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 ...

There are some old AC units ... servo to the AC control panel. Attached to the servo horn is a slotted cylinder sized appropriately to fit the shape of the control knob. An Arduino measures ...

But with a little sweat-equity you can fix it with this Arduino thermostat and temperature ... allowing him to use a small relay to control the AC. The control algorithm is simple enough ...

“Arduino Self-Timer” project presented here is a low-component count 60 seconds countdown timer. It can be powered from a 9V compact battery or suitable Arduino AC mains adaptor ... An optional RELAY ...