just use an RC circuit honestly...

 

wait is this going to be just a pulse or a constant signal?

 

Constant signal.

It's going to control an electric fan for my compressors. The fan will turn on when the compressors turn on, but I want the fan to run an additional time after the compressors have shut off. Otherwise they would bake in the enclosed environment.

 

ok so you want the relay to stay ON for a time after the signal is removed? by using a resistor in series and then a cap in parallel, the relay wont click until the cap charges up and then it will keep the relay on for a bit more after the signal has stopped

 

Yes. What's a cap? and put the resistor in series with what?

 

cap = capacitor

http://www.cvs1.uklinux.net/cgi-bin/calculators/time_const.cgi

using a 100k resistor to get a delay of 20s, u need a cap of 200uF

http://en.wikipedia.org/wiki/RC_circuit

 

wouldnt a thermal sensor be more ideal?

 

sure.. but he asked about a delay.. he didnt ask about a sensor lol

 

true, HE needs to think about a sensor or rather, a switchi guess.

 

I figured this would be more reliable then a sensor. It can be placed inside the cab, out of the elements.

I think I'm just going to go with a premade one, I want to be able to adjust it rather then re-wire it.