Rongge

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.  This hobby circuit below was designed for a 12v DC system.  But, it could be modified for other voltage as well.  It uses an inexpensive photo-transistor 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) Source: DiscoverCircuits

Compact circuit, Can be wired in parallel to existing switches This device allows one or more lamps to illuminate at sunset and turn off at dawn.Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operation.   Circuit Diagram: Dark Activated 220 volt AC Lamp Circuit Diagram Parts: R1 = LDR R2 = 100K R3 = 200K R4 = 470R R5 = 12KR6 = 1KR7 = 470R C1 = 10nF-63V D1 = TIC106D D2 = 1N4007D3 = 1N4007D4 = 1N4007 D5 = 1N4007 Q1 = BD327 Q2 = BD337 SK1 = Female Mains Socket   Notes: R3 allows fine setting of operating threshold and R2 value can be raised to 150K maximum. Several lamps wired in parallel can be connected to the circuit, provided total power dissipation of the load does not exceed about 300...