This Window Open Sensor (WOS) has a great WAF because it makes use of a nice case, harvested from a very cheap window alarm. This alarm module holds two AAA batteries and has a switch, a buzzer, a magnet and a reed contact.

We remove the electronics, the buzzer and the switch. Then we replace the electronics with a Sensebender with NRF24L01+ module, which just fits inside.

You can use the delivered magnet, but the one in the BOM is much smaller. So I glue that on the window instead. The module can be fixed on the window with two of the loop/hook (velcro) pads listed in the BOM, which works far better that the self-adhesive tape that comes standard on the alarm module.

Preparation of the alarm module

Prepare for fitting a SensebenderMicro and radio:

• Cut the two plastic studs.

• Carefully cut the edge of the round plastic border away, until you can lift the buzzer up. Do not leave the buzzer in place; it would cover the antenna.

Preparation of the Sensebender

• First, solder a rectangular header to the Sensebender for programming it. Not all pictures show this header, but it makes uploading the sketch much easier. See the last picture.

• Solder the SensebenderMicro to the NRF24L01Plus, so that they are close together.

• Cut the pins short, they are too long.

• Salvage the reed contact. Careful! The glass breaks very easily if you bend the wires. This picture shows the original electronics inside the alarm module.

• The reed contact goes between D3 and ground (no resistor needed).

Putting it all together

• Solder the power wires to the SensebenderMicro

• Program the sketch: "WindowOpenSensor.ino".

• Mount the module with the hook/loop (velcro) on the window.

Remarks

• I always use rechargeable batteries. As a consequence, the batteries condition is always indicated as less than 50%.

• Not all alarm modules are the same. The ones I had first, only barely fit a sensebender micro, the ones I ordered later, have a more shiny plastic, and fit a Arduino mini pro - something to try out later.