Circuit - 12v Light/Dark Switch - Circuits designed by David A. Johnson, P.E.


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Circuits designed by David Johnson, P.E.

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Last Updated on: Sunday, August 31, 2008 02:27 AM

12v Light/Dark Switch
Often, for certain low voltage lighting systems; you would like to turn off the lights during the bright light of the day. Most commercial day/night switches are designed for AC lighting. The circuit below was designed for a 12v DC system. But, it could be modified for other voltage as well. It uses an inexpensive phototransistor as the light detector. An n-channel FET is used to switch power to the lights. A transistor circuit is included to provide some hysteresis. This keeps the circuit from fluttering the light during the transition from day to night and night to day. It is recommended that a plastic tube be placed over the transistor to prevent it from being illuminated by the lights it is controlling. By selecting the appropriate power FET, the circuit could control over 100 watts worth of 12v lighting. (July 22, 2008)

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