Light/Dark Activated Switch

These are switches used to turn on lights at dusk and turn them off again at sunrise. Generally, a light sensitive device like a photocell, optical sensor of some sort is ties to an opamp comparator which energizes a relay to switch on the lights. A less sophisticated approach might be a simple mechanical timer. The timer does not need to sense light levels.

An even simpler solution is to use a low voltage mechanical relay energized by a solar panel. This approach assumes that a solar panel system is already in place. Even a 5 watt panel can develop enough power to energize a 5 volt, 150ma relay without significantly effecting its ability to provide useful electrical power.

For the solar panel solution to work reliably the relay needs to be sensitive enough to energise even on a cloudy day or the lights will stay on. Also, the relay needs to provide both normally open and normally closed contacts. The idea is to allow the solar panel energy to turn ON the relay which turns OFF the lights.

To prevent the 18 volts from the panel from over powering the relay a series resistor and 5 volt zener across the relay will keep the relay coil at a maximum of 5 volts. A capacitor across the relay coil would introduce some hysteresis to prevent the relay from cycling on and off at certain light levels.