Make A Cheap (And Dangerous) Automated Flamethrower

Nothing lights up the night like a quick blast from a flamethrower, but there is a reason why you can’t buy them in the Halloween decoration aisle at Target. They are dangerous, for fairly obvious reasons. [Erco] seems to have no particular fear of death, though, and he shows how you can build a simple flamethrower with a small candle, a servo, Arduino and a can of hairspray. Tresemme Extra Strong Hold, in particular, although we don’t think the exact type matters that much. All he did was to mount the candle in front of the hairspray, then mount the servo so the arm presses the spray head down. The candle does the rest, lighting the highly flammable propellant in the hairspray to produce the flamethrower effect. [Erco] is using four of these, which are co-ordinated to fire in time with music.

This one seems a bit risky. Servos have a habit of locking, and there is nothing stopping these from locking in the open position, or sticking there if the Arduino crashes. A relay or other switch that reverts to an off position when the power is removed would have been more suitable here. Secondly, there is no emergency off switch. [Erco] has wired the Arduino up next to the flamethrower itself, so you are going to have to reach in to disconnect it. That is risky enough, but he also tried a 4-way configuration that would have been impossible to disable in the event of a problem (shown in the accompanying images). Thirdly, there is no fire protection between the can of hairspray and the open flame, so if the spray head melts or fails from the heat, it’s game over. Finally (and most importantly), where are the fire extinguishers? We’d like to hear how you’d build this with safety in mind. Let us know in the comments below.

We’re big fans of flames and explosions: we’ve have seen a couple of Survival Research Laboratory shows and were blown away by their destructive pyrotechnics. But, as SRL head Mark Pauline said in a recent talk, “when things blow up at an SRL show, it’s on purpose”.

21 thoughts on “Make A Cheap (And Dangerous) Automated Flamethrower

  1. The trick to dangerous hacking is to quit while you’re ahead. I made an 8kv MOT power supply at age 16 and it wasn’t until college that I learned all the reasons that it was a terrible idea and decided I was done with high voltage and flames.

    1. The trick to dangerous hacking is to know what you’re doing, anticipate potential problems, and take appropriate safety measures.

      That 8kV supply – it was galvanically isolated, yes?

        1. You are underestimating the MOT – 100 to 200mA are only just lethal. . A normal MOT has a working current of at least 500mA@2kV. The short circuit current while arcing must be around 2A – 2 MOTs, giving 4kV drew 35A@230V .
          Normally MOTs are not galvanically isolated as the HV winding is grounded.
          So for 8kV you need 4 MOTs, but at least two of them MUST NOT have the HV winding grounded. And this is an absolutely deadly amount of power, if you touch it.

  2. I’ve been trying to do this safely and easily for years!

    Start with one of the automatic air freshener things that spray every minute or whatever. You can find a point on the small control circuit that will generate a spray, usually with a pulse to ground.

    The ignition is not as straight forward – I’ve got one of the sparkfun Spark Gap Igniters – https://www.sparkfun.com/products/retired/11218 – but I haven’t put the two together yet.

  3. Aside from the issue of a flaming jet of hairspray, it’s not insanely dangerous.
    Even if, as suggested in the article, the hairspray valve melts open it’ll just vent the contents of the can in a pillar of fire. It’s not gonna explode. It’s just propane, natural gums, and some scented alcohols. It’s not a thermobaric grenade.

    At worst you’ll get a spinning flame thrower that may skitter around a bit as it vents.
    Not an experiment to do in your garage or basement, but not life threatening either.

    1. Is there no chance of the flame going into the bottle at some point? Forgive my ignorance but I remember everyone saying this is a way to lose a thumb.. although this was around the time that me and my friends used to light hairspray all the time and we kept our thumbs..

  4. OK nerd time – four flame sources and the first thing I thought was “X-wing afterburners”…

    Take 4 small motors, add fan blades. Replace the hairspray with USB powered humidifier, then feed those with the fuel base of your choice. Ignite the fuel mixture as it leaves the fan engine.
    If the engine actually produces enough thrust, make a centrufuge with the fuel reservoir at the centre, gavity feeding fuel to the spinning, burning jet.

      1. I was contemplating kerosene, turpentine and methylated spirit [wood alcohol] as potential candidates. If the motors blow hard enough for a venturi effect, you don’t even need the atomizers.

  5. Had a house mate tell me a story about his 16-yo self trying to sell wind-up fire alarms (spring had bi-metal release). Family was primed – another part of the family had died in a house fire. The sales pitch included a can of hairspray and lighter to trigger the alarm. He did the demo (in the living room). Family did not look at him, but to his side – at the (now carbonized) bird in the cage. He just packed up and left quickly. It was the only sale he made that summer.

    Be careful with this thing. I have the most respect for the burn medical folks.

  6. I used an old kitchen sieve to protect the spray can from the fire. works perfectly fine :-)
    Instead of a servo I used an old car lock actuator that gets turned on by a relay and automatically releases pressure when powered off.

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