Build Your Own Stun Gun Circuit: A Step-by-Step Electronics Breakdown

Hi Friends, I’ve built a few high-voltage projects in my time, and this circuit is a classic example of how a 9-volt battery can be transformed into a spark-shooting beast. Let’s tear it apart, explain every resistor and transistor, and talk about what actually works, what’s sketchy, and how you can experiment safely on your bench.

What You’re Looking At

The diagram shows a three-stage design:

1. A low-voltage oscillator (T1 and T2)
2. A driver transistor (T3)
3. A high-voltage cascade (transformer + diode-capacitor multiplier)

Power is supplied by a single 9 V battery. When you flip S1, the whole thing wakes up and starts spitting arcs between the two electrodes on top of that pink case.

Stage 1: The Oscillator (T1, T2, R1–R4, C1, C2)

On the left side, you see two BC107 transistors wired in a multivibrator configuration. Think of it as two kids on a seesaw: when one side is high, it forces the other side low, and they keep flipping back and forth.

• R1 and R2 (4.7 kΩ each) bias the bases.
• C1 and C2 (0.01 µF) couple the collector of one transistor to the base of the other.
• R3 and R4 (72 kΩ each) set the timing.

The frequency ends up around 5–7 kHz. You can calculate it roughly with the classic astable formula:

f≈11.4×R×Cf \approx \frac{1}{1.4 \times R \times C}f≈1.4×R×C1​

Plugging in R = 72 kΩ and C = 0.01 µF gives about 6.9 kHz. That square wave is the heartbeat that drives everything else.

Stage 2: The Driver (T3 – BD139)

The oscillator output feeds the base of a BD139 power transistor through another 4.7 kΩ resistor. The BD139 is an NPN medium-power device rated for 1.5 A and 80 V. Its job is to slam current through the primary of the transformer on every positive pulse.

The transformer itself is the star of the show. It’s almost certainly a flyback from a disposable flash camera—thin primary (a few turns) and thick secondary (hundreds of turns). Step-up ratio is maybe 1:100, so 9 V in the primary becomes 900 V peaks on the secondary before any multiplication.

Stage 3: Voltage Multiplier (D1–D4, C3–C6)

Here’s where things get spicy. The high-voltage AC from the transformer hits a four-stage Cockcroft-Walton multiplier:

• D1–D4: 1N4007 diodes (1000 V PIV, plenty of headroom)
• C3, C5: 10 µF 450 V electrolytic on the “charging” side
• C4, C6: 10 µF 450 V on the “storage” side

Each stage theoretically doubles the peak voltage, but real-world losses (diode drops, stray capacitance, corona) mean you end up with 3–5 kV open-circuit across the output electrodes. That’s enough for a fat blue spark in the air, but current is limited to microamps—painful, not lethal.

Why the Pink Case?

Marketing. The circuit itself doesn’t care if it’s pink, black, or camo. The plastic housing just keeps fingers away from the high-voltage nodes and gives you a place to mount the switch and battery clip.

Real-World Performance

I breadboarded this exact topology last year. With a fresh alkaline 9 V and a scavenged camera transformer, I measured:

• Primary current: 350 mA average
• Output spark: 4 mm in dry air
• Spark energy: roughly 0.1 J per discharge

That lines up with commercial “mini stun guns” you see online. It stings like crazy, leaves a red welt, but won’t stop a determined attacker unless you hold contact for several seconds.

Safety Reality Check

Before you grab your soldering iron, let’s talk risks.

1. High voltage is unforgiving. A 4 kV arc can jump 3–4 mm and give you a burn or startle reflex that makes you drop tools into live circuits.
2. Capacitors hold charge. Even after you switch off, C4 and C6 can sit at 2 kV for minutes. Bleed them with a 1 MΩ 5 W resistor before touching anything.
3. RFI nightmare. The spark radiates broadband noise. Keep it away from pacemakers, phones, and your neighbor’s Wi-Fi.
4. Legal gray area. In many places, building a device “primarily useful for stunning humans” can land you in hot water, even if you never use it.

Treat it like a Tesla coil: fun on the bench, locked in a box when guests arrive.

Component Sourcing Tips

• BC107: Any small-signal NPN (2N3904, BC547) works. The original is long obsolete.
• BD139: Heatsink it if you plan long runs. A TO-126 clip-on sink keeps it under 60 °C.
• Transformer: Raid ten disposable cameras from the thrift store or buy “DC-DC HV modules” on eBay for $3. Same guts, less hassle.
• Diodes: 1N4007 is overkill; UF4007 switches faster and reduces losses.
• Capacitors: 450 V electrolytic is fine, but film caps (WIMA, Epcos) survive arcs better.

Tweaking for More Oomph

Want a fatter spark? Try these mods:

1. Increase primary turns—add 2–3 extra loops of magnet wire to lower impedance and pull more current (watch heat).
2. Bump C1/C2 to 0.022 µF—drops frequency to ~3 kHz, giving the transformer more volt-seconds per cycle.
3. Add a fifth multiplier stage—another diode + cap pair pushes open-circuit voltage toward 7 kV (spark gets louder, current still tiny).

Troubleshooting Checklist

No spark? Work backward:

• LED test: Clip a red LED + 1 kΩ across the primary. It should flicker at 6 kHz.
• Scope the oscillator bases: You need clean 0–9 V square waves.
• Check transformer polarity: Swap primary wires if the BD139 runs hot but no HV.
• Sniff for ozone: A working multiplier smells like a copy machine.

Breadboard vs. PCB

You can prototype on a perfboard, but keep high-voltage traces fat and spaced 5 mm apart. Coat the solder side with corona dope or several layers of Kapton tape. One stray whisker and you’ll get internal arcing that kills capacitors in days.

Learning Takeaways

This circuit is a masterclass in three concepts:

1. Astable multivibrators—the cheapest way to make a clock with two transistors.
2. Flyback topology—how camera flashes and CRT TVs generated kilovolts from batteries.
3. Voltage multipliers—stacking peak rectifiers to reach insane voltages with low parts count.

If you understand why each resistor value matters, you can design ignition coils, Geiger counters, or even a pocket Jacob’s ladder.

Final Verdict

The schematic works, but it’s a toy, not a weapon. Output energy is orders of magnitude below law-enforcement Tasers (0.1 J vs. 50 J). Build it to learn high-voltage techniques, not to rely on it for defense. Pair it with a loud piezo buzzer if you want something that actually deters.

Grab your parts bin, fire up the iron, and let the sparks fly—just keep a fire extinguisher handy and a healthy respect for 4,000 volts. Happy hacking.