In this quick Arduino tutorial I will explain how you can control a relay using the Arduino Board, one 1K and one 10K resistors, 1 BC547 transistor, one 6V or 12V relay, one 1N4007 diode and a 12V fan ...

If the battery voltage is too low (e.g. 5 or 6V), the relay may drop out immediately due to undervoltage. To get around this problem, connect the solar panel directly to the battery until the voltage ...