Schematics of an 1980 German (Merten 10A 5449, Dämmerungsschalter mit Schaltverzögerung) daylight switch, still works and in use.
Specs:
Mains 220V/50Hz, switching capacity 10A at 220V.
The potentiometer simulates the photoresitor (range from 100 Ohm to 400k Ohm). The relay switches between 50% and 70% resistance only, per design, to prevent oscillating.
Wait for the power unit to stabilize; then play with the wiper position of the potentiometer to simulate dusk and dawn.
The original PCB design has a design flaw however, because the 24V zener is placed almost directly beside the filter capacitor 100µF, reaching temps in the +120 °C cooking the cap. Therefore I've reduced the blocking-capacitor from 660nF to 433nF, decreasing the zener temp 30-40
°C, easing the overall stress, too.
Photoresistor Basics
High resistance = night
Low resistance = daylight
Watch the PNP base voltage, on state change to ON 200mV are added.
PS: Sometimes the engine complains, it can not find a solution.
