retriggerable one shot

Thread Starter


Joined Nov 20, 2012
Hi All
Looking for a simple circuit for 12 Volt on a car to do this - preferable 555 type with relay driven output.

Requirements :
  1. When +12V is applied (switch on) the output should be a pulse (neg. or Earth) - easy with 55 monostable....but....
  2. When +12V is removed (switch off) it should again give a pulse output (or neg.)
  3. Both pulses in 1. and 2. above, MUST be separated - not from same output, as 2 different circuits are to be driven from each of the two pulses.
The idea is : When my car's Ignition is switched on (+12V) this cct. must then give a negative output pulse to drive a relay and switch on load A momentarily and when the ignition is switched off after the journey, it must again give a 2nd negative pulse to drive load B momentarily.


Joined Aug 1, 2013
1. Is it possible to have the timing circuit powered continuously, and triggered by the ignition being switched on, or must the timing circuit b powered by the switched ignition signal?
2. What are the widths of the two pulses?
3. What are the loads being switched?
4. To drive the loads, are you switching 12 V to them (grounded loads) or switching them to ground (12 V loads)?



Joined Oct 2, 2009
Not every nail requires a hammer with 555 stamped on it...

You never specified what the load was. I'm switching 0.4A


Current draw from battery with Ign switch off is zero.


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