The circuit uses a micro switch that is attached to the fridge. When the fridge door is closed the reset pin of the 555 timer is low meaning that the timer wont start until the door is open. When the fridge door is open pin two becomes high triggering the timer. The timer will the start to count down to the set time. The time is set using by the potentiometer. Once the set time is reached the buzzer will turn on. It will then turn off when the fridge door is then closed.
To keep the circuit small you can build it on a PCB I have included the GERBER files if you want to make your own. I made the PCB single sided so that you could etch your own at home. I used screw terminals so the power supply can be easily connected.
The circuit can run of a 9 volt battery. Just simply insert the red wire from the snap connector into the positive screw terminal and then the black wire onto the negative terminal.
Solder all the parts together either on the PCB or on some perfboard. Solder wires to the micro switch if you switch does not come with them pre-attached. Then solder the wires from the micro switch to the PCB. There is a mistake in the schematic so swap the NO and the NC wires around.
Position the microswitch in a suitable place so that it will be pressed when the door is closed. you recommend using some hot glue to attach the switch so that it does not damage the fridge when it comes to removing is.
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Check out more of my other projects on my website.