Guest Teflon Posted September 23, 2013 Report Share Posted September 23, 2013 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? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted September 24, 2013 Report Share Posted September 24, 2013 You need a permanent 12V power source for the relay.Your description is a bit vague. A microcontroller should be able to do this. Quote Link to comment Share on other sites More sharing options...
Guest Teflon Posted September 24, 2013 Report Share Posted September 24, 2013 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. Quote Link to comment Share on other sites More sharing options...
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