Day Night Activated Triac Switch Circuit

The purpose of the discussed triac based day night activated switch circuit is to automatically switch OFF a given  electrical appliances and peripherals during day light and vice versa, this could be a lamp, an alarm, etc.

Generally speaking, this is a type of photoelectric relay which is driven directly through the home’s mains electrical system, with no need for any kind of mechanized device like relay or other component.

This day night switch circuit using triac and LDR is extremely straightforward, therefore its inexpensive and simple to build, only there exists one particular drawback, it is not flexible and is not going to enable electronic adjustments to drive or modify the switch. That, by the way, is not a mechanical switch, rather an SCR or triac.
Photoelectric relay cell
Interconnection of a photoelectric relay cell
Photoelectric relays

There are numerous remedies developed to automatically switch off the lamps as soon as it’s dark. Generally we require a DC power supply and an electromechanical relay, however in case you come up with a circuit which can be directly coupled to the AC power supply, the quantity of components necessary is usually minimized down and that is exactly what reveals in this project displayed below.

day night automatic triac switch circuit


The key component of the day night triggered triac switch circuit is a LDR photoresistor of sensitive resistance, resistance in Quebec may differ its resistance According to the strength of the light it obtains, it possesses a resistance of 200 ohms at nighttime and during daylight a few thousand Ohms.

The capacitor C1 is a voltage divider for delivering the photosensitive LDR and the triac gate.

In Normal light, voltage declines as LDR resistance diminishes, however, at night, resistance to photoresistor gets higher, causing improved triac activation current, operating and lights.

Parts List

C1 = 105/400V PPC capacitor

R2 = 1K, 1/2 watt

LDR = high sensitive type

Triac = BT136