Electric/Pneumatic BB gun

[Peepsalot]

I have an idea for a BB gun that is powered by batteries. Basically it uses a small compressor motor that can produce up to 28psi. It pressurizes a small chamber, and when it is ready a momentary switch(trigger) is pressed to activate a solenoid that releases the pressure out through the barrel.

I have ordered a solenoid for this purpose, and it is supposed to be run at 12vdc. My compressor motor runs at 9vdc. I would like to use a set of 8 Nimh batteries at 1.2v ea which would give me 9.6V. Do you think this would be sufficient for powering the solenoid and motor? I want it to be as quickly opening as possible, not only halfway open by the time the round gets out the barrell. What would be the simplest way to convert ~9vdc to 12vdc if I wanted to do so?

 

[jbeng]

You could charge a capacitor, then dump that charge through the solenoid valve coil. Think of how a photoflash unit operates. They're often ran from just a couple of small AA cells, but they dump a LOT of current through the strobe tube in a short amount of time. Anyone who's ever accidentally burnt the end off a screwdriver by shorting out a photoflash cap knows what I mean ...
JB

 

[Peepsalot]

yeah I could charge a cap, but it would still only be 9v, not 12v. And as far as I know Nimh have a pretty low internal resistance anyways.

 

[jbeng]

yeah I could charge a cap, but it would still only be 9v, not 12v

How many volts do you think the average photoflash (which runs on 2 AA cells) stores in the capacitor? It's not just 3 volts. Its more like 200-300 volts. They accomplish this by using a DC-to-DC converter. That's the whine you hear coming from the flash unit while the cap is charging. A dc-dc converter is about the only way you're going to change 9 volts DC into 12 volts DC. The problem is, you probably won't be able to operate the solenoid directly from the output of the converter, as it won't be able to supply enough current.
As for the low internal resistance of the NiMH cells, where does that figure into the equation?
JB

 

[Someone Electro]

You can always try conecting diretcly if that dosent work try addidng a few cells.And if it still wont triger then use DC-DC converter.Dont complicate stuff uneserely.The more complicated it it the more can go wrong.

 

[Oznog]

It takes a decent amount of pressure to propel BB's. Are we talking old school shoot-your-eye-out metal ones, or 6mm plastic Airsoft pellets?

An Airsoft pellet can get decent speed out of lower pressures. Small metal BB's, not so much. Basically remember the force from 28 PSI is proportional to the area- a projectile in a 1 sq in cross-section area barrel will see 28 lbs of force, a projectile in a 0.1 sq in barrel gets 2.8 lbs. The smaller metal BBs see less force and are much denser too.

There are already electric airsoft guns. The motor pulls back an air cylinder on a spring, then lets it go and it compresses air rapidly. No air storage necessary.

Other notes:
A compressor may take a significant amount of current.
A problem you may run into is that you're dumping all the air you've compressed which is terribly inefficient. Like, ideally you'd want to only dump air close to 28 psi and then just stop the flow when the projectile leaves the barrel and the pressure's still at 25 psi. If you dump it all, then it doesn't make it go any faster but it takes forever to rebuild the pressure from 0 psi.

 

[stevez]

The speed at which a solenoid valve operates is likely to be related to the amount of power applied but if it's pilot operated, as many are, the solenoid only opens a small passage - air pressure operates the larger valve.

Back to the solenoid - applying too much current will simply the coil. They do make valves that operate really fast but they aren't common and are expensive. You might want to do some experiments to see if the solenoid you have will operate fast enough for your purposes with a large supply that can deliver all the current the solenoid wants. If that doesn't work then it won't work with a battery.

An alternative might be to use the compressed air to push the piston back into the cocked position and use an electric "trigger" to release a mechanical piston.

As suggested, the 28 psi might or might not be enough depending on the projectile and your requirements. You might make some assumptions - that the bore is frictionless and that you have 28 psig as the projectile moves down the bore - see how fast it might go. That's a pretty direct calculation as I recall.

 

[evandude]

yeah, bear in mind that a "normal" bb gun which runs on those 12-gram CO2 cartridges is running at a much higher pressure... CO2 cartridges are normally hundreds of PSI. That's a far cry from 28.