Is this safe?
Just in case someone tries this and burns something down, I must give a disclaimer that I hold no responsibilty to any damages caused by attempting this! You should take the following precautions:
- Ensure the environment is not too dry and will set on fire with the slightest amout of embers.
- Bring a fire extinguisher.
Principle of Operation
It is quite well known that steel wool, especially the finer grades, will ignite and combust when they are shorted with a 9V battery. You should experiment with this first if you haven't tried it yourself. Any grade finer than #0 will do, the finer the faster and more vigorous it burns. However, this means a shorter burn time, not necessary what you want.
You can easily attach steel wool to a drone and simply run up the drone with a 9V battery and brush the steel wool to get it to burn. However, in the interest of safety and usability of other applications, I developed a remote ignition system called the 'Fireshaker', which is what this article is about.
Components
The Fireshaker is composed of two parts:
- The bulb: This holds the steel wool
- Driver circuit: Electronics that control ignition
The driver circuit connects to the radio receiver via a RC switch (to translate between PWM and on/off). It also connects directly to the drone's battery.
The Bulb
The most common item used to hold steel wool as it burns is a whisk, hence why the bulb resembles are whisk. The only additional feature are that two of the rails have electrical connections. When switched on, one of them is connected to the +ve side of the battery and the other to ground. Have not experimented on whether direction affects the quality of sparks.
You'll need:
- 2mm thick steel wire (if thinner, the lost of rigidity adds the risk of rails shorting out) about 200mm long
- Small section 10-15mm thick wood
- Electrical cable (choose a gauge used normally for power systems)
To start the build, cut the base of the bulb:
- Cut the wood into a circular piece approximately 40mm in diameter.
- Drill 6 holes around the perimeter.
- Strip a decent amount of sheathing off electrical cable.
- Wrap the copper part around the steel wire.
- Wrap a little bit of plastic sheathing around the steel wire as well.
- Bend the steel wire to clamp down on the electrical cable, ensure it clamps down onto the plastic sheating for a more durable connection.
- Finish the connection by soldering the copper wire onto steel (yes this does work).
- Repeat on another steel wire.
Attach the rails onto the base:
- Feed the steel wire that has the electrical cable attached to it through one of the holes on the base.
- Bend it into a loop then feed it into the opposing hole.
- Repeat for the other holes.
- The end result should look like:
If you use screw terminals on the driver circuit, you don't need to solder any connectors onto the electrical cable of the bulb. However, I used bullets.
Driver Circuit
This is the schematic of the driver circuit:
Which I built using verobard:
Then heat shrinked over to prevent accidental short circuit:
It's split over two heat shrink tubes because I didn't have one big enough to fit over all of it.
The large 1ohm resistor is used to limit the max current in the circuit in case the resistance of a blob of steel wool becomes too low. Without it, the steel wool system can draw too much current from the battery. This will cause the whole system to overheat, but your biggest concern is that because the battery is so overworked, it can no longer supply enough current to keep the drone flying. More details on how to build to electrical system can be seen in my 'Build' video for this device:
Attaching onto the drone for best sparks
Attach about a foot it beneath the props to maximize the airflow due to the drown draft of the props. Of course, if you plan to attach this to a fixed wing, you can make the whole system burn brighter just by flying faster (on second though, I think this system will look way better on a fixed wing).
How well did it work?
My first prototype didn't have the 1ohm resistor. So the relay blew up during ground testing (thank god it was only ground testing). I didn't pay enough attention to how much current the system was drawing because I though that if a 9V battery was sufficient, I wouldn't have to worry about it. After adding the 1ohm resistor, I had a 100% igntion success. However, the steel wool didn't suddenly all burst into flame at once like I expected. You can see the Uavdude episode about this here:
In terms of improvements, I'd probably put steel wool on a reel and basically have a fully automatic spark gun!
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As you know, this was shot in the winter away from dry trees. Steel wool is relatively safe because it burns so quickly and the fact that the combustion products are solid means that it doesn't tend to float away.
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