Analig circuit, explanation


Diamond Member
Feb 21, 2004
Capacitors charge exponentially; on chargeup, their voltage is Vin(1-e^(-t/(RC))). 555s trigger at about 1/3Vin or 2/3 Vin (depending on the setup) assuming the same supply as is charging the capacitor, so the equation relating your time to your resistance and capacitance would be:

2/3Vin = Vin(1-e^(-t/(RC))) -> Vin cancels, good - the time is independent (roughly) of the supply).

2/3-1 = -e^(-t/(RC));
1/3 = e^(-t/(RC));
ln(1/3) = -t/(RC);
t = -ln(1/3)(RC)

Done! That's your turn-on trigger time given R and C.

For 1M and 220uF, t = -ln(1/3)((10^6)(220*10^-6)) = -ln(1/3)220 ~= 241.7 seconds, or 3 minutes.

Plug in your numbers and have at it.