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Snap action night light...

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

  1. Externet

    Externet

    710
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    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.
    [​IMG]
    [​IMG]
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    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

    Alec_t

    2,699
    715
    Jul 7, 2015
    This might suit your complementary MOSFET IC if its FETs are logic-level types with low turn-on threshold voltages :
    Dusk_Sensor.PNG

    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

    9,395
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    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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