I am a electrical contractor we have gps devices on our service trucks the gps device pulls very little current but if the truck is let set for a week the battery will be run down.
I have come up with a timmer relay that, after the timmer times out,thru a holding contact will drop power to both the timmer relay and the gps device.
my problem is that the relay i am using is expensive and too big in size.
I have seen pneumatic relays that deactivated based on time after the power to the device is taken away.
I need something that is small in size and can be mounted under the hood. It needs to hold the relay contact closed for 4-8 hrs after the power has been turned off. thanks for any help DTC
The 555 isn't suitable for very long delays like this as it needs a large capacitor, but there are a number of other circuits. I searched Google for "Long delay timer" and came up with a bunch.
You can use a dual 555 chip, the LM556, as a cascaded timer to greatly extend the standard period of an LM555. Details are in the datasheet.
Another choice is to detect the voltage of the battery and drop out the relay after it falls below a certain point, say 12.5V. Use a zener diode of the appropriate voltage to drive the base of a transistor, which controls the relay that powers the GPS unit.
12v dc timmer
Posted - Jan 06 2010 : 2:46:28 PM
