Full/Semi Auto Mini Marshmallow Gun - Compressed Air - 1 Week Classroom Project

After nearly two years of development, we are proud to announce the release of the Marshmallow Gun V1 and V2. These were originally inspired by the classic 1/2" PVC marshmallow blow gun in which 3 pieces of PVC were jointed at a T shape, this allowed marshmallows to be loaded in the top like a magazine and be fired continuously by blowing through one side. We decided to take this concept a little further by adding two 120psi compressed air chambers behind this simplistic firing mechanism along with a timing circuit to allow fully automatic and semi automatic firing. The total cost of this project is ~$36

Looking for an EPIC one week classroom project? This project was also a part of our Inventing Reality 2017 summer program which we taught at Redwood Day to 6th-12th students for a week. It was a blast (pun intended) with all students For more information for lesson planning, preparation, and tutorials, check out the educational sections below.

Youtube Tutorials:
-Tank assembly tutorial
-Grip wiring tutorial: coming soon...

Before we get into the details, I just want to clarify the exact meaning of full/semi automatic modes. Note that modes are selected via a mini rocker switch on the grip assembly.

Full Auto Mode As long as the trigger is held down, marshmallows will shoot out of the barrel in a continuous stream. This method of firing is significantly more inefficient and will only allow the emptying of one 20 round marshmallow magazine (approximately 12" long 1/2" PVC) before your air tank is empty. You can also shoot mini full auto bursts of marshmallows to make it a little more efficient. The range of full auto is around 50ft though accuracy significantly decreases after around 20 feet where the mini marshmallows begin to be affected by wind.

Semi Auto Mode Each time you pull the trigger, a timing capacitor drains into your solenoid valve firing a quick burst of air. To fire again, the trigger must be released and re-pulled. Depending on timing capacitor size, solenoid valve size, and air tank size this should allow you to fire around 1-3 20 round magazines and release 1 to 2 marshmallows each shot. The range of semi auto mode is similar to that of full auto mode (around 50 feet)

Safety Mode This mode essentially turns off your marshmallow gun. If the trigger is pulled in this mode, nothing will happen. Note that when you switch your marshmallow gun into safety mode after having it in semi-auto mode, there will still be one charged shot in the gun that needs to be discharged before it is really "safe".

Over the course of designing and refining this project, a couple of variations surfaced.

Grip V1 The grip V1 allows the installation of two 9v batteries directly into the grip itself. This simplifies the grip assembly though also doesn't allow the mode switch to select full/semi automatic firing or safety mode. This means that you either have to wire the 18v from the batteries through the micro switch directly to the solenoid valve to allow only full auto firing, or use a simplified timing circuit to allow only semi automatic firing.

Grip V2 External Batteries The second version of the grip mounts the batteries externally in a battery mount which slides over your solenoid valve. This creates a little more room inside your grip to add the necessary electronics for a mode switch to toggle between full and semi automatic firing.

Single Air Tank The original marshmallow gun only had a single 2"x12" PVC air tank. This air tank supplied enough air to fire one full 20 round magazine but no more. Benefits of using this simpler tank design include a significantly lighter gun which is easier to carry as well as a more sturdy gun which is less likely to break (because of the weak joint between the two tanks on the dual tank version)

Double Air Tank To take this project one step further, we added a seccond 2"12" air tank. These air tanks are joined at the rear by some 90 degree elbows and a short 2" connector piece. The double air tank will allow you to empty around 2-3 full 20 round magazines in semi auto mode and get a little extra punch in full auto mode. The dual air tanks also allow you to install a 3D printed rear cap which allows you to snap on and hold up to four extra magazines on the side of your gun.

3D Printed Parts List:

Grip V1 Explained above

Grip V2 Explained above

Trigger This trigger screws into your grip with an M3x35 bolt and lock nut. It presses into your micro switch when you pull on it!

Battery Holder (only if using Grip V2) To easily and conveniently mount your two 9v batteries as well as protect your solenoid valve wires, we've designed a handy battery holder to slide onto the top of your solenoid valve and screw on to the front of the V2 grip with an M3x35 bolt and lock nut.

Rear End Cap (only if using dual tank version) The rear end cap serves multiple functions for the dual tanked marshmallow guns. First, it's a safety necessity. Because the the two tanks are only joined by one small 1/2" PVC pipe, they are very susceptible to breaking apart. Since the tanks are pressurized at 120psi, if they were to break, it would be... explosive. The rear end cap covers the ends of both tanks and makes sure they they are protected during accidental drops and normal usage. The rear end cap also has four handy clips on the side to store four extra magazines so you aren't constantly reloading!

Dual Tank Clamp As an additional safety measure, we designed two "tank clamps" to ensure both of your air tanks stay nice and aligned and don't get twisted and break. These clamps simply go on either side of your dual tanks and screw together with an M5x35 bolt and lock nut to keep them nice and tight.

Magazine Tops Just to save a few extra dollars, instead of buying 1/2" PVC caps, you can instead print these handy caps!

Magazine Bottoms To allow you to store multiple loaded magazines without all the marshmallows falling out, these handy end caps are easily removable with a little hole to see if the magazine is loaded as well as a handy string to attach them to the magazine so they don't get lost.

Electronics & Hardware List:

-1 1/2" metal solenoid valve
-1 Three position rocker switch (only if using grip V2)
-1 Micro switch (for trigger)
-2 220uf capacitors
-2 9v batteries
-2 9v battery connectors
-Some 24awg silicone wire
-2 M3x35 bolts (only need one if using grip V1)
-1 M5x40 bolt](https://amzn.to/2LvnV72) (only if using dual tank clamps)
-2 M3 lock nuts

Single Tank PVC Pipe & Hardware List (buy at local hardware store):
-1 12" length of 2" PVC pipe
-1 2" length of 1/2" PVC
-1 12" length of 1/2" PVC
-1 2" PVC socket cap
-1 2" PVC coupler
-1 2"-3/4" PVC reducer
-1 3/4"-1/2" threaded reducer
-1 1/2" PVC T joint
-1 1/2" PVC coupler
-1 1/2" PVC threaded to non threaded coupler
-1 1" wood screw
-1 Quick release air valve

Dual Tank PVC Pipe & Hardware List (buy at local hardware store):
-2 12" lengths of 2" PVC pipe
-2 2" lengths of 1/2" PVC
-1 12" length of 1/2" PVC
-1 2" PVC socket cap
-3 2" PVC couplers
-2 1/2" PVC street elbows
-1 2"-3/4" PVC reducer
-1 3/4"-1/2" threaded reducer
-1 1/2" PVC T joint
-1 1/2" PVC coupler
-1 1/2" PVC threaded to non threaded coupler
-1 1" wood screw
-1 Quick release air valve

Other Tools n' Stuff:
-A LOT of mini marshmallows (see good and bad marshmallows section below for more info)
-Plumbers tape
-1/4" drill tap for quick release valve (not necessary but helps keep things air tight)
-Duct tape
-PVC primer
-PVC cement
-PVC pipe cutters
-Safety glasses
-Air compressor

Good and Bad Marshmallows
After some extensive testing of many brands of mini marshmallows to see which ones performed the best, we concluded that Dollar Tree's "Fireside" branded marshmallows and GIANT supermarket's mini marshmallows performed the best because of their slightly larger and more consistent size compared to other brands such as Jet Puffed.

Proper Marshmallow Loading
When using your marshmallow gun, it is critical to load your magazines properly so they don't jam. The trick is to make sure that each marshmallow you load can easily slide down the magazine to the bottom without getting stuck. If a marshmallow is too large and doesn't want to slide down easily, roll it in your fingers a bit until it is just the right diameter. Remember, even one oversized marshmallow can cause a magazine to jam.

IMPORTANT SAFETY NOTICE: This project is potentially very dangerous if not done properly! Make sure that everything is glued, dried, and protected properly! YOU MUST WRAP YOUR AIR TANK(S) WITH DUCT TAPE TO HELP PREVENT THEM FROM SHATTERING IF THEY BURST! We are not responsible for any injuries obtained from building this project. Additionally, don't shoot people in the eye or from point-blank range because these can cause bruises! Lastly, wrap the tip of your barrel with bright orange tape or spray paint an orange band around it so it is not mistaken as a dangerous weapon.