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light activated LED circuit

Discussion in 'LEDs and Optoelectronics' started by supak111, Feb 7, 2013.

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  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    Apr 29, 2012
    Making a light activated LED circuit like this one:


    Circuit works and the LED does turn "ON with light" and "OFF without light", but the problem is that when there isn't any light the LED is still ON very tiny little bit.

    What do I do to make it turn OFF all the way? Replace the 10k resistor with say 20k? Or do I get an LDR that has higher DARK rating resistance?

    Or is this not possible and maybe transistors always have some leakage?

    PS. Need to make the circuit less sensitive. My circuit/LED only has to turn ON when there is a LOT of light falling on it, say if put a flash light right up against the LDR. That's the only time I would like the LED to activate.

    Thanks everyone
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    In order to decrease sensitivity, you can
    - reduce the resistance of the 10k+10k resistor/pot combination from base to GND (not increase)
    - add a resistor in series with the LDR

    A more stable solution would use a Schmitt-Trigger circuit to ensure the LED is either on or off, but not dimly lit.
  3. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    Apr 29, 2012
    Right, that makes more sense. I'll just go with a lower size resistor to keep it simple.

    And Schmitt-Trigger would def be better for what I'm trying to do but kinda looks complicated and uses more parts which I'm trying to avoid.

    Anything else out there that's simple/small/cheap?
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
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