The box contains a HEPA filter, a 12V fan, and 3 cell 18650 Li-ion batteries along with a BMS circuit. We've tested the setup work for over 10 hours which is more than enough for a working day.
Here's a mock-up of the electronics. At first we used cheap charge indicator modules but that wouldn't be as readily available as the rest, so we set out for a custom PCB.
We've designed a circuit with a speed controller and charge indicator with Arduino Nano as a controller, but found the soldering part too unreliable and time-consuming. We are now looking for an alternative solution.
This is the design so far. The filter fits inside and is held tightly with a plastic frame. Holes are sealed with hot glue.
A small number of units was produced and given out to three local hospitals to test.