This isn't really A/V but I thought some of you gents here might be able to help.
I'm looking for a way to run a 12v device (low current draw - 0.2A) while the ignition is on and continue to run for a period of time after the ignition is turned off. I'm thinking 60~90 minutes after turning off the key but anything over 15m would be OK.
Any ideas, before I break out a 555 and breadboard something?
What kind of current draw Tom ? I have a few schematics I could send ya.
and if you want to save yourself the trouble there are several pre built DC timers. let me know.
Could you use a time delay relay such as. http://www.naii.com/products/viewPro...?productID=141
if the current draw is too high for something like this maybe use a relay to handle the current.
T.
Power-Off Time Delay Relay
The two circuits below illustrate opening a relay contact a short time after the ignition or ligh switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to +12 volts. The circuit on the left is a common collector or emitter follower and has the advantage of one less part since a resistor is not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input. The common emitter configuration on the right offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out voltages less of a concern but requires an extra resistor in series with transistor base. The common emitter (circuit on the right) is the better circuit since the series base resistor can be selected to obtain the desired delay time whereas the capacitor must be selected for the common collector (or an additional resistor used in parallel with the capacitor). The time delay for the common emitter will be approximately 3 time constants or 3*R*C. The capacitor/resistor values can be worked out from the relay coil current and transistor gain. For example a 120 ohm relay coil will draw 100 mA at 12 volts and assumming a transistor gain of 30, the base current will be 100/30 = 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1.4 = 10.6. The resistor value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K. The capacitor value for a 15 second delay will be 15/3R = 1327 uF. We can use a standard 1000 uF capacitor and increase the resistor proportionally to get 15 seconds.
http://www.***boat.com/image_center/data/500/timer_.JPG
But that's a hella capacitor bank for 60 min
C.T. :wink:
Or go buy a DC-DC carputer power supply...those have timers built in.
For 41 bucks it sure would save a lot of trouble soldering up a 555.
http://www.factorymation.com/s.nl/it.A/id.765/.f
http://factorymation.info/relays/821TD10H-UNI.pdf
T.
These are all good suggestions, particularly this last item... the weed delay timer. It looks perfect.
The issue I have with the weed device is the quiescent current is a little high. I'm concerned I'll always have a dead battery if I don't charge it every couple of weeks.
I really appreciate all the help and ideas.
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