Help with sensing falling coins!

Hi! For a school project I'm making a money box where there's a milestone where you set an amount of money, (e.g. $3) and when you add a coin in the corresponding slot like a coin in the $1 slot, it adds that amount to a counter. When the counter and milestone align a door opens and allows you to take the money out of the box.

Right now I need help with finding an easy way to create the mechanism that recognizes the falling coin, I originally thought of a light gate with an LED and an LDR but am unsure how to make this work. This is the first time I've ever done anything Arduino based and my project is at a halt until I can solve this problem, help!

Look at "slot sensor" in Google. These use a LED to shine across a gap to a phototransistor. While the gap is clear, the phototransistor conducts, when object blocks the gap the output turns off.

They are cheap and easy to program for.


You have not said, but I assume you are NOT trying to detect the type of coin.

Detecting the type of coin or detecting counterfeit items would be complex.


when you add a coin in the corresponding slot like a coin in the $1 slot, it adds that amount to a counter.

Suggests that there are individual slots for each denomination.



Could you not detect the coins by weight? I'm not sure if I'm correct, but I thought that different coin denominations had unique weights.

So you could do something along the lines of:

  • have a weight sensor in the bottom of the box
  • when the weight changes, compare it to the previous weight
  • from the weight, work out the coins denomination
  • add the coin value to an ongoing tally; calculating the amount from the total weight alone won't be easy.

Not sure if this is the right approach, but it might be something to consider...

Could you not detect the coins by weight?

Not a good first project. Too complicated.

Could you not detect the coins by weight?

How could you tell it was a coin rather than a piece of used chewing-gum?

What about coins from other countries?

Coin detection is a complex business.


They sale coin machines can't think of better word then machine But there not really that much as in cost and they handle the coins I got one off a dryer but it only handles one coin size.

You can buy programmable "coin acceptors", like this one fairly cheaply. Much easier than building your own!

I've worked with coin mechanism's in the past, and can give you an insight into how they work:

As the coin rolls down an inclined plane, it passes three coils of different dimensions/shapes.

The coils are each part of an oscillator circuit, operating at various frequencies. The geometry of the coils, and the frequencies used are chosen so that each coil will respond differently as the coin passes.

By measuring the change in frequency/amplitude of the oscillators it it possible to determine the diameter, the thickness, and the type of metal present. The results are then compared with minimum/maximum values for say 6 different coins, stored in a non-volatile memory.

Only if the coin passes on all three parameters, will it be accepted as a valid coin, and it's value determined.

There are other simpler methods that are used, which might only check the diameter by the method previously described, or optically, using a custom photocell array.,

This is a school project. Think using the KISS method.