One of 10 educational circuit boards we designed for a private training institution. The circuit senses ambient light with a photoresistor (LDR); as it gets dark, the LDR's resistance rises and triggers a transistor, which in turn energizes a 5V relay. The relay output directly switches an external AC lamp, turning it on automatically at night. An adjustable trimmer lets the exact darkness threshold be fine-tuned.
Our customer's goal was to teach students light-sensing principles, transistor switching, and how a low-voltage circuit can control a higher-power load through a relay, using a concrete and easy-to-follow example. Because the circuit uses a relay, its output isn't limited to a lamp — it's general-purpose and can also control other AC or DC devices such as a fan or a heater. We handled the full schematic and PCB design ourselves, using DIP components and connector-based wiring to make assembly simple and safe for students.
After the needs analysis, we designed the circuit combining an LDR-based voltage divider, a transistor switching stage, and a relay output, adding a trimmer for sensitivity adjustment. We improved the circuit's robustness with a flyback protection diode across the relay coil and a reverse-polarity protection diode on the power input. We delivered the schematic and PCB design to the training institution.
