configuration is used here in which all transistors remain off when the circuit is in its stable state, so consuming virtually no current.
|click on circuit to Enlarge|
small current to flow from the positive ine, though the finger and into the base of TR1. It is advisable to insert resistor R4 between TP2 and the positive line to prevent damage to TR1 should the two pads be shorted accidentally by an object with a low resistance. If the unit is found to be too sensitive, a high value resistor of about 10M can be connected from the base of TR1 to the battery negative. This will prevent the circuit from switching on inadvertently, especially in areas where the mains field is high.