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Dark Activated Transistor Switch

Discussion in 'Electronic Design' started by [email protected], Oct 24, 2007.

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  1. Guest

    Setup: Voltage divider using a potentiometer and a cds photoresistor
    controlling the base current to a MPS2222 NPN transistor. 12v
    supply. Common emitter. Load is a cold cathode light driver circuit.

    Problem: When the photoresistor is put into darkness, the cathode
    tube lights up only half-way. I can adjust the pot to get the cathode
    tube to light up all the way but when the photoresistor is brought
    back into brightness, the cathode tube only turns off half-way.

    I'm guessing the transistor is not fully saturated. Is there a way to
    increase the range of the photoresistor (without getting a new one) or
    some other way to turn this fully on when the lights go out?
  2. Ecnerwal

    Ecnerwal Guest

    Dumb question before you get too far into the driver electronics - is
    the light sensor adequately shielded from the light it turns on? What
    happens if you put your hand, or something else opaque, over the sensor?
  3. John Larkin

    John Larkin Guest

    Use a mosfet instead of the 2222. You'll have a lot more margin for

    A schmitt trigger might be nice, too, so the fet snaps on or off.

  4. Try using 2 transistors (darlington).
  5. Jamie

    Jamie Guest

    cascade another transistor behind that one.
    look up darlington transistors. What you need is more gain.
    I'm assuming you're trying to keep the part count down and
    advancement to a minimum ? Other wise, I'd use voltage
    comparator IC.. or an OP-AMP with a little schmitt trigger on
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