Much of the backstory to this part of the project can be read in the project details section: https://hackaday.io/project/191066-dementia-support-unit-bedroom-door-monitor#menu-details

Initially the plan was to have a magnetic reed sensor and arduino unit on each door frame.

Much time and resources were "wasted" (though much was learnt) developing, testing, purchasing materials only to realise it was not necessary... as it turned out there was already a superior solution installed ! I just never noticed it there, as it was a very discreet PIR roof mounted motion sensor.

The plan for the nodes was to use an atmega 328 chip running low voltage brown out, on 2 AA batteries with deep sleep between the times the reed sensor triggered an interrupt pin. Along with an NRF24L01 module. It was estimated the batteries would last for 6-12 months depending on doorway activity, but something i didnt realise at the time was that the radio modules didnt like running at less than 3.3v (unlike the arduino which could go down to 1.8v) so 2xAA batteries would not have been enough....

But before finding this out I designed and had printed my very first pcb:

As well as purchase all the components needed to make up about 30 of these:

I even bought all these window/door sirens off ebay, just for the magnets, as it turns out reed sensor need a particular magnet polarisation, not the normal type you get from a fridge magnet. But it turned out these magnets were inferior strength for what i needed and couldnt reliably bridge the gap i needed between door and doorway...