Light sensor to power LED?

Discussion in 'General Electronics Chat' started by Mshadow, Jun 3, 2008.

  1. Mshadow

    Thread Starter New Member

    Jun 3, 2008
    I am trying to use a light sensor to control a simple circuit: battery, current limiting resistor, LED. How could I use this light sensor without passing the current through it (and thereby cooking it)? Essentially the sensor would act as a good old CdS photo resistor or photo diode.

    Battery is 3.7v and supplies 20ma to the LED.
  2. mik3

    Senior Member

    Feb 4, 2008
    Use a transistor to detect the photocurrent and control your circuit.
  3. Wendy


    Mar 24, 2008
    A CDS with a current limiting resistor might do it as is, have you tried it? How much current are you planning on running through your LED? And when is it lit, when it's dark or bright?
  4. Mshadow

    Thread Starter New Member

    Jun 3, 2008
    I am not sure they make CdS's small enough for the footprint I am shooting for, but I have an old pack from the shack and their size might be workable--needless to say the have no parameters on the packaging. The resistor I have matched with the LED allows 18ma to the light.

    As for the light vs. dark...when the ambient light is very low or non-existent I would like the LED to be on.
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    From the data-sheet: "Photocurrent is proportional to illumination." So you'll want the LED on when current through the plight sensor is at minimum current, and off when the light sensor is above a certain threshold.

    I would do something like this:
    R1 would be sized to develop gate threshold value at your chosen light level threshold. Experimentation ahead of time with the SD211 (or whichever MOSFET you choose) will pin down the needed gate voltage, and therefore the needed resistor value.