I'm back at it again with an update on my AI Coin Sorting Robot. Version 4 is now complete, and I wanted to share some lessons learned and a breakdown for anyone interested in pursuing a similar project.
Here is a video walkthrough of Numi v4 in action. You'll see the entire process, from dumping a handful of coins into the hopper all the way through AI analysis and automated sorting.
www.youtube.com/watch?v=9ImLmteQgm4
You can find previous posts about Numi v1-3 at the end of this post.
To be upfront, v4 is not perfect. The real goal of this version was to find the gaps in my own knowledge and close them before version 5.
Numi v3 was a Raspberry Pi 5 running about 14,000 lines of Python, bolted to a chassis made from more than 500 LEGO Technic pieces, with two 16-megapixel Arducam cameras looking at the coin.
It was a wonderful proof of concept and a genuinely terrible machine. LEGO pieces flex under pressure. Every time the frame shifted a millimeter, the cameras lost focus, and the coin chute lost alignment. I spent more time re-squaring the build than improving it. So v4 got a full teardown. The LEGOS are gone, replaced with a 3D-printed frame I designed and printed myself. The Raspberry Pi is gone, replaced with a microcontroller about the size of a postage stamp.
The clearest way to explain v4 is to follow one coin through the machine.
1. The hopper drops a coin
Knowing when to stop feeding is harder than it sounds, so Numi uses a laser rangefinder. It's a small sensor that fires an invisible laser at the coin stack and measures how long the light takes to bounce back. When the stack reaches about 90mm away, the hopper stops. When it drops back past 108mm, the hopper starts again.
2. A pusher slides it under the camera A second motor pushes the bottom coin out of the chute and onto a small lit stage. The pusher then retracts until it physically stalls against a wall, which is how it finds its home position again.
The stage is lit by a ring of 19 white LEDs. Numi photographs the coin at 5 megapixels.
3. It photographs both sides Numi has one camera, so to see both faces of a coin, it has to physically turn the coin over. A flipper arm rotates the coin, and the camera takes a second photograph.
The flip completing
This works, but it also turned out to be one of my biggest mistakes.
4. The AI analyzes the coin Both photographs are sent together to Google Gemini Flash (the same AI you can chat with in a web browser) along with a set of written instructions. Gemini sends back the coin's series, year, mint mark, a list of descriptive tags, and a decision about where it should go. All this info is sent to an easy to read dashboard.
The part that surprises people the most is that the sorting rules are not code. They are written in plain English, in a text file, and I can change them without touching the robot at all. An excerpt from the actual instructions Numi sends with every coin:
Save: Silver, Key Dates, or Semi-Key dates.
Inspect: ANY known US Variety Candidates. Use your numismatic training to cross-reference the Series, Year, and Mint Mark; if this combination is associated with a specific valuable variety (e.g., 1955 Lincoln Cent, 2004-D Wisconsin Quarter), tag as "Variety Candidate" and set Action to "Inspect".
Dump: Common face-value US coins, standard clad, and severely damaged coins.
Misc.: Foreign coins, casino chips, tokens, non-coin objects, etc.
Retry: Machine anomalies (Double-Feeds where images show different coins, overlapping coins, empty frames), or if BOTH Year and Mint Mark are unreadable.
If I want Numi to start pulling War Nickels tomorrow, I add a sentence and go. That is a fundamentally different way to build a coin sorter than training a machine to recognize specific coins, and it is the single thing I am most convinced about after four versions.
5. Positioning the sorting carousel Once the verdict comes back, the carousel spins the coin cup over the correct bin, and a dumper arm drops the coin in.
Numi processes roughly 250 coins an hour at a cost of around $0.25 per thousand coins.
The speed didn't come easily. Numi v4 started at about 60 seconds per coin. Getting to 15 seconds took weeks, and most of the gain came from overlapping steps that used to happen one after another.
Lessons Learned
A mechanical flipper will eventually jam. Any moving part that has to physically manipulate a coin is a part that will eventually fail.
Use two cameras, not one. The flipper only exists because I dropped from v3's two cameras to v4's one. I saved a camera and paid for it with a moving part that touches every single coin. That was a bad trade.
A funnel will eventually jam too. Another moving mechanical part that will eventually fail. Writing a function to un-jam your machine is a sign you designed the wrong machine.
Lighting and camera angle are much harder than I expected. Coins land at slightly different distances from the LED ring, and by my own measurements the brightness of the subject swings by roughly ten times between the near and far cases. Too bright and the lettering blows out to white. Too dark and the date disappears.
Bringing Numi to shops and shows is the best part. Nothing in the code is as valuable as watching a dealer's face when the machine pulls a silver quarter out of a junk bin, then hearing feedback on how to improve the machine to be useful for them.
Version 5 is already sketched out, and it's shaped entirely by these lessons.
I'd rather tell you what broke than pretend I got it right the first time. Four versions in the goal remains the same: raise awareness of AI and its impacts on our hobby.
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If you have ideas, I'd love to hear from you. And if you see me at a show, come say hello. The machine will be running. If you want to keep up to date about Numi's latest developments, [as well as Dansco research], feel free to check out my Instagram page. https://www.instagram.com/dansco_dude/
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Posted Yesterday 06:06 am
