This automatic solar light circuit using relay changeover is designed for charging a battery during day time or as long as the solar panel is generating electricity, and for illuminating a connected LED while the panel is not active.

Upgrading to a Relay Changeover

In one of my previous article which explained a simple solar garden light circuit, we employed a single transistor for the switching operation,

One disadvantage of the earlier circuit is, it does not provide a regulated charging for the battery, although it not might be strictly essential since the battery is never charged to its full potential, this aspect might require an improvement.

Another associated disadvantage of the earlier circuit is its low power spec which restricts it from using high power batteries and LEDs.

The following circuit effectively solves both the above two issues, with the help of a relay and a emitter follower transistor stage.

Circuit Diagram

Automatic Solar Light using a Relay Changeover

How it Works

During optimal sun shine, the relay gets sufficient power from the panel and remains switched ON with its N/O contacts activated.

This enables the battery to get the charging voltage through a transistor emitter follower voltage regulator.

The emitter follower design is configured using a TIP122, a resistor and a zener diode. The resistor provides the necessary biasing for the transistor to conduct, while the zener diode value clamps the emitter voltage is controlled at just below the zener voltage value.

The zener value is therefore appropriately chosen to match the charging voltage of the connected battery.

For a 6V battery the zener voltage could be selected as 7.5V, for 12V battery the zener voltage could be around 15V and so on.

The emitter follower also makes sure that the battery is never allowed to get overcharged above the allocated charging limit.

During evening, when a substantial drop in sunlight is detected, the relay is inhibited from the required minimum holding voltage, causing it to shift from its N/O to N/C contact.

The above relay changeover instantly reverts the battery from charging mode to the LED mode, illuminating the LED through the battery voltage.

Parts list for a 6V/4AH automatic solar light circuit using a relay changeover


  1. Solar Panel = 9V, 1amp

  2. Relay = 6V/200mA

  3. Rx = 10 ohm/2 watt

  4. zener diode = 7.5V, 1/2 watt

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