Sam D'souza wrote:
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Well, since you have been such a sport.....

It is a kind of metal detector, with the envelope of the sweep of an oscillator coupled with the coin used as one of the detection mechanisms. There are often several addtional systems and complex logic to ensure no false positives ! Optics and mechanical scales are also used.

In the UK,with the introduction of the pound coin, it was discovered that a lead copy would work the existing mechanisms. One manufacturer put in a microphone which listened to the sound the coin made as it hit the first set of stops in the equipment. The right coin went "ping", the lead coin went "thunk".

Steve