Sunday, April 20, 2014

Secret Knock Gumball Machine

I want you to know that just because I haven't posted much recently it doesn't mean that I haven't been busy. I've been working on a housewarming present for by brother John and his wife Doreen. They are building a house and it's supposed to be finished in August. That project is well along now, but it's a secret so I can't blog much about it until I give it to them.

I do want to tell you about something I made a while back, though. It was adapted from an article in Make Magazine, issue 25 called Secret-Knock Gumball Machine by Steve Hoefer. As the name implies, it is a gumball machine, but instead of putting in money, you knock on it in a particular pattern. If you get the pattern right it gives you a gumball. Below is a video of one of them that I made.

The default knock, as shown is the video is "shave-and-a-haircut" but can be changed to anything the user would like. I heartily encourage you to look over the article. I think the programming that Hoefer did is particularly clever and could be used to open a door or a treasure box, or to trigger any other thing.

I built the one shown in the video for my brother Jim. However, I modified the design in the article in several respects. First, I made the dispensing wheel horizontal, rather than vertical. I did this so that it was more patterned after conventional gumball machines. Second, I added a switch so that the machine knew when it had fed a gumball, and I changed the software to add some strategies like reversing the feed wheel if a gumball did not feed in a certain amount of time, thus making the dispensing process more dependable.

I made the one for Jim out of wood. I was happy with it, and it worked pretty well, but it would occasionally jam. Nothing would break, but on rare occasions you had to turn it over and then set it back upright to clear it.

While I was building Jim's machine my sister Monica and her husband Jim visited, and I showed them how it was going to work. Usually when I show my family what I'm building they watch and say something like "Oh, that's very interesting. . . . What should we do for lunch?" but Monica and Jim asked me to demonstrate it a couple of times and asked a number of questions about the details of the design.

Therefore, I decided to build a second machine, this one for Monica. I wanted to make out of acrylic, and I wanted the feed to be more dependable.

I knew that the thing that was causing the jamming was the hopper mechanism that funneled the gumballs to the feed wheel. I was experimenting with a bunch of sculpted designs that would guide the gumballs to the holes in the feed wheel but wouldn't allow two or more gumballs to block up the mechanism. My son-in-law Tim looked at it and said, "You're over-engineering it. Get rid of the hopper and just let the gumballs fall down the throat of the machine and drop onto the feed wheel. They'll fall into the holes without jamming."

I realized that he was right. The only time it might not feed continuously is when there are only one or two or three gumballs left, but that is only a fraction of the time. Mostly it will have a bunch of gumballs and they'll easily drop into the feed wheel.

Above is a picture looking down the throat of the machine. The wheel with the holes in it turns and as it does a gumball that is in one the holes passes over the feed chute and the gumball falls out, rolls down the chute, and comes out the front of the machine. The last time I talked to Monica about it they had just about gone through the entire box of gumballs that I had given them with the machine (the standard size package has 850 gumballs) and there had been no jamming or mis-feeding.

This is a great project. People are fascinated by it. Again, read Steve Hoefer's article and look at his blog posts on

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