Light sensitive switch delay timer

Discussion in 'The Projects Forum' started by RodneyB, May 6, 2014.

  1. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012

    I am trying to increase the time delay for the light sensitive switch. I have attached my drawing.

    The problem I am experiencing is that if I increase the resistor the base voltage never gets high enough to switch the transistor.

    Any help will be greatly appreciated.
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  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    You could try doubling the 470uF to say 1000uF, use a low leakage 1000uF cap.
    Or Add another 470uF in parallel with the existing one.
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  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    use a Mosfet, like Irf830
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  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    If you connect R7 to pin 3 of the opamp instead of pin 2, the circuit will change from a very high gain amplifier to a comparator with hysteresis. You will get cleaner switching action between light and dark conditions.

    Also, the two resistors in Q1' base total 130K, for an drive current of only 70 uA. You are starving Q1. Start with RL1's coil resistance, calc the 12V current, divide by 10 to get Q1's base current for firm saturation. Next use the opamp's max output voltage, Q1's Vbe, and the base current to calc the total base resistance. Then divide the total into two resistors to get the delay performance you want.

    The answers to all of that will be lower resistor values, so you're going to have to increase the timing capacitor.

    Taking a different approach, you can change the effective time delay by changing the trip point of the detector circuit. Adjusting R3, R4, and/or R6 will affect the circuit's response time by requiring a greater change in the LDR value.

    And now the news: A 741 is not the right amp for this application. Its minimum output voltage is greater than Q1's Vbe, so Q1 might not turn off in the light state after you lower the base resistor values. Try something with a better low output voltage range such as an LM358, most of the TL07x amps, or even a comparator like an LM393.

    This might be more of an overhaul than you wanted to get into, but replacing the 741 with 1/2 of an LM393, and changing Q1 to a FET like a 2N7000, will let you have a longer timer range with one less resistor and without changing the timing cap.

    Last edited: May 9, 2014
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