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Testing different values for a fridge door alarm which sounds after 5-10 mins of the door being open. The edge of the fridge door has a magnet glued to it, and the fridge has the alarm attached to it, in a small box, with a reed switch which is held closed by the proximity of the magnet (on the closed door) and current is shunted, via the 470K resistor, to ground, bypassing the transistor base, but when the door is left open the magnet moves away, the switch opens and the current fed from the 470K resistor is diverted from to the 1K resistor which charges the 220μF cap very slowly, and when it reaches enough potential to turn on the Q1/Q2 darlington pair, the relay (or buzzer/sounder) turns on, alerting the owner to the fact that the door was left open.
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