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Another "school" coilgun

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LAN_403

_Eugen_

Thu Apr 12 2018, 08:42AM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58

This simple coilgun was made for my little 3-form son who had something like contest on physics at his school. The development didn't take a lot of time because all basic units had been tested before.
One-stage scheme commutated with MOSFET was used accomplished with LM78S040-based converter boosting up to 63V (which preserves safety enough to be handled by children). Principle scheme of EM-5 is given on fig. below.
Schoolgun

The controllable switch eliminates the need for converter-strobing circuits (which stop the booster during a shot) and power diode (which is conventionally mounted on caps). Only one indicative element is incorporated (D1 LED signalling that caps are ready for shooting). Any accu is suitable in range of 5...16 V and nominal discharge current 1-2 A (I used 7.4 V Li-Po, connected through backlighted toggle switch). RESET input of 555 timer is connected to simple R7-C7 circuit with the resistor parallelled by internal diode of U2 IC - this allows instant grounding of the timer's output at power switch-off, and delayed permission at switch-on - this prohibits unintended shots. The capacitor are six 2200 mkFx 63 V, connected in parallel.
All the components are assembled on two-side printed board of 50x70 mm.
The only regulation of the scheme is attenuation of the accelerating pulse duration by R6 potentiometer. The optimal duration depends on the length of projectile ( I used 5 mm nails sawn and grinded).
Accelerating coil has outside diameter 15 mm, inside one 7 mm and 14 mm in length, it is wound by 0.6 mm caliber copper wire. Its geometry and timing is calculated with a help of FEMM - this procedure is described here.
Total turns count is 112, resistance - 240 mOhms.
Current and voltage forms at shot are shown on a graph below. Maximum current value doesn't exceed 180 A which is allowable for IRLB4030 MOSFET.
VCF

_Eugen_

Thu Apr 12 2018, 08:49AM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58

Laser-cut plywood and plexiglass panels were used for the case. They were painted black before accembly, and the handle was additionaly covered with black "liquid resin".
683407707

The case assembly process is shown below. Only one strength element (steel profile) is used to connect upper and lower panels. As a result, total mass of the coilgun makes 620 grams, 420 of them is the case.
363516280

_Eugen_

Thu Apr 12 2018, 08:56AM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58

The projectiles as I said are pieces of 5 mm nails, their length is about 14 mm and mas approx. 2,2 grams.
They are accelerated to about 18 m/s which is safe for kids playing, but fast enough (you couldn't throw that bullets so fastly by hand ). Efficiency makes about 2,7 %, and about half of energy stored in capacitors is wasted per shot. Recharging takes about 5 sec.

As a result, we have nice small coilgun. My son was fully satisfied. I amuse myself by shooting EM-5 sometimes, too :)

More detailed description with more photos and videos is here.

Sulaiman

Fri Apr 13 2018, 01:12PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A very nice project, I'm sure your son is happy.

========================================= =======
None of my business, just a suggestion for similar projects by parents/guardians in future;

It looks like a gun and it fires projectiles - in many countries this could cause trouble

Do you really trust children to behave sensibly ?

----------------------------------------------- ------------------------------------
For less risk, may I suggest making an electromagnetic accelerator more as a tabletop device than hand held,
trying for heavier slower projectiles.
For fun it could look like a cannon or some other artillery.
Adding a tape measure, and protractor or similar, may encourage experimentation.

A table top cannon is definitely not as much fun as a gun,
but as none of the other kids will have an electromagnetic cannon, it will still be 'cool'

A few pairs of cheap safety glasses would be a good idea,
. we only get one pair of eyes per lifetime
. encourage safety
. putting on safety equipment makes thinking ahead more likely.
.

_Eugen_

Fri Apr 13 2018, 08:38PM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58

Sulaiman, thanks.
My experience shows that the most dangerous thing in coilgunning is not an accelerated projectile, but HV in caps.
As for this project, I used only 63 V just for the safety reasons. 18 m/s in the projectile are quite harmless, too.
Nevertheless, I completely agree with your warning.

AndreiRS

Fri Apr 13 2018, 11:49PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

Good to see that school is still not paranoid about guns. I like how it looks with plexyglass. Nice project.

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