2.5 Electronic Controls with DC Relays.

The circuit for an electronic control is the most complicated but the best for the utility. The incoming AC line is rectified, filtered, voltage dropped to 5-22 volts and then sent to an electronic trigger circuit. This trigger circuit is an analog to digital converter (A to D) that takes the slowly rising (or falling) signal that the sunrise (or sunset) is sending the CdS or silicon light sensor. It converts this analog signal to a fast rise or fall , ON or OFF digital signal. This signal drives a transistor that drives the relay coil with either zero volts or full voltage. The result is instantaneous switching with no chatter or buzz. Hysterysis is added to the trigger circuit so that minor variation of light will not cause cycling. Typical turn-OFF Hysterysis (or ratio) is about 1.2 to 1.5 times the turn-ON value; this small ratio will not cause excessive morning burning time but is a large enough ratio to give positive switching even with fast moving clouds.

An advantage to this circuit is that with the addition of a diode, resistor and capacitor, the control can have a short 3-5 second turn-OFF delay and instant turn-ON. This feature makes the control "blind" to lightning or occasional mis-aimed car headlights. It also allows a quick glove test in the field. In my archives, I have a 1954 electronic photocontrol that uses vacuum tubes and a DC relay. It still works as designed: 1.5 footcandles (16 lux) turn-ON, close OFF/ON ratio, instant turn-ON, and 5 second delay for turn-OFF. What was ideal 43 years ago is still ideal today.