I wanted a simple reliable circuit which would turn on a light when it was dark and turn it off when light. I also didn’t want the switch to detect the light from the lamp it controlled.
A Schmitt trigger switch action was needed, to produce a clean on/off switch operation. Also, a short delay might be nice to insure the light does not turn off from car headlights at night. The circuit below performs these operations.
Instead of a CdS photoresistor, I used a photo-transistor as the light detector. I could then use a single fixed or an adjustable resistor to define the light sensitivity I needed for a specific light installation. Instead of a triac or some other solid state switch, I chose to use a DC latching relay. Such relays are very reliable and require only a short 25ms pulse to change their on or off state. The relay I chose had a single coil and could handle up to 10 amps of AC current.
The circuit is powered by tapping into the AC power line. A “voltage dropper” type circuit is used to draw power from the AC line using a capacitor C1. The capacitor is connected to a bridge rectifier then to a capacitor C2. Energy stored in that capacitor is then used to power the rest of the circuit. I used a 555 timer as a Schmitt trigger. The timer was cheap and had a good output drive current rating, needed to power the latching relay. During daylight hours, the phototransistor Q1 conducts current producing a voltage across R2. When the voltage across R2 reaches about 10 volts, the pin 3 output of the 555 timer changes from a high logic state to a low state. This low state signal applies a negative 15v to the latching relay coil through a 10uF capacitor C4. Resistor R3 and capacitor C3 form a time delay, filtering out sudden light changes. During night, the voltage across C3 slowly drops. When the voltage reaches about 5v, the 555 timer output changes from a low state to a high state, applying +15v to the relay coil through C4. This change latches the relay coil, turning on power to the lamp. The resistor R2 can be selected for any desired light sensitivity.