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Circuits designed by David Johnson, P.E.
Last Updated on: Wednesday, October 18, 2017 05:04 AM

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More:  Switch Circuits   Light Dependent Circuits 

24vac Powered Day/Night Switch

Many low voltage outdoor lighting systems use 24vac to power the devices.  The low voltage is much safer to use outside than standard 120vac or 240vac power systems.  The circuit below is designed to turn off power to outdoor lights during daylight and turn them back on at night.  The circuit uses a phototransistor to detect daylight.  I would suggest you use a phototransistor which has a built in daylight filter. These are colored a deep purple. The filter helps insure the circuit reacts only to sunlight.

The circuit uses a unique bidirectional switch network made from two n-channel MOSFETs, wired in opposite directions.  The DC control voltage needed to turn on the two FETs is developed by tapping into some power from the 24vac source through two capacitors.  A small Schmitt trigger inverter provides some hysteresis, so once the lights turn on, the extra illumination does not turn the switch back off.  With the components shown, the circuit can easily handle 10 Amps of current.

Click on Drawing Below to view PDF version of Schematic

24 VAC Day-Night Switch designed by Dave Johnson, P.E.


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