EMPi – Raspberry Pi

A raspberry Pi that will stand the most intense electromagnetic and electro static environments and cope with EMP (Electro Magnetic Pulse). The sort of environment around tesla coils.


I started with a Pi and a Adafruit Pi-Plate. My first job was to create a optic-fibre link (115Kb/s) for comms to my tesla coils and it also doubles up as a remote and insulated tty console for the PI. The chips I used for the fibre are the Avago 1524/2524 1Mb/s fibre transmitter/receiver.

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The transmitter is driven from a Microchip TC4428 which takes the 3.3v TX to 5V and gives the current drive for the TX LED .The RX is level shifted and inverted using a BC337 (update – a speed up cap was added to the base as at 115kb Rx errors were seen)


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I realised that the Pi would have to run on its own power supply, so 6 rechargeable AA cells giving 7.2V were added, along with a 7805 regulator to give 5V for the Pi and a couple of diodes to allow the batteries to smoothly take over when the external power was removed.

As the power connector is open to the EM noise on the out side of the case, there is a diode , a cap and a 30V TVS across the power input.

It also became apparent that there was no way of cleanly switching off the Pi without connecting a network or terminal. So I added a button and a script to shutdown the Pi (although not cut the power).

A Pi Cam was attached and bolted to the case, A small hole lets it see the outside world without letting in too much radiation. I also added in a sedcond button which allows the camera to be turned on and off. The script makes the camera take photos every 10 seconds. (I hope to add video too)














I added a removable panel that cover’s the USB, Ethernet and Fibre ports. This is not ideal as I can’t close it fully when I’m running on fibre.

I also added a shutter that goes across the external power/charging connector to further protect the circuits.

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A quick test proved that the Pi can run when sat on top of a small tesla coil.


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The camera also worked, but the breakout was too close for the camera to give good photos.








A monocle (50mm lens with 50mm focal length) was added







Giving a couple of goodish photos of the discharge from under the breakout point. (breakout is the wiggly black line to the LHS)




Tesla Quotes

Quotations from the great man himself

“Invention is the most important product of man’s creative brain. The ultimate purpose is the complete mastery of mind over the material world, the harnessing of human nature to human needs.”

“What one man calls God, another calls the laws of physics.”

“The scientists of today think deeply instead of clearly. One must be sane to think clearly, but one can think deeply and be quite insane.”

“If your hate could be turned into electricity, it would light up the whole world.”

“I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success . . . Such emotions make a man forget food, sleep, friends, love, everything.”

“All that was great in the past was ridiculed, condemned, combated, suppressed — only to emerge all the more powerfully, all the more triumphantly from the struggle.”

“But the female mind has demonstrated a capacity for all the mental acquirements and achievements of men, and as generations ensue that capacity will be expanded; the average woman will be as well educated as the average man, and then better educated, for the dormant faculties of her brain will be stimulated to an activity that will be all the more intense and powerful because of centuries of repose. Woman will ignore precedent and startle civilization with their progress.”

“It’s not the love you make. It’s the love you give.”

“Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more.”

“I don’t care that they stole my idea . . I care that they don’t have any of their own”

“That is the trouble with many inventors; they lack patience. They lack the willingness to work a thing out slowly and clearly and sharply in their mind, so that they can actually ‘feel it work.’ They want to try their first idea right off; and the result is they use up lots of money and lots of good material, only to find eventually that they are working in the wrong direction. We all make mistakes, and it is better to make them before we begin.”

“The human being is a self-propelled automaton entirely under the control of external influences. Willful and predetermined though they appear, his actions are governed not from within, but from without. He is like a float tossed about by the waves of a turbulent sea.”

How do tesla coils make music

I have been asked this question so many time I thought I aught to write down the answer.

Musical tesla coils are fairly common now and there are many videos of them working, but what causes the noise.

So lets start with the basics. Most electronic tesla coils don’t run continuously (Ill come back to CW coils later). To maintain a large spark output current in the primary circuit is key, to get the best out of the components the coils are driven for short periods of time, at high currents, quite often exceeding the continuous rating of the drivers.  During this short ‘on’ period, power is stored as in the primary and secondary coil. As soon as the power is turned off the stored energy is transformed into a high voltage high frequency pulse. It is this pulse that you see as the discharge from the top of the coil.

Creating hot plasma in the atmosphere has an effect, the air heats up to thousands of degrees C in a microsecond or so. This expanding air causes a sound wave that you hear as a click. (or a bang with real lightning, its just a matter of scale.)

So, if you repeat this process at say 263 times a second, you will hear an audio tone at middle C

Music can be played by controlling the length and frequency of the pulses you send to the coil. With these coils getting the frequency and length information is relatively simple from a MIDI audio source via a suitably programmed microprocessor.

Due to the pulsed nature of the coil real audio (e.g speech,singing etc) cannot be rendered.

OK, I mentioned CW (Constant or Carrier wave) Tesla coils
With CW coils the power isn’t interrupted they are driven constantly. Their output is essentially silent (You do get a hiss at lower frequencies due to variations in the spark output). To get music from these coils you vary the power going in to them in a very similar way to an AM radio transmitter. There is two ways of achieving this.
1) You control the voltage to the driver therefor controlling the power
2) You vary the frequency of the drive (FM modulation), The tuned circuit formed by the secondary gives maximum output only at the central frequency, so the effective output power is varied as the frequency is varied.

As the power is continuous, you can’t gain any power advantage by short pulses and neither do you get the voltage gain by the release of the stored power in the secondary so the sparks are smaller than a pulsed coil.

But, you can render speech, singing and any instrument can be played due to the almost radio style modulation. Plasma speakers are made using this technique.

Violet Ray Machine – Warning – Get it CHECKED before use.

I see many Violet ray machines on Ebay (and other sites) and haven’t really thought about the dangers, until I bought one that is.

Of course, the one I bought was cheap. To be fair to the seller someone had cut both the power cable and the wand cable before I bought it. That action probably saved someones life. 1. Because it was cheap on ebay, so I bought it and 2. If some one else had bought it, they may not have checked it before use.

The device, like many on sale on the internet is nearly 90 years old. The internal construction is wood and the primary insulation of the internal wiring is rubber. This rubber had over time become brittle and in many places fallen off.

Here are the pieces of cable I have replaced.

They are all suffering from noticeable wear!. The wires from the capacitor were for the most part bare.

The cable from the wand was wired with only two wires. Considering that the third is the “earth” and that the high voltage developed by the wand would have traveled back via the other two cables (one is connected directly to neutral the other via a capacitor to live) the sparks from the wand could easily have been backed with 240V

When properly repaired these devices can be used safely, but GET THEM CHECKED, not just by an electrician, but by someone who knows about high voltage and tesla coils. There are a number of possible gotchas in there.

Also, don’t run a vintage violet wand for more than 10 minutes. They were not designed to be continuous use devices, they WILL over heat, and when some of the components are insulated with wax, heat is not a good thing.

Stay safe and please pass this warning on.


More on the rebuild

Kay Induction coil toy

A toy from the London based Kay Sports Co (who made toys) from around the 1930’s

A small induction coil powered from a 4.5v battery with a pair of steel tube handles to give a shocking experience.

Advert from Meccano Magazine (found at Meccano Index) Jan 1933

KAY appears to have been taken over by S.E.L (Signalling Equipment Ltd) as this advert appears to show. Very similar product with a different logo. (from Brighton Toy Museum)

No.1700 Induction Coil (Signalling Equipment Limited, Potters Bar)
No.1700 Induction Coil (Signalling Equipment Limited, Potters Bar)



The manufacturing company J & L Randall of Potters Bar (Middlesex/Hertfordshire) made military equipment as SEL before moving into the toy trade. They are also known as Merit for more mainstream toys.