555 delay before on - UPDATED - diode

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. CHANGES: Replaced the transistor with a diode. Added diode to bridge the capacitor to ground (bypassing the resistor in the RC network pair) when pin 3 (out) goes high (the light goes on). This drains the resistor ensuring its not partially charged when the switch is thrown quickly. The gate inline between the capacitor and diode is there so one can see what the behavior is without that feature. This change is not really necessary for my application, but it bothered me that the capacitor lost charge so slowly as a result of the resistor between the capacitor and ground. The result was the light would come back on quickly if you opened and closed the switch before the capacitor had time to discharge. Modifed from circuit seen here: http://www.learningaboutelectronics.com/Articles/555-timer-delay-before-turn-on-circuit.php

published 7 years ago