Blowing a hooey here, tidying up exercise ready for tomorrow, new boiler being fitted.
One of those days to mull over a 'round tuit', Hall Sensors and a method to trigger one, being a devout tightwad looked for a cheap DIY method.
Yellow insert, from one of these kits,
Metal Construction Vehicle Kit | Construction Toys at The Works, pick one up whenever I come across one on my travels.
Insert drilled out for an interference fit over a wagon/coach axle, couple of small magnets super glued on to it.
Hall Sensor module, cheap and cheerful for experimentation purposes, fairly tolerant in the Horizontal & Vertical plane for the magnets to activate it, anything between half and quarter inch away from the magnet, as for side play from the axle anything between on eighth or a quarter of an inch, precise not a requirement!
Signal (VOUT) and POS hooked up to a Green LED, as magnet gets close/ and near far away, Hall Sensor activates/deactivates LED turns on or off.
Once the magnet passes the activation zone, Hall Sensor off and LED off.
Basis for further tinkering, and as one thing leads to another, another 'round tuit' may even get started!
Hall Sensor activated, LED ON
Hall Sensor deactivated, LED OFF.