Using Photodiode 50mV output to turn on circuit

Discussion in 'The Projects Forum' started by davedmw7, Aug 22, 2008.

  1. davedmw7

    Thread Starter New Member

    Aug 21, 2008
    Is there a circuit or device that can be used to switch on a low power circuit without itself using power? I am trying to take the output (min. 30-50mV) from a photodiode and use it to turn on a circuit that is connected to a +5V power source (4-AAA batteries). However, the switch itself can not draw any power.

    I have tried using an NPN BJT. But it took 735mV to turn on the circuit and I can only use about 30-50mV. Thank you for any help.

  2. mik3

    Senior Member

    Feb 4, 2008
    No you have to take a small amount of power from the 5 volt source to make it work otherwise you have to use another voltage source to power the control circuit.
  3. kubeek


    Sep 20, 2005
    You could use some FET and have the diode as a part of a resistor divider, but that would still consume some current all the time.

    Maybe you could use a phototransistor?
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    How about a small solar cell? Shining light on it will produce a voltage and current.
  5. davedmw7

    Thread Starter New Member

    Aug 21, 2008
    kubeek, thanks for the phototransistor idea, I will look into that. I'm not sure if a JFET would work. From what I understand it controls current and I need a way to control voltage, however I could be wrong. I guess another possibility is trying a MOSFET like the IRL510, but I have yet to figure that out.

    I actually did try a solar cell, one from a calculator, and it did work. But what I forgot to mention is that the reaction time needs to be in the microseconds range and unfortunately the solar cell was too slow. I also looked into photoresistors but they were too slow as well.

    Is there a way to possibly store voltage in a capacitor and then "knock it out" using a photodiode?
    Last edited: Aug 22, 2008
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    I think it would help if you say exactly what the application is.