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Its a bad photo but it conveys the idea.
The red piece of fiberglass sheet has the reed switch placed so the magnet swings by it. There are three adjustment screws with springs keeping it in place. They are black humbucker mounting screws and springs and the three screws are in a triangle to enable adjustment of the distance and the relationship of the reed switch to the magnet. This is important because if it is not adjusted correctly, you'll get what is called switch bounce from the magnet being too forcefull which results in false counts. The reed switch is attached to a simple voltage divider which creates a 5V and zero volt output from the switch. You can also see in this photo a 9V DC power supply input into the counter. The power supply is just a typical 9V DC wall wart. I think the counter draws something like 10ma.