Driving Pneumatic Solenoids

So, I'm a member of a group that does regular Nerf battles at a local park. I've recently started constructing a defusable Nerf bomb (complete with cutable wires) for our games. I've finished the timer, display, keypad, and defusing wires but I still need to develop a pneumatic system to fire the darts when it "explodes". My problem is, I'm having trouble picking out a pneumatic solenoid for the project.

I'm aware that the solenoid will need a power supply that's separate from the actual Arduino output. I was wondering if anyone hand recommendations on how to go about designing the circuit and if they had any specific solenoids they'd recommend.

I'm actually thinking of using this. It's fairly inexpensive. I don't actually care about the solenoid direction, only if it's open or closed. Any thoughts?

Metasyntactic: develop a pneumatic system to fire the darts when it "explodes".

More detail please.

Don't nerf guns use springs normally

Boardburner2: More detail please.

Don't nerf guns use springs normally

Yes, but this is not a nerf gun. The way this works is the Arduino has it's whole countdown mechanism (an LED display and wires that use are using pull-up resistors to detect if they're cut) and then it drives a solenoid. The pneumatic mechanism has a CO2 canister (though I'm probably upgrading to a refillable air tank) that, when the solenoid is open, directs pressure to a bunch of PVC tubes that are capped off at one end and filled with darts.

The actual design is inspired by Think Geek's April Fools Nerf bomb product. I'm actually doing the mounting for the darts the same way, using a plastic bubble bowl with the PVC tubes epoxied inside in a radial manner. The main difference is, instead of some bomb you shoot, this will have a countdown that you defuse.

So yes, no springs or clunky mechanical mechanisms, just pressurized air.

While that looks to be a lot of fun

Metasyntactic: , just pressurized air.

That would indicate considerable inexperience.

You have failed to mention the second solenoid needed to charge the firing chamber of calculated size and pressure prior to firing.

just pressurised air could mean anything.

Get it wrong and apart from the danger, under UK law you could end up being charged with 20 separate counts of possessing an illegal firearm

Ill explain further. You can legally posses a crossbow or bow (massive spring) capable of taking down a bear , if that's your thing.

However if its pneumatically powered you are limited to 12 ft lbs of energy.

Although a nerf dart is light and loses energy rapidly the measurement is done at the barrel and its very easy to exceed.

Someone close could sustain serious injury both from the dart and air blast.

You may have seen tennis ball practice machines. They use a rotating tyre to project the balls. Easier to do with pneumatics but then they would require an FAC and secure storage.

My favorite has always been potato cannons, but they are illegal for the same reason.

This is UK though and may not apply but there is still the safety issue of compressed gas.

Boardburner2: That would indicate considerable inexperience.

Actually, I worked with pneumatics for a couple years via FIRST robotics when I was in high school

Boardburner2: You have failed to mention the second solenoid needed to charge the firing chamber of calculated size and pressure prior to firing.

Why would I have a secondary firing chamber? I can literally have direct release from the tank I'm using to store the air. A flow regulator will allow me to fine tune the output PSI. To help you envision the system (since I have no clue what you think I'm talking about) it'll involve a pressurized air tank feeding through a flow regulator to a solenoid, and out through a union tube to a number of PVC tubes capped at one end.

Boardburner2: just pressurised air could mean anything.

Well, in this case, it means air that is stored under pressure.

Boardburner2: Get it wrong and apart from the danger, under UK law you could end up being charged with 20 separate counts of possessing an illegal firearm

Well, I'll keep that in mind if I ever decide to take this to the UK. About the only real danger this device might realistically have is if a dart hit someone in the eye but that's true of any Nerf gun. It's not like these are being shot at 340 m/s. The flow regulator will allow me to control the pressure output of the system.

Boardburner2: You can legally posses a crossbow or bow (massive spring) capable of taking down a bear , if that's your thing.

I can comfortably say this will not be taking down any bears.

Boardburner2: Someone close could sustain serious injury both from the dart and air blast.

I would be quite surprised if that were actually the case. In addition to the flow regulator, you also have to factor in that the air is being released out of many barrels at once. It's going to be pretty widely distributed.

So, I do in fact appreciate your safety concerns. Safety is always paramount when building any project. That said, between the low mass of the darts, the wide distribution of pressure through multiple tubes, and the control of the output pressure, I think your fears are largely unfounded for this project. I'm reasonably confident I will not be being brought up on possession of lethal weapon charges any time soon. I'm more concerned with how to safely drive the solenoid without frying anything by accident and making sure that the solenoid I buy is capable of being driven by the Arduino.

ok Sounds like you know what you are doing. Original post provided little detail Wondered if you were a kid outside of his experience. It does happen here. No one is likely to give useful information if they are concerned about inexperience or safety.

To answer your original question. The solenoid you referred to is 6.5 W at 12 v so I would suggest a digital transistor of 2A collector current would be suitable. Do not forget the commutation diode across the solenoid though. You have explained things better than in your original post and I would not be surprised to see more replies now. I'm off to bed and revisit tomorrow if you need more help.

Metasyntactic: Why would I have a secondary firing chamber? I can literally have direct release from the tank I'm using to The flow regulator will allow me to control the pressure output of the system.

Direct release from the tank even with a pressure regulator implies an uncontrolled release of gas.

Charging a chamber of known volume to a known pressure gives a precisely metered energy.

Your solenoid is incapable of the speed required to act as the controller.

This system is used on PCP air guns but the solenoid valve is something very special.

Im not too worried if you are using a sparklets bulb for this but you implied an upgrade to a compressed tank later. Size and pressure of that needs specifying.

PCP guns work off 3000 psi dive tanks and they can deliver terrific amounts of energy if something goes wrong

Boardburner2: Direct release from the tank even with a pressure regulator implies an uncontrolled release of gas.

That's quite literally exactly what I want. I really couldn't care less about the amount of gas used. I mean, I could release the entire quantity of the tank and the device would still function as desired... if also release air for a very long time. The quantity of air released does not impact the function of the device.

Boardburner2: Size and pressure of that needs specifying.

PCP guns work off 3000 psi dive tanks and they can deliver terrific amounts of energy if something goes wrong

I think you keep imagining something incredibly more sinister than what I'm building. This thing is launching foam darts. Why on earth would I be using dive tanks or anything close to 3000 PSI? I'm sorry if I'm sounding argumentative, it's not my intent, I'm just a little baffled. I couldn't tell you exactly how much pressure I plan on using without experimenting but I can't imagine it'll be much more than 50 PSI, if that, but that honestly shouldn't even have to be stated. To reiterate, I'm shooting small foam darts a couple meters, not pellets at 3000 PSI or bear killing crossbow bolts. This is meant to be a toy. You defuse it or you get showered with Nerf darts, not shot dead. I think you're over thinking this by a fair degree.

Really, the only thing I'm particularly concerned with at the moment is how to safely drive a single solenoid with an Arduino without accidentally frying the Arduino.

Its a common question here if you search around driving servos and motors you will find various answers.

Your solenoid will however require a separate power supply to the arduino.

Im not implying anything sinister.

If it were my project however I would calculate the required size of the air receiver as a starting point.

Metasyntactic: That's quite literally exactly what I want. I really couldn't care less about the amount of gas used. I mean, I

The commonest form of air injury weather it be from a fan or high pressure air supply is a bit of grit in the eye. I have seen a fair few and in a lot of cases regulation eye protection was being worn.

In the spirit of trying to be helpful.

Mate fires missile salvos off his rc boat.

He does it using spring and an escarpment release.

Its cheap and easy to reload.

He tried pneumatics , the result was first out the tube went the distance with remainder littering the deck and some in the tubes.

Increasing pressure makes first out go further but result is the same.

Only way is to have separate reservoir and solenoid for each tube.

Alternative would be captive pistons but a spring performs much better.

One of your darts will have less friction than the others. Air will follow the path of least resistance, said dart hogs the air and when it leaves barrel an open port will drain the pressure