> 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

You have two criteria to meet

A valid coin must be the right size and it must be the right metal.
It might not be the right currency though. Unless you want to use
recognition s/w (and I don't think you do), detecting foreign coins
is just something that you'll have to forget

"invalid" includes anything metal that's a different size to either of
the two valid coins, but perhaps the same weight, and anything the
same size but perhaps a different weight, eg plastic

Diameter is fairly easy to measure with optos or trip-switches.
Slightly more tricky would be weight. Differences between similarly-
sized coins could be small. Thickness would be another parameter.
So would colour. For example, NZ $1 and $2 coins are "gold", all the
others are "silver"

Determining the metal would need something more electronically
sophisticated, as Simon suggested

I did a repair job on some coin mechanisms a long time ago when
I had part-time work at a fun fair. There were two common types.
One was quite light, electronic, black, and based on an MC68HC11.
Don't recall the name, but do recall they were NZ$700. The slot
would accept any coin but the micro would reject (by moving a gate
with a small solenoid) any it wasn't programmed for. The other was
mostly mechanical. I have one here. It has a sticker on it

COIN CONTROLS
Royton
Oldham
England

That one has a narrow slot to prevent insertion of $1 and $2 coins. 5c
and 10c coins fall through because they're thin, a 50c (the largest coin)
pushes a microswitch to open a reject gate, and so the only one to get
through is the 20c. There's a manual reject button to clear blockages

Hope that helps you in some way to decide what you need to do. It's
going to depend a lot on the characteristics of the coin(s) you need to
detect. For example, extreme sizes, like the NZ 5c and 50c, are the
easiest. Those of average size and weight may have a lot of potential
pretenders

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