I need to create a timer switch that can run off 12v or 5v dc battery powered circuit. The circuit will likely carry 2.5 amps max. I would like it to be open at night and clsoed during the day. The device using the power in the circuit is dumb and can't turn itself off at night. So I need to open the circuit so it doesn't run my battery down at night.

 

You need a light activated switch. Don't have anything handy for 12 volts 2.5 amps, but shouldn't be to hard to do.

 

Perhaps you could turn a MOSFET on with a photovoltaic cell.

 

I need to create a timer switch that can run off 12v or 5v dc battery powered circuit. The circuit will likely carry 2.5 amps max. I would like it to be open at night and clsoed during the day. The device using the power in the circuit is dumb and can't turn itself off at night. So I need to open the circuit so it doesn't run my battery down at night.

Use a comparator IC (op-amp), an LDR and a few resistors to make a dark detector. Then with this drive a relay to control your circuit.