Coil Gun - Optical Trigger Circuit


Introduction: Coil Gun - Optical Trigger Circuit

I designed this coil gun optical triggering circuit,  this is used in conjunction with and IR led and IR Photo Transistor  mounted on the barrel.   When a projectile is heading down the barrel as soon as it passes through the beam it will trigger the next coil to fire further accelerating the projectile.  This was designed for a coil gun but I am sure it could be used in many other ways.

The part of the circuit which handles the triggering is the op-amp  IC1 which is used as a comparator,  pot R4 is used to adjust the comparison voltage going in to the op-amp so you can set the level at which it will trigger.   If this was all that was in the circuit it would still work and we could use it to fire the coil gun however the coil would continue to create the magnetic field until the capacitor was empty in some cases this is not desirable, we need to be able to turn it on and off at an exact time.

You will see in the circuit I have also included a 555 timer in a one shot configuration,  Ill explain that later but first I want to go in to a little bit about what we want the coils on our gun to do.   When we fire a coil an intense magnetic field is created around the coil however when we shut the coil off the field does not immediately stop its slowly decays due to eddy currents,  if the projectile has passed the midpoint of the coil and the magnetic field is present it will then attracting the projectile backwards slowing it down and reducing its velocity. In the ideal world the magnetic field would be completely dissipated as the projectile reached halfway through the coil allowing it to coast without resistance through the barrel.

The coil can be dampened with a resistor to help the magnetic field collapse as soon as possible but this may not be enough to get the perfect timing.  That is where our circuit come’s in there are two things we can do with this circuit the first is we can control when the coil fires,  we want this to happen as soon as the projectile is in the area of  influence of the magnetic field generated by the coil this can be adjusted by moving the position of the Ir trigger closer or further away from the coil.

The second thing we can control is how long we provide current to the coil if timed correctly we can make sure the coil is shutoff to give enough time for the magnetic field to decay before the projectile reaches the other side of the coil.   This is a really simple process all we do is set the length of time the coil is on using the pot on R7.  How long should the time be ?,  well you could make a ghetto chronograph using two of these optical trigger modules and micro controller this could measure the projectile velocity then knowing this and the distance between the trigger and the coil you could work out the rough time the coil needs to decay in  and then use a scope to make sure the coil is decaying in this time.

Sound complicated right, well it is and their is a much easier way.    Using the same ghetto chronograph described above you could simply step through multiple time settings on the 555 timer from fast to slow and then see which setting gives the projectile the highest velocity and bingo its set perfect no tools no scope nothing.  Once you have the one coil dialed in then add another coil for higher velocity the same setup applies,  in all my coil gun design’s I am trying to make a modular system so it is expandable to any size gun you like.

You can download the Gerber and Drill Files from my blog - to make your own pcbs.

If you dont have the time or means to make a PCB, This Blank PCB is avaliable at



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    And do you think the circuit would be triggered by a cal 4.5 mm steel BB traveling through a 5 mm (7 mm outer radius) acrylic barrel? Or is it possible, that the projectile would be to small and fast to be measured?

    Neat instructable! I'd like to buy one of your blank PCBs for a powerful coil gun (i live in Denmark), but my avast! blocks your site for some reason. What to do?

    What is a good site to order the components for this from?

    what is the size/material of projectile that you use with this? how far do they go?

    1 reply

    I have not built the full gun yet, however I will be using projectile .5" diameter and approx 20mm long. I will post more info when complete.