How to Make a Coil Gun

Did you always wish you had a gun? Well just make one for yourself!

Somewhere in the video I mentioned it was a “Rail Gun”, but it is actually a coil gun. They both work with magnetic force, but differently.

Here I describe the detail design of the coil gun, so you may turn away if you hate details:

Basically as we all know, when we turn on or energize a coil, it starts attracting ferromagnetic material, in my case a steal bullet I made.

The issue is that if we keep the coil on, it keeps attracting the bullet. So what happens is that the bullet accelerates towards the coil and as soon as the magnetic center of the bullet reaches the magnetic center of the coil, the force of the coil over the bullet becomes zero. But as the bullet has momentum, it passes the center, at which point the coil starts pulling it back and decelerating the bullet, or accelerate it back towards the center.

You see in the video that this results in the bullet oscillating as if it was attached to a spring, and eventually stops at the center.

So the best way to make sure the bullet reaches maximum speed is to turn the coil off as soon as it reaches the magnetic center of the coil. If the coil and bullet are both symmetrical, their magnetic center will be the same as their physical center.

There are two ways to do it: one is to perfectly time the turn-off moment of the coil, or another easy way is to use a sensor and detect when the bullet reaches a sweet spot to turn the coil off.

In some designs they just use capacitors and discharge them in a coil and the capacitor runs out of charge quick. This way the coil won’t have time to pull the bullet back, which is sort of like timing the coil.

The problem with all the timing designs in my opinion is that it is not possible to easily predict the required time to keep the coil on, as it depends on the bullet mass, coil field, original distance of the bullet from the coil and etc… and therefore the coil either turns off too early or too late, both resulting in a slower bullet.

So I believe to make the on time of the coil independent to all these factors, the best way is to have a sensor and detect the location of the bullet and turn it off on time.

In my design, to turn the coil off I placed an infrared (also known as optical) sensor to detect the tip of the bullet right where their centers meet, and this would turn the coil off using a simple circuit.There are other options for the sensor such as Hall Effect sensors, which can be much faster. You need a fast response time as any delay in response will cause the bullet to pass the center and result in some deceleration. Especially when you have many coils rather than one, the speed of the bullet in the final stages is super fast.

The following circuit is what I designed and made.

Circuit Schematic of the Coil Gun

Circuit Schematic of the Coil Gun

In the circuit above I have added D1, which I forgot to show within the video. Not having this diode caused me grief and explosions which I didn’t capture on video!

This circuit is just a quick way of doing it for demonstration that came to my mind. It is by no means the best way to do it. Especially since the infrared sensor I have has limited response time (3kHz).

I purchased the parts from Digikey and here’s their part numbers for your reference (you may choose your own):

OP1: EE-SX1070 from Omron
Q1: IRF7739L2TR1PBF from International Rectifier
Q2: Generic 2N3904 NPN Transistor or similar
D1: Generic Schottky Diode Capable of >30V, >10A
SW: Generic push button, normally open
R and C: Generic
L1: Hand wound

Here’s the circuit description:

Q1 transistor is basically the switch that turns the L1 inductor on and off. When Q1 turns off, L1 will try to release the energy stored in it by continuing the current in the same direction. Without D1 this would cause the voltage at the drain of Q1 to jump to a huge number breaking Q1 and shorting it. This will short the supply through Q1 and will result in exploding Q1 and damaging the coil. Having D1, when Q1 turns of D1 turns on closing the loop of the current and protecting Q1. Another side effect is reversing the magnetic polarity of L1 that could help accelerating the bullet.

When the switch SW closes, it sends a high level voltage through the 1uF capacitor to the gate of Q1 turning it on. The 1M ohm resistor will help discharging the 1uF capacitor.

OP1 must be biased such that when the sensor is not blocked, the transistor output is at a low voltage below 0.4V to make sure Q2 is off, and when the bullet comes into the sensor blocking the infrared light, the optocoupler transistor output jumps above 0.7V turning the Q2 transistor on.

So Basically when the switch SW is pressed, the gate voltage of Q1 jumps, turning it on. This will attract the bullet into the coil and as soon as the bullet reaches the sensor, Q2 turns on pulling the gate of Q1 low and turning it off.

And it works as good as you saw in the video. There are some issues with this design though. If there is no bullet inside and the trigger is pressed, the gate voltage of Q1 drops as the capacitor at the gate is discharges by the 1M Ohm resistor. At some point this results in increment of the Q1 drain-source resistance, meaning a high power drop across Q1 that will blow it up. So there should always be a bullet. We can’t remove the 1M Ohm resistor because without it the gate voltage may never return to 0 volt.

Like I said, this circuit is only for demonstration and a better and faster circuit could work much more reliably. But the functional concept remains the same.

Now more coils can be put together in series, everyone with its own fast sensor. The bullet can be smaller and more light weight to help accelerating faster. Also there can be different ways to turn the series coils on and off. For example:

– They can turn on and off one at a time, as soon as the first coil turns off, the second one turns on and so forth.

– They can turn on two at a time. For example if we have 4 coils from L1 to L4, L1 and L2 can turn on together. As soon as the bullet reaches between L1 and L2, L1 turns off and L3 turns on, This way there can be better acceleration, but more power usage.

– All coils can turn on together, but turn off one at a time as the bullet passes them.

And the rule of thumb is: the more number of winding and more power over coil (voltage and current) the greater acceleration of the bullet. Also of course turning the coil off at the perfect spot is very important too.

I hope you like what you read. If not, please drop a comment and I will write some update when I can. Thanks and stay safe!