Simple Timer Circuit - help please

[Teflon]

Sorry this is so simple, but I can't solve it myself so need a circuit to achieve what I'm after.  I've been trhough loads of 555 diagrams but can't find exactly something to work.

I need a 12V timer, about 5 minutes, adjustable with a pot and low power so that it doesn't drain a lot of current when not triggered.

I want to trigger a relay and hold it on while ever 12V is applied.  When the 12V goes to 0V I want the timer to trigger and hold the relay on for 5 mins or so, then drop it down.  If 12V is re-applied during the timing period I want it to hold the relay on and reset the timer so that it doesn't start timing again until the 12V drops to 0V again.

I can find loads of circuits that can be one shot negatively triggered, but not if it stays at 0V.

Can anyone help me please?

 

[Hero999]

You need a permanent 12V power source for the relay.

Your description is a bit vague. A microcontroller should be able to do this.

 

[Teflon]

Okay, here's a bit more specific.  It is a timer for a dash cam in a car.

I have a permanent 12V supply for the timer.  I also have a switched 12V supply (the ignition) which will power a relay when live.  When the switched 12V supply is dropped (ignition turned off), I want a timer to kick in and hold on the relay, using the permanent 12V supply.  Hence, my dash cam will remain powered for about 5 minutes after the ignition is turned off.  The absolute time isn't critical, a minute or so either way is accurate enough.

The timer will always have the permanent 12V supply available, so after the end of its timer cycle I need it to draw very little current itherwise it will flatten the battery.

Of course, when the ignition is turned off, there is the possiblity of a slight current drain backwards, and maybe this could be the trigger for the timer?  But ideally, I want to put a diode into the ignition feed, so that nothing goes back up the supply from the timer.

I hope I've explained this clearly enough.