Category Archives: Blog

Tesla Coil Tuner

First light of a Tesla Coil tuner.

Old TC TunerSo I needed to get a better Tesla coil Tuner. I have been using this NE555 pinger for 10+ years.

Always said I’d put it in a box.. Never happened…

With some of the tuning problems I’ve had recently I needed a better solution.







PIC powered USB controlled Tesla Coil Tuner using AD9850 DDS Frequency Synthesis.

Built on a PIC low pin count, USB development board (hence redundant RS232 connector)

Direct connectiontctuner to the Primary Coil with current measuring.

Three ways of measuring.

Primary and cap on its own(Secondary removed)

Secondary on its own (driven by primary with no Primary cap)

Primary Cap and Secondary (across spark gap) – Should see both current dips.


Tesla Coil Tuner - sec only

First plot of a small Secondary coil, using VB to drive the USB.

Plots at ~15 seconds for 1000 points (e.g will scan from 500Khz to 1.5Mhz in 1khz steps in ~15 seconds.)


Range 50Khz to 4Mhz (with some droop at each end)

I’m having some issues getting a plot with just a primary coil, not sure why at the moment. Also my driver (tc428) is damaged, its not driving correctly to ground. Which may be causing me some issues. But not bad for a first go.

More developments

Added an Ariel input (B)

Improved the display, and removed a whole load of noise on the traces.

Also, added the ability to take snapshots of the traces (pale colours) so you can see any change from scan to scan

Tesla Coil Tuner - sec-pri
This shows a current scan in Dark blue, with a snapshot in light blue and the Ariel input in dark green with a light green snapshot. The current scan was done with my hand near the topload hence the lower resonant frequencies.

This clearly shows the primary (C & L) as a large dip on the left and the secondary / topload as the smaller dip on the right. I’m feeding the primary coil and primary cap in parallel as this gives the best indication of the current change in the primary. For some reason driving the coil with Cpri in series shows a lumped resonance (or doesn’t show the primary resonance up at all)

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.


Mercury Rectifier – Cambridge Museum of Technology

After many years of sitting in a rack gathering dust, one of the volunteers at Cambridge museum of technology has resurrected their mercury rectifier.

It used to run a large DC Motor from the new-fangled AC mains. But this was soon superseded by a replacement.

The Rectifier has lain dormant since at least the 70’s. The Museum has managed to get a supply to the anodes and enough current via it’s ignitor circuit to get the tube lit up and working (although at much lower power).

A fantastic achievement that shows the engineering (and beauty?) of these rare devices. Much better than an odd shaped dust covered glass thingy in a rack.