Weekend Mini-Project: Pneumatic Gun with Electric Trigger
This page details the basics of how you can make a fun little pneumatic gun from PVC and add an electric trigger system (so that you can launch with the press of a button) using a sprinkler valve. All the materials were sourced from Home Depot and only cost about $20 altogether (+$5 if you don't already have PVC cement). I also used the schraeder valve from an old bicycle tube. Anyone who's into bikes will have some lying around.
WARNING: This is a powerful and therefore dangerous device. Be responsible. Be careful. Don't shoot at people.
- 1' of 2" PVC pipe
- 1 x 2" end cap
- 1 x 2" to 3/4" adapter
- 3/4" PVC pipe and elbows (of amount depending on personal choice, used for barrel)
- 2 x 3/4" unthreaded to threader converter
- Sprinkler valve (I used the Orbit WaterMaster Automatic In-line Valve)
- Wire, momentary push button, and 2 9V batteries
- Schraeder valve
The gun has three main parts: The chamber, the launch valve, and the barrel. The projectile loaded down the barrel and then then launched as the pressure in the chamber is released quickly by the launch valve. The main parts are labeled in the below diagram.
The chamber is made of a 2" diameter pipe with one end capped. I used ABS pipe for the chamber instead of PVC because I didn't find any 2" PVC, but PVC is preferable because bonding the ABS to the rest of the PVC parts requires a special type of transfer cement. Using PVC throughout is simpler because you'll only need PVC cement. The length of this pipe (which determines the size of the chamber) doesn't actually need to be all that big, a foot should be more than enough.
The chamber also needs a way to be filled. This is accomplished by drilling a hole in the side of the the chamber and inserting a schraeder valve up through it from the inside (the size of the hole should be determined so as to be as snug of a fit for the valve as possible). The valve should be cut out from an old bicycle tire so that there is a circle of rubber around the valve. The valve then comes up through the drilled hole and rubber is sealed to the inside of the chamber using super glue (as shown below). This valve allows the chamber to be pressurized using a standard bicycle pump.
Red outline shows where the rubber around the valve is glued to the inside of the chamber
The other end of the chamber pipe must have a 2" to 3/4" converter since the launch valve and barrel are 3/4". In my build I have elbows turning the flow of air 180 degrees so that the barrel faces the other way and the whole thing is more compact, but this is not necessary. I do, however, recommend that a manual ball valve is placed between the chamber and the sprinkler valve. This is also not necessary, but it can be closed as a safety against the sprinkler valve going off accidently (which is a real concern).
The launch valve is a sprinkler valve that is held closed by it's own pressure and triggered by a voltage being applied accross a solenoid that unbalances the pressure and causes all the pressure to be released at once. One can easily look up how they work; it's an beautifully effective mechanism. For our purposes though, all that needs to be done is the sprinkler valve be put between the chamber and the barrel and wired with a way to turn the voltage across its leads on and off. The valve will likely have threaded ends, which can be connected to the rest of the launcher using threaded PVC adapters and PTFE tape. The below picture shows the valve with leads soldered in series with two 9V batteries (I found 18V was plenty strong to trigger it reliably) and a momentary push button.
The barrel is just a length of 3/4" PVC. Greater length does mean that the projectile is propelled for longer, but PVC pipe isn't usually totally straight and so long barrels can throw off accuracy. I used 2.5' of pipe for the barrel, but could have done with less.
To fire, the chamber is first pumped to anywhere from 20 to 60 psi through the schraeder valve using a bicycle pump. Then a projectile (grapes work nicely for 3/4" barrels) is loaded down the barrel. A wooden dowel or the like may be used as a ram-rod. Be careful not to look down the barrel; a misfire would put out your eye and, depending on the pressure and the projectile, probably make it well into your brain. As a precaution, the manual safety valve can be closed while loading the projectile and then reopened afterwards. Then simply aim, press the button to put voltage across the sprinkler valve, and do your best gleeful mad scientist laugh as your target is decimated!