555 delay before on - works

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10:46:23

Using for fridge door open alarm (replace light with buzzer).  Replace switch with a reed switch that is discontinuous in presence of magnetic field. 3V source will work for buzzer, but won't light an led with the 220Ohm resistor. Led is just there in this diagram to give the circuit something to do.

RC network brings pin 2 (trigger) and 6 (threshold) low once the capacitor has fully charged.  Low on 2 pulls the "trigger" and sets 3 (out) high.

As seen here:
http://www.learningaboutelectronics.com/Articles/555-timer-delay-before-turn-on-circuit.php

published 8 years ago

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