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Snap action night light...

Discussion in 'General Electronics Discussion' started by Externet, Jun 15, 2018.

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  1. Externet


    Aug 24, 2009
    What considerations should be taken to build a dusk-to-dawn switch that will turn fully on abruptly at dark instead of gradually ramping on ?
    I prefer to use a phototransistor and a mosfet. No LDR, no transistor. Will turn on a tiny 3V motor, no leds.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Use a comparator with a fairly large hysteresis.
    The comparator will give you the on/off signal instead of a ramp.
    The hysteresis will prevent the circuit at dusk and dawn from repeatedly turning on and off during the transistion from light to dark and vice versa.
  3. Alec_t


    Jul 7, 2015
    This might suit your complementary MOSFET IC if its FETs are logic-level types with low turn-on threshold voltages :

    R1 value depends on the photodiode sensitivity. R3 provides hysteresis feedback. Either could be replaced by a trimmer for set-up.
    Last edited: Jun 15, 2018
    Externet and Harald Kapp like this.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    NIce simple comparator wit power output.
    The current from the photodiode may be a bit low to provide the necessary drive on a 200 kΩ load resistance. R1 may need to be increased.
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