Counterfeit coins detection

[noname06]

Hi, I am doing a project that needs to detect counterfeit coins. I've plan to use the speed of the coins passing through a magnetic field to differentiate the types of coins. What are the components that I need to detect the speed of a coin? Is there any other simpler way besides using speed or weight?
Thanks

 

[unclejed613]

some counterfeit coins used to be made from the punch-out slugs in electrical boxes (maybe some of them still are because coin boxes still have magnet traps in them), and can be stopped from going into the trip switch of a coin box with just a magnet that grabs the coin as it goes through.

as for other types of coins, you could use optical sensors to measure the diameter, thickness, and speed of coins as they pass, and a hall effect sensor to measure their magnetic properties. most coins have specific magnetic properties that can be measured to determine the range of magnetic properties that are normal. anything outside of the normal range would be an indication that the alloy is different.

 

[noname06]

The coins used are made of same material therefore i don't think i can test the magnetic properties. If i would like to test using speed and eddy current what are the components that i need to use?

 

[unclejed613]

often the materials coins are made of are chosen in order to make counterfeiting them counter-productive. in other words, to counterfeit a $1.00 US coin would require $2.00 worth of the same materials (obviously the government buys the materials at a huge discount). so counterfeit coins are made from cheap materials, and so have different physical and magnetic characteristics. measurable characteristics would be magnetic, size, mass, optical reflectivity and absorption (not reliable because of oxidation and wear). the mix of ferromagnetic and paramagnetic characteristics would probably be the best method), and measuring the density (by determining the volume and mass, and then calculating the density). you should see if you can find out how coin counters work. most large businesses use them. there are a very small number of tests that can be done in the few hundred milliseconds that it takes a coin to pass through a coin counter. if you're trying to come up with a better way to detect counterfeits, more power to you, but most of the tests already done have been around for a long time, and work quite well

 

[dr pepper]

Payphones have a little winding where the coin passes on its way to the collection box, if the phone isnt happy then a solenoid moves a gate and returns the coin.

I would have thought there is a constant ac voltage on the coil and it would be part of an oscillator maybe around 100kc, recognition of a coin can be done through a signature analasys, a sample of the coin passing over time could be taken and then fed into an fft algorithm which would result in a signature file, the same thing is done on your stereo with the spectrum analyser display, you would make a load of measurements on one type of coin then store this in the coin checkers memory, so long as a coin has a similar signature it can be classed as valid.

You can buy piggy banks that tally up the total cash in the jar, these from what I've seen use a similar technique.