I worked for a company that made indoor payphones for worldwide
distribution. The coin mechanisms used 2 or 3 coils to detect the coins and
analyse their material in combination with a PIC.
The units also had a number of mechanisms to detect and reject coins that
were too small/big, coins on a string, too many/too few coins etc.
The angle of the ramps, the coil design and the electronics behind the coils
(oscillators etc) were quite critical to the function of the units and took
some considerable time to develop.
When a new country wanted a unit, they supplied a minimum of 1000 coins
(preferably 5,000) of each denomination to allow for metal variations along
with any coins previously in circulation and any from neighbouring countries
to prevent any fraud. During the development cycle, a 5-10,000 coin test per
denomination was carried out.

If this unit is going to be anywhere where people can try to defraud it,
don't underestimate the lengths they will go to. We had one situation where
'ladies of the night' used to insert hundreds of coins (obtained in bulk
from the bank) through the payphone hunting for a misread coin since it was
known that one coin occasionally misread to another 25x the value. This was
their pastime whilst waiting for their clients.

If your application involves money, the longer term benefits of buying a
mechanism may be more cost effective!

Simon


-----Original Message-----
From: Gabriel Caffese [mailto:gcaffese@HOTPOP.COM]
Sent: Friday, February 20, 2004 2:45 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Coin detector circuit


I4ll explain a bit:

I need a coin detector circuit / mechanism that should detect
two types of coins (or at least, if the circuit/mechanism gets
too much complex, detect just one).
It should reject invalid ones, so just like a Vending Machine.
Don4t want to buy one, for cost reasons, I should made it by myself.

Any clue ??

Thanks to whom answered before.


  Gabriel.-


-----Mensaje original-----
De: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]En nombre de Jinx
Enviado el: Jueves, 19 de Febrero de 2004 17:47
Para: PICLIST@MITVMA.MIT.EDU
Asunto: Re: [EE]: Coin detector circuit


Hi Gabriel, what exactly do you want the detector to do and
what sort of application is it ? For example do you need to

- detect any metal coin
- detect counterfeits
- differentiate and count coins
- have any sort of reject system
- comply with any commercial laws re performance

etc etc

I have made my own coin detectors (not strictly what's known
as a "coin mechanism"). A sloping chute with opto-interrupters
at different heights to detect and count the 6 NZ coins. Basically
it's a digital system. If you can do the engineering and wire up
diodes it's pretty easy

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