Dark-detecting circuit trouble, scaling 9v to 16v

Discussion in 'The Projects Forum' started by poopscoop, Dec 22, 2012.

  1. poopscoop

    Thread Starter Member

    Dec 12, 2012

    I am building a dark detecting circuit for a lamp. I originally designed it to use a 9v battery, the diagram of which is attached. When I try to use it with 16v the transistors cease turning off completely, and I always get some degree of light through the LED's. I've tried connecting all of the bases to ground through resistors, to no avail. All transistors are 2n3904, and are rated to 40V.

    How do I make this circuit function at 16V?

    I would like to keep the circuit functioning at 16v or very near it, and tune the transistors and resistors to lower the voltage to the LEDs.

    The 57k Resistor is specifically matched to the level of darkness before the circuit activates the LEDs. In the interest of getting more current I may add a third transistor as an amplifier to drive the LEDs.
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  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    First use a resistor in series with the LED string to limit the current, otherwise you risk damage to the LED's.

    The photo transistor leakage is probably enough to turn the first transistor on- even in total darkness. Try putting a 100K potentiometer from the first transistor's base to ground, you can then vary the threshold.