Building a Timed relay

[EZSnow]

I need to build a timed relay that energizes with a momentary switch and stays on for 15-20 minutes, then relaxes until the momentary is closed again. Any ideas or schematics?

Even better, I'd like a setup in which the momentary will toggle between energizing/relaxing the relay, but if left on, it will time out in 15-20 minutes.

[wyldman]

Why build when you can buy.Get a rear window defrost relay\switch assy from any vehicle.I'm sure you can find one from a Ford which matches your truck pretty well.

[EZSnow]

I know, I know... but why buy when you can build?:grinz

I just want to use a small momentary button on my armrest by the mirror control, with a lil' LED to indicate function. Can it be as simple as a loop-circuit relay with a capacitor and a resistor?

[norrod]

15 minutes is a long time for simple r-c loop.


For solid-state solutions I have used a lot of 555 timer circuits in the past..

But 15 minutes is a bit long for that without driving and 8-bit counter or something like that. And you would need a driver( transitor ) to pull in the relay.

here is a page with details on a 555....

555-timers (http://www.uoguelph.ca/~antoon/gadgets/555/555.html)

OR here is a kitted IC timer

1-255 Sec timer & Dirver (http://www.hobbyengineering.com/H1990.html)

[wyldman]

It can be done,but why make it complicated ?

If you want it clean and simple,then use a small hidden switch to trigger a rear window defogger switch\relay.You can hide the relay anywhere.