Power Off Delay circuit

Question

I'm looking for a timer that starts when power is removed.

Here is the use case:

The head light on dirt bike my turns off with the engine.

This good when it's parked.

This is very bad when doing 45 mph and running out of gas at night.

So I would like to hook up a timer to the head light so that the light will come on with the engine and stay on for 30~60 seconds after I hit the kill switch. Or run out of gas, which ever comes first.

I know they exist out in the wild: http://www.grainger.com/Grainger/MACROMATIC-Time-Delay-Relay-5WMK1

Building my own would be a good opportunity to learn something. I could do it with an micro controller but that seams like over kill.

Any ideas on where to start?

Answer 1

A RC circuit with long time constant + buffer with Schmitt trigger + relay will probably do the job. When you flip the lamp switch a capacitor is charged. When you flip it back, the capacitor will start to discharge slowly. When its voltage reaches certain voltage lamp is turned off.