The basic principle is very simple: an audio signal goes in, it’s amplified, and then sent to a wire that’s wrapped a single time around a large coil. This single wrap is a rudimentary primary coil and the varying electrical flow in this (being created by the music signal) creates a much larger voltage in the secondary coil. This secondary coil is finished with a loose wire that sticks into the air, and a sufficiently large voltage is created in the secondary to produce a spark.
This spark heats the air around it, so as the spark comes and goes with the pulsing electrical current, this heating causes a vibration in the air. This vibration is the same frequency as the audio signal going in. In other words, it’s an elaborate speaker system.
The tesla coil speaker – also known as a Zeusophone or a Thoramin, after the Greek god of lightning and the Norse god of thunder – is widely available in kit form from direct-from-China electronics sites. We got a kit from Yi Ma Trading Company Ltd on Ali Express, though identical kits are for sale elsewhere. Our test kit cost just £4.96, including postage.
Our kit came as a PCB and a handful of through-hole parts. The instructions were in Cantonese, but it was easy enough to follow along as all you really need to know is which part goes in which holes in the PCB, and they were labelled in English on the board. There are heat sinks for two power transistors, but again, these were easy to attach. We’ve seen some similar kits advertised as coming with thermal paste, though ours didn’t, and it doesn’t get very hot under moderate use.
The only thing that the kit didn’t come with was a power supply. It takes a 15-20 V barrel adaptor (the same shape as an Arduino Uno). This can also be supplied through headers soldered into the PCB if you’ve got a bench power supply.
Connecting a music player to a device capable of producing enough voltage for this spark is a little nerve-wracking – especially since we’d soldered it ourselves with the instructions written in a language we don’t read. We didn’t want to risk frying our phone, so we found an old MP3 player in the back of a drawer and fired this up. A small (approximately 5 mm) spark danced from the end of the loose wire on the coil. You’ll need to dim the lights to fully appreciate the majesty of the raw electricity flicking to the tune of your favourite song. It’s not compulsory that you play Electric Six’s ‘Danger! High Voltage’, but it is strongly recommended.
The sound produced is audible but quiet. It’s just about possible to make out the tune being played, but from an audiophile perspective, it’s just about the worst quality sound output we’ve ever heard. That’s not really the point though – this is music from lightning and that’s cool at any quality.
The main issue with the kit is that it’s made of weak parts. The secondary coil is made of very thin wire that is delicate when soldered onto the board. The tube that the coil is wrapped around doesn’t attach to the board in any way (unless you glue it down), so this joint is very exposed and vulnerable to breaking if the tube is knocked. We found that hot glue didn’t adhere well to the PCB surface and we had to glue all the way to the edge of the board in order to get a solid joint (see images). Some other adhesive may work better, but this produced a strong, if unsightly, joint.
The power socket failed after several uses. The flimsy metal connector that had been slightly bent in transit snapped in two, despite gentle handling. This is fairly easy to replace (and there are additional points on the PCB to add wires to supply power), but it’s another source of annoyance. For the price we paid, we can live with the fact that it’s prone to falling apart as it was a fun project to make and even with a few uses we feel we got our money’s worth. However, it does make it a little hard to recommend this to hackspaces and clubs unless you’re willing to take some measures to toughen it up.
Obviously a certain amount of caution is necessary with a kit like this. It will output plenty of electromagnetic interference, could cause a fire, and generally needs treating with the respect any high voltage project requires, but for anyone with the ability to use one of these safely, it’s one of the most impressive electronics kits around.