power off delay- need help to use timer for specified use

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gnomeman

Joined: 02 Nov 2008
Posts: 22

28 Jun 2009 0:51 555 timer off delay

Hi gang. Can some one point me in the right direction.
This is what I have now. In my truck I have an air comp and air tank setup. I have a home made water cooling setup for the compressor. I have it wired so that the cooling setup water pump starts and stops with the compressor ( the compressor starts and stops automaticly using a pressure switch).
What I need is a curcuit that will allow the water pump to continue running one or two minutes after the air compressor shuts off. The water pump will need to start with the compressor but continue to run a little longer after the compressor shuts off.
I have made one shot timers before with the 555 ic, but can't figure out how to use this timer for what I need.
Any help would be greatly appreciated.
Bill

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IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

28 Jun 2009 4:54 Re: power off delay

When the compressor stops you have the falling edge of 12V signal that operates the compressor ..
You can use it (through a capacitive coupling) to trig the 555 timer (simple negative edge one-shot) to generate a 2-minute delay ..
Then, you can connect the output of the 555 and the voltage that operates the compressor through two diodes (OR-configuration) and combine it to one signal that will control the pump ..
Of course, this signal is most likely to be to "weak" to control a pump, so all you need to do is to use a relay, and you’re done ..

Rgds,
IanP