Integrated Fridge / Freezer door alarm

thingiverse

This alarm uses a window security alarm magnetic switch, along with a 555 timer project board and buzzer to enable an alarm to be sounded if either of the two doors are left open for a pre-determined time. A magnetic window switch is installed on one of the doors, and the corresponding magnet is installed on the other, such that if either door is opened, power is applied to a circuit board. The circuit board uses a 555 timer in astable mode. The time between on pulses is adjustable, as is the duration of the pulse. As a typical buzzer is relatively low current (<200mA), the buzzer can be driven directly by the 555 timer. Educational projects are available, and these provide a quick and cheap route.