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I'm always taking things off of this forum, maybe I can put something back?
Here is a working circuit design that I have that may be of some value to the home experimenter but of little value to the industrial engineer. This circuit approximates the performance of my old Whites Coinmaster 2/db
with only a handful of components from Radioshack.
The coils are mounted on a hard plastic dinner plate concentrically. The Tx coil is 75t of 30g wire 7" dia. The Rx coil is 95t of 30g wire 4" dia. and the bucking coil is about -22t of 30g wire around the rx coil. The coil can be shielded with a piece of electrostatic plastic (the kind you would use as a disposable ESD mat.
Tx frequency is about 13Khz.
The coils can be unbalanced using a potentiometer across the tx coil with a tap between the tx+ and tx- coils. This allows a ground balance or disc control, but not both.
The Rx section is a 2 stage amplifier using a common LM324. This is followed by a DC filter and a high gain DC amplifier that only allows fast changing DC signal through. This functions sort of like a SAT circuit.
A comparater circuit is used as a threshold detector and it drives a transistor to gate an audio signal from a 555 timer. It looks kind of crude but it functions reasonably well and will detect a U.S. nickel at about 8".
Any comments for improvements would be appreciated.
Thanks..
Don
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