Setting Up a Coilgun Demonstration for a Science Fair Project
Most kids building a Gauss rifle for their science fair end up with something that either doesn't fire consistently or burns out their MOSFETs on the third shot. I learned that the hard way when my judge walked away from my booth because the projectile kept veering left and I couldn't explain why without pulling out a schematic that would've taken ten minutes to draw. Start with the coil geometry before you buy any components. The standard setup uses five to seven coils spaced at increasing distances from each other. That's not arbitrary — it accounts for the projectile accelerating through the barrel. If you space them evenly, your timing gets off after the second stage and you lose efficiency fast. I ended up measuring spacing with a caliper and adjusting based on the estimated velocity after each stage rather than guessing. The power supply matters more than people realize. A 12V lead-acid battery sounds fine until you try firing and the voltage sags to 9V under load, which drops your magnetic field strength enough that weaker coils can't pull the projectile through. I switched to a dedicated 12V wall adapter rated for at least 5A and the consistency improved dramatically. Your coils need to handle the current surge without heating up, so use 22 to 24 AWG magnet wire and keep your windings tight and even. Loose windings create resistance hotspots.
Timing is where most projects fail. You need photodiodes or Hall effect sensors to detect when the projectile passes each coil so your microcontroller can fire the next stage at the right moment. IR-based sensors are cheaper but sensitive to ambient light. Hall effect sensors cost more but work reliably in any lighting condition. I used four TSOP31238 IR receivers wired to an Arduino Uno and spent three hours debugging why they'd fire randomly during demo because the gym fluorescents were triggering false reads. Switching to a TLE4905 dual-axis Hall sensor for each stage fixed it completely. The PCB or breadboard setup needs to handle switching currents around 5 to 10A per coil. Regular breadboards will arc and melt under those conditions. I made my first board on strips and discovered the copper traces lifting off after about eight shots. Perfboard with heavy-gauge wire runs and a decent heat sink on your MOSFETs will get you through a full demo without anything failing. Use IRLZ44N logic-level MOSFETs instead of regular 2N7000s — the Rds(on) difference means less heat and faster switching, which matters when you're trying to fire a coil in under 10 milliseconds. Projectile selection isn't as simple as using any steel ball bearing. You want something magnetically soft with consistent mass and diameter. Standard 5mm ball bearings vary by about 0.1mm between batches, which throws off your timing calculations. I ordered C3-grade bearings from a bearing supplier and the precision difference was noticeable in the consistency of each shot. The projectile should also be non-conductive coated or you'll get eddy current losses that slow it down and generate heat in the barrel.
Documentation for the display board should include a block diagram showing signal flow from sensors to controller to MOSFET drivers to coils. Judges want to see you understand the system architecture, not just that you built something that shoots. Include coil resistance values, wire gauge, number of turns, and the timing gap between stages in your poster. I found that listing the actual measured velocities at each stage rather than calculated values looked more credible, even though measuring velocity required setting up two photogates and a stopwatch. If your budget is tight, skip the custom PCB and use a single-layer board with thick traces. The performance difference between a custom four-layer board and a hand-wired perfboard setup is minimal at this scale. What actually makes or breaks the project is the timing algorithm and the mechanical alignment of the coils. I've seen properly funded teams fail because their coils weren't perpendicular to the barrel axis, causing the projectile to graze the coil wall instead of passing through the center. A simple test is to drop the projectile through each coil individually and listen for a satisfying thump rather than a scrape. Power management gets overlooked in these builds. The capacitor bank between the power supply and the coils smooths out voltage sag during the high-current firing pulses. A 1000F electrolytic capacitor rated for 25V across each MOSFET source connection usually handles it without needing a massive supply. I added a 470F tantalum capacitor in parallel with the electrolytic and saw cleaner waveforms on my oscilloscope, though the practical difference in projectile velocity was probably less than 2 percent.
Get the Full Details
For the actual presentation, be ready to explain why you chose your specific timing method. Sequential firing with delay is easier to code but wastes energy. Sensor-triggered firing is more efficient but requires more hardware. Most judges won't ask about the physics, but if they do, you should be able to discuss the Lenz's law effect and how your timing accounts for the back EMF from collapsing magnetic fields. I kept a laminated sheet with the relevant equations at my station and used it when someone asked about energy efficiency, which came up exactly once during a three-hour judging period.
