Cap Launcher

Ok so don't ask how I inherited almost 4,000 2 liter (or 20 ounce) soda bottle caps...

Anyway my plan is to make a robot that walks around and randomly shoots caps at things. I think I can fashion something to hold the caps out of coat hanger wire and use a servo to feed them one at a time. I have some high powered motors that should be able to launch these things too. Work will start in a few days since I will have to dismantle my first robot to harvest the board and servo. :(

Any ideas for other robots that could use these things? I was wanting to build one out of caps and hot glue, but that could take a while and probably wouldn't be stable.

I ordered some gears that should be here mid next week then I can start building the cap feeding mechanism. I think I will use a servo to make a lever move back and forth that moves a cap out of a gravity feeder and into the path of a propellar that is mounted on a small yet fast motor. The cap will shoot up a ramp that gives it a nice arc until it crashes into something. I was thinking it would be fun to make two of them on radio controls that adjusted speed and arc so you would have to try to hit a target on the other person's robot. Either that or make it so you drive around and try to shoot them on the run.

Added some pictures of minor progress. I put the belt tracks together (if it is going to be a mobile weapon what better than tank tracks!). I also began working on a mechanichism for moving a cap away from a gravity feeder to where it will shoot. A servo will move the right part in a semi circle which makes the other part move up and down thus pushing a cap from the feeder to a spot where a motor will shoot it. I'm not 100% sure how well it works I need to harvest the servo off of my other bot I only bought 1 servo :( More to come!

I wonder why I couldn't find any yellow tracks...? Hmmmmm

A simple cardboard base. The metal is a cut up coathanger.

It still needs a little work but here is how it should load caps one at a time. As you can see it still needs some work. The wire rubs against the holder and needs a larger end on it so it will push caps all of the way out.

Made some more minor progress. The tracks aren't mounted all that well, but it should be able to move. I need to get some better stability since it curves a bit. It will get there eventually! I made a small fan and mounted it on a motor (not in the picture) the cap flew about 3 feet but since it is light it wasn't that spectacular. I'll work on it more when I can...

This is a companion discussion topic for the original entry at

Ok so how did you inherit

Ok so how did you inherit almost 4,000 2 liter (or 20 ounce) soda bottle caps?




You could get some light

Why not use gravity feed? You could get some light weight PVC or aluminum tubing that is just slightly bigger than the cap. A set of rails below the pipe could be a landing pad. A servo could spin an arm when aimed at the target and it would smack the cap and launch it down the rails.

If you wanted a magazine feed, you could put a section of tubing on a funnel and would need to find some way to make sure they lined up right. A small pager motor attached to the funnel may do it.

Instead of using caps, why
Instead of using caps, why not use a coil gun? Coil guns would require only electrical components, not compressed air, etc. I have used and made several coil guns.

Silly Ideas

You could have a set of lighting gels on a wheel and rotate them round while looking through them with an LDR or something. Then by knowing which colour parts of the gel were at which rotations you could guess at the colour using only one sensor. You could then try and lay a coloured trail or mozaic.

It might also be good fun to make a messy robot and a tidy robot. One that lays them out and one that chases it around scooping them up. The tidy bot could try and put them back into a hopper on the messy bot when it has collected enough, so the cycle could continue.

I also once saw an interesting example of emergent behaviour with a group of very simple roving robots. Each had a little scoop on the front and would drive around pushing little pucks about until the pressure on the scoop was too much, then they would pack up and go in a random direction. The end result was that the robots zoomed about and collected an bunch of scattered pucks in an arena, and pushed them into a big pile.

The best way to launch caps

The best way to launch caps may be to have some sort of wheel or mill spinning, and then "drop" them onto this :slight_smile:

(Warming up Cap-Gun… Full speed… Fire! :D)

/ Fritsl

I figured I would make two
I figured I would make two wheels. One on a servo is a full circle minus a wedge (Pacman shape) to move a cap to the firing area. The next would look like a fan that had four blades (mounted on a motor) that would send the cap flying. My only concern is getting a motor that can power up fast enough to shoot the cap fast enough to get some distance. I have some small motors that may be able to do the job. I’ll start building the cap holder tonight and see if I get inspired.

"Coil guns" you say?

Tell me more about coil guns. Sounds like lots of ampage. Why not make a walkthough?

God created the integers; all else is the work of man - Leopold Kronecker

I think it is a cool project

I think it is a cool project :smiley:

Next you’ll have to make a bot that picks them up again!

/ Fritsl

That sounds cool, but I
That sounds cool, but I haven’t though of a way for a robot to detect a cap and go toward it. The SRF05 doesn’t detect objects that fine. It would have to roam around and scoop everything then do a dump. I would almost have to put a RFID or something like that in each cap. When I get my gears next week I can start work. Til then all I can do is wait :wink:

If it was me, I think I

If it was me, I think I would make one that was touching every little thing it found. And if it moved by the touch, then try to pick it up and throw it in my sack :smiley:

/ Fritsl

PS: I send you a goodie-bag - Wrote mail about it as well :slight_smile:

Coil guns operate on a force

Coil guns operate on a force of energy called Joules. You can convert joules into any force of energy, such as horse power, etc. The basics of a coil gun operate by storing a lot of Joules in a capacitor. The best way to generate a lot of Joules, is to use lots of voltage and capacitance in capacitors. We are talking on the range of 300+ Volts. The formula to calculate how much joules something has by relating voltage and capacitance is:



J= Joules

V= Voltage

C= Capacitance in Farads


This means that if you had 100 Volts and a 1000uf capacitor, you would have 5 Joules of power, which is not a lot. If you were to boost the voltage to say, 300 Volts and 1000uf capacitor, the power would be 45 Joules. Thats a sinifigant increase in power! That’s because voltage is exponental. It is a common practice to increase voltage by sacrificing capacitance. Here is an example:



45= Joules

300= Voltage

0.001=1000uf, but it had to be in a F format so you add a few zeros before it.


That above was a control. We will now decrease the capacitance by half, and increase the voltage by double. We do this by simply adding the capacitors in series, like so: -||–||-

Here is the increase in power


90= Joules

600= Voltage


As you can see, by increasing Voltage by sacrificing half of the capacitance doubles the output power!


for more on coil guns, you can go to wikipedia for more on coilguns.


I will post more on the specifics later.

I am assuming it requires a
I am assuming it requires a metal projectile as it appears to use magnetic force. If so this wouldn’t work since I am using plastic caps…

Yes it requires a
Yes it requires a ferromagnetic projectile.

Thanks! Odds and ends are

Thanks! Odds and ends are always nice to have even if you don’t know what to use them for. I like to get an idea then sit and figure out how to do it with what I have. I rarely if eevr plan out way ahead, order parts, and make something. I like to look at a pile of parts and think… what can I do with this? I’ll call it the McGuyver approach… :wink: I ordered some gears and with the goodie bag you sent I should be able to work on my cap launcher.

Tomorrow I’m goign to show my parents their first grandchild grandrobot from me. My sister has 2 kids and I won’t be married for another 2 months so they will have to make due. Once they see it and I get home to make a video or two for archive, I will harvest the parts and work on something to feed caps from the holder to the firing range. I think I know how it will work…

Gluuue… Nice! / Fritsl

Gluuue… Nice!

/ Fritsl

Yes I did use Frisl glue. I
Yes I did use Frisl glue. I took the wire frame off and will use PVC. The caps were getting stuck on the glue and not moving down smoothly. I’ll attach it later tonight and replace the video with it feeding caps (hopefully).

It looks really cool!One

It looks really cool!

One good advice: Drop making it on a base of "paper"! Find some wood now! It will kill the project in problems to base it on what you are now! Seriously.

I realized that after I put
I realized that after I put the tracks on it. I thought it would be solid enough, but the cardboard is curved. I need to find some scrap wood to use. I only wasted a couple glue <Frits> sticks :wink: I was hoping it could be used as a good base since I am trying to make the robots as cheap as possible so I can keep them around a bit longer and not always have to rip them apart for parts for the next one. I’m going to try to wire it up on perf board so I dont use a project board for every bot. When I get back in town I’ll hunt down a better base. I just don’t have a garage to cut wood and such.

I know you went back to

I know you went back to college but have you got back to this project?

LMR would like to see your cap shooting robot!!!