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Snap action night light...

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

Jun 15, 2018 #1

Externet

736

158

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.
Jun 15, 2018 #2

Harald Kapp Moderator Moderator

10,025

2,138

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.
Jun 15, 2018 #3

Alec_t

2,822

754

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

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Jun 15, 2018 #4

Harald Kapp Moderator Moderator

10,025

2,138

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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