Hi.
Can a sawed-off slot type optointerrupter, of the fax paper detection type (5V logic output) turn on/off a mosfet for making a dusk-to-dawn 12V switch ?
The sensing half to drive the mosfet gate. Partial turn-on undesired, that is why the logic output type.

[Trying to do it the simplest possible way only with these parts in my bins. Beefy mosfets salvaged from PC switching power supplies; optointerrupters from fax machines]

Considerations ?

 

You might need a power supply and a switching transistor to make the 5V signal from the opto compatible with the gate drive of the MOSFET. You could also use a bootstrap device to create the gate voltage from the +5V supply. The short answer to the question of can you use a logic level signal to directly control a non-logic level MOSFET, is NO.

 

You need to put the photo transistor across the gate,source of your fet, but you will need to put a preset resistor from the gate to the positive supply to create a voltage divider,

It may work better of you use an extra npn transistor as a Darlington pair with the photo cell, and put that across the fet gate,source.

http://i.stack.imgur.com/tCeKN.jpg

 

You need to put the photo transistor across the gate,source of your fet, but you will need to put a preset resistor from the gate to the positive supply to create a voltage divider,

It may work better of you use an extra npn transistor as a Darlington pair with the photo cell, and put that across the fet gate,source.

Darlington idea is good except for the case where the optointerruptor has a buffered output. Check the part number to see if it's a simple transistor detector or a buffered detector.

And bear in mind that depending on where the sensor is situated it may be too sensitive, in which case you may have to put a short tube in front to collimate incoming light (narrow down the angles) and maybe even put an aperture in front. But I could be wrong about the sensitivity.

 

Another possibility.
http://www.edn.com/design/led/4363842/Use-LEDs-as-photodiodes