Trigger a +12v ~1 second pulse (time delayed)

Thread Starter

M J Schwartz

Joined Nov 13, 2018
Hello friends, I was looking for a device where after about 10 seconds a 1-second +12v pulse would be generated.

This is an automotive application that would turn on the defogger about 10 seconds after the remote start beings.

The reason I'm looking for a 1-second pulse is that I'm trying to simulate the physical button press of the momentary defogger switch.

I don't see any devices like this in production and I wouldn't mind making it myself if someone could point me in the right direction.

My experience is limited to installing premade +12v car accessories, but I think it would be fun to build a circuit for this use.

Thanks for your help.


Joined Mar 30, 2015
Welcome to AAC!

An LM556 would do. Don't want to take away too much of your fun, so will only give a couple hints.

You'll need an inverter for the 10s one shot because 555's are negative edge triggered. Both trigger inputs will need to be AC coupled.

Automotive power is very dirty. Protect your circuit well.


Joined Aug 12, 2014
I decided to try to do an alternate version of this without any 555s. Here's my attempt at a Schmitt inverter circuit. Only requires one IC, plus 8 passive components (9 if you include a decoupling cap for the IC power line, not shown in my sim.) The diodes may not be necessary. And maybe there needs to be a bleed resistor across the input cap to make sure floating power supply when vehicle is actually at ground potential for each restart. I'm not totally sure what real-world problems this circuit might have, but I thought it would be fun to try something besides the usual 555 answer. I'm curious how overall component count compares - haven't worked out details on a 555 version for comparison.