Build a coin counter from particleboard and a Pi Pico
Feed the Meter
© Lead Image © Andrey Burmakin, 123RF.com
Parking meters and vending machines detect and count the coins you insert, but how do they work? We'll show you how to mimic the functionality with some particleboard, a Raspberry Pi Pico, a few extra chips, and some Python code.
Counting coins is a problem that was solved decades ago, but I was wondering whether I could come up with a solution of my own. While I may not have the same set of sophisticated tools that a parking meter manufacturer has, it should certainly be possible to create an electronic coin counter using just a Raspberry Pi Pico and some common components. Writing software that can count is trivial, but how do you detect what type of coin was inserted? If you assume that only legal US coins will be used, the easiest way to recognize them is by measuring their diameter. Table 1 lists the diameters of US coins, both in millimeters and in inches [1].
I decided to sort the coins using different sized slots. The coins roll down a small ramp and pass several cut-out slots. If the coin is the correct size, it falls through the slot. The required width of the slot depends on how fast the coin is rolling past whereas the height of the slot needs to be slightly larger than the actual coin while still being smaller than the next largest coin. In my case, the slots were approximately twice the width of the coin (see Figure 1).
It is not difficult to draw a small rectangle, but it is tricky to try and cut it out with straight edges that accurately follow the rectangle. Making a cardboard prototype was no problem, but it took some more effort to cut the slots out of particleboard.
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