Step 1: The Plan
I also saw this as an opportunity to start to train them how to plan and manage a project. So, the first image is our schedule and resource plan. The second one is a list of the things that could be learned from the project.
The third image is my original attempt to draw out the circuit for the trigger. I will describe it in a later step. The fourth is of our plan for making the documentary and the supplies we would need. The next is the rest of the supplies list. The next shows the design layout for the gun and the trigger box, with the parts list for the gun. The last shows the prep'd raw parts for the gun.
Step 2: Chamber
1) Measure the openning size for the pressure gauge and the valve, then drill end caps and insert them. The pressure gauge was (if I remember right) was a water pressure gauge with "1/4" fitting. I drilled the hole as if I was tapping it, then screwed it in and put a pipe fitting on the other side to hold it. The valve is a standard auto tire valve (I walked into a local tire shop and asked for one - they just warned me that they had a 120PSI limit).
2) Cut, clean and glue together the main chamber shafts with the PVC fittings.
Step 3: Barrel
The main thing to do on the barrel is to bevel a sharp edge on the end of the barrel so that is cuts the potatoes down to a cylinder when you breach load them.
We also made a matching ram for loading. We used an old table leg (from a X Mart round wooden table) that already had a threaded shaft on the end. We drilled a hole in a 3/4 pvc end cap and screwed it onto the ram.
The final picture shows the completed chamber, two pieces of the barrel, the ram and the parts for the trigger and valve.
Step 4: Testing
The initial test showed a slight leak in the pressure gauge fitting. So, I think we either epoxied the fitting or used silicon caulk.
Then we tested both the auto valve and a manual valve. The auto valve was triggered with a single 9-volt battery.
Step 5: Trigger
a) Three 9-volt batteries in series to provide the 24V that the circuit should safely need.
b) A power safety-switch for a two-step firing sequence (poswer up, then fire).
c) An indicator light to show when the power is on.
d) An intermittent switch for firing the valve.
e) Everything linked together on a terminal strip (all of the components have spade terminal leads).
The secret to the circuit was the diode in paralell with the firing switch. This made the light come on when it was powered-up shorting the firing switch.
Here are the construction steps:
1) Lay the trigger box components.
2) Measure where the external ones will go.
3) Drill the openings and test the fitting.
4) Add the interlocking connectors to the pig tail of the valve and the end of the trigger power cord.
5) Add leads to all of the components.
6) Wire it all together on the terminal strip.
7) Test the fit of the box.
8) test the circuit to make sure everything is flowing right.
It worked great, at least for a few weeks. It got us through the school fair and a bunch of firings at home. Alas, eventually the wires on the intermittent switch somehow shorted out or just got bent off hitting the terminal block. I have a larger project box in the garage to put it all back together some day. This time I will probably also add some battery holders.
Step 6: Platform
The first pictures shows my son's rendering of the plan and steps.