coin selector

[Nicholas]

Hi, all
I am thinking of a circuit for a coin selector.
The coin first go through a coil when it is inserted into the coin selector. I think that when the coin is in the coil, it changes the L value of this coil and so it really changes the oscillator frequency of the whole LC oscillator circuit. And then, the 8bit MCU on board can find this change of oscillator frequency to detect the coin. Am I right?
But I found that when there is no coin in the coil, the LC osc at 60.5KHz. It changes to 58.5KHz when I place a 1 yuan coin in the coil. and 61.2KHz for a 1 jiao coin. It's only a small difference, right? The 8bit MCU works at 18MHz. How can the MCU detect this small differenct to validate the real coin and to reject a counterfeit coin?
Maybe I'm wrong. Does anybody know how does the coin detector work? Really need your help, thanks!

 

[Russlk]

Feed the oscillator frequency and 18mHz clock to an AND gate, count the pulses coming out and do the math to determing the frequency.

 

[olly_k]

I think you need more coils to properly validate a real coin - and keep counterfiets out - if I get a minute I shall dig up a couple old validators and take a snap of the coil arangements - at least you are on the right track, most people seem to think the coins are weighed!

 

[Nicholas]

Feed the oscillator frequency and 18mHz clock to an AND gate, count the pulses coming out and do the math to determing the frequency.

Thank you!
But I think that a 8-bit MCU such as Microchip PIC16C57 working at 18MHz cannot do this counting job well. Because the pulses coming out is too fast for the MCU.

 

[Nicholas]

I think you need more coils to properly validate a real coin - and keep counterfiets out - if I get a minute I shall dig up a couple old validators and take a snap of the coil arangements - at least you are on the right track, most people seem to think the coins are weighed!

Thank you!
This is a real coin selector product. So maybe the coils are enough.

 

[plot]

Yes, most people think it has to do with weight, some say it has to do with the circumference of the coin. I've also heard the conductance of the coin is measured.

Anyways, you are definatly on the right track with the frequency, I'm thinking you arn't going to see many if any counterfit coins unless you are building this for a country that has strayed away from paper money? Anyways, if we are talking small values for each coin, I dont think anyone is going to take the time and effort to figure out at which frequency a coin oscillates, and try to replicate it.

Givin that, I'm thinking that any extra circuitry used to detect a counterfit coin verse a valid coin, would cost more them if someone actually did put in a few counterfit coins...

So... all you really need, is a surefire way to detect the value of the coin that is inserted.

 

[Nigel Goodwin]

But I think that a 8-bit MCU such as Microchip PIC16C57 working at 18MHz cannot do this counting job well. Because the pulses coming out is too fast for the MCU.

A PIC running at 4MHz is commonly used for a frequency counter with a 50MHz range, you don't need an 18MHz chip. There's an old MicroChip application note about how to do it. Basically you use the hardware timers, which work upto 50MHz.

 

[Nicholas]

A Technical Documentation I found on the Internet says
"
Measuring principle of the coin validator
Coins inserted into the coin validator pass inductive sensors which check the coins and there they generate individual measurement values. Due to a special construction and arrangement of the sensors, each coin is checked for its material properties and dimension. If a number of coins of the same type are inserted (e.g. 2 Euro), for one measurement, different measurement values are generated which are close together. For coin acceptance, the coin type is assigned an upper and lower limiting value, which forms a so-called acceptance band. The acceptance band determined for a type of coin represents the coin channel.
"

 

[Nicholas]

But I think that a 8-bit MCU such as Microchip PIC16C57 working at 18MHz cannot do this counting job well. Because the pulses coming out is too fast for the MCU.

A PIC running at 4MHz is commonly used for a frequency counter with a 50MHz range, you don't need an 18MHz chip. There's an old MicroChip application note about how to do it. Basically you use the hardware timers, which work upto 50MHz.

I found that application note.
But in this circuit, the LC OSC output is feed into a general purpose I/O of the MCU. So it just can't do that.