Half logic-output optointerrupter for driving mosfet dusk-to-dawn ?

Discussion in 'General Electronics Chat' started by Externet, Sep 29, 2015.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
    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 ?
  2. Papabravo


    Feb 24, 2006
    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.
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    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.

    Last edited: Sep 29, 2015
  4. InspectorGadget

    Active Member

    Nov 5, 2010
    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.
  5. ronv

    AAC Fanatic!

    Nov 12, 2008