With a freezer in the basement and the risk of rotten meat due to a blown fuse while we are away, I designed this simple alarm circuit so that our neighbors will be alerted to fix the fuse. As can be seen in the photo the door bell is chiming since the USB-charger has no power.

Step 1: The Components

A low cost doorbell with remote battery operated push button (7 USD in Sweden)

Relay, 5 VDC 0,5W (= 100 mA)

Old USB Charger from the scrap heap

Cable with USB-A connector in one end (from the scrap heap)

Step 2: Relay Mounting

Begin by finding out which two of the three relay pins are closed when there is no power, and also the pins for the coil.

If you want to mount the relay on the push button case drill holes in the top and solder wires from the relay pins.

Yellow - the switch

Red and Grey - the coil (the polarity is not important)

Step 3: Connect the Relay Switch

Solder the cables to the micro switch pins on the PC-board, ( I drilled a hole in the circuit board for the wires).

Of course you can add another relay and connect it to some kind of temperature sensor, in order to monitor freezer compressor failure.

Step 4: The USB Cable

Drill a hole in the case and connect the black and red wires of the USB cable to the grey and red wires from the relay

Step 5: Fix the Wiring

Use a "glue gun" to melt plastic to fixate the wires. Note that I moved the LED from the PC-board to the case and put a 10kohm resistance in series with the LED to reduce the current to 100 uA since the alarm possibly will be active for hours, and the battery is a small CR3032.

Step 6: The Drawing

A simple drawing of the complete alarm. The switch is in open position as long as 5 V is applied to the coil. When a mains power failure occurs the switch closes and activates the push button radio transmitter.

This brand of door bell is said to handle 100 meters (300ft) between the push button unit and the main unit,