External hardware is not strictly
within the scope of Beltower software, but a few hints are offered here.
Alan Griffin has come up with a simpler design which works on his RISC OS computer.
This is his contribution.
I am using a Hall effect chip, with a very simple circuit, with only five components.
The circuit and further details are in a PDF file HallCct.pdf
It is designed on veroboard, and I fit it into a 35 mm photo cassette.
The chip bends down over the end of the board, and touches the lid of the cassette.
When the magnet is near, the red LED comes on, which you can see
through the translucent cassette, so it is very easy to set up.