Hi,
I would like to make (or find an off the shelf device) that can power a small LED for a specified time ie adjustable from 1 to
30minutes, after the main 120VAC power is disconnected. The LED brightness should degrade over that time, starting at full brightness, and then to no light after the specified time. The circuit should also be able to operate down to -40C.The LED initial power draw can be under 2watts, and should degrade somewhat linearly over the specified time down to zero watts, and the circuit only needs to run once per day.
I was thinking maybe a supercapacitor or a small rechargeable battery powered from a small 120VAC constant current supply, and then adjust the power draw of the LED by adding resistance to get the required time.
Assuming linear power decay and 2watts initial, that is an average of
1watt for 30minutes max time, which is 0.5watt hours energy storage, or 1800 Joules required. I think a matching supercapacitor would be better than a battery to do this as the charge cycle energy storage decay might give more consistent results than with a battery.This supercapacitor has 400F capacitance and 2.7V = 1458Joules would work maybe:
Expensive though $17CAN each.
With a battery it might make more sense to use a timer for the LED and not rely on fully discharging the battery, which is a more complicated circuit but probably cheaper.
As simple and cheap a circuit (off the shelf preferred) as possible would be nice, any ideas? :)
cheers, Jamie