555 Timer Based Plasma Speaker

This Instructable will show you how to make an audio modulated plasma speaker using a flyback transformer out of an old CRT display, and the all time hobbyist favorite which is the 555 timer chip.

Not only can this circuit be used to produce audio modulated plasma arcs but it functions as a high voltage power source for other projects.

I will be updating this Instructable over time.

Parts list:

1x Flyback tranformer

A flyback transformer, sometimes called a line output transformer are used in CRT TV's and computer monitors to produce the high voltage needed to power the CRT and electron gun. They also have other auxiliary windings built into them that power other parts of the TV.

You can get flyback transformers out of older CRT monitors and TV's. They are the ones that have a big chunky chassis. There are also other instructables on this website showing how to remove them from the chassis and circuit board.

1x Power MOSFET,
I used an IRF540 as that is all that I had lying around. I strongly recommended using a MOSFET with a higher drain-to-source voltage than the IRF540, which is only 100v.

Just for an example IRFP460 would be well suited for this and IRFP250N and IRFP260N would also work. Any MOSFET that is rated for high voltage, has a low on resistance and can take more than 15 amps would be fine.

1x Heatsink

You will need a large heatsink as it will get very hot (more on why later). The TV board you got the flyback from is a good source for heatsinks.

1x NE555 timer chip

I also recommend using an IC socket (8 pin) for the 555 so you can easily remove the chip without de-soldering it.

2x 47 ohm resistors

1x 22 ohm resistor

1x 470 ohm resistor

2x 50K potentiometers

3x 1nF capacitors

1x 220uF - 1000uF electrolytic capacitor (16v will fine).

1x 10nF capacitor

1x 100nF capacitor

1x fast diode, such as UF4007

1x NPN and PNP complementary bi-polar junction transistor pair (if you are following the first schematic). BD139 and BD140 can be used here.

Stripboard

12V power source

Fuse (recommended to protect the power source/supply).

Audio source (This could be an MP3 player or old phone for example).

Solder and soldering iron + some spare wire.

Disclaimer
I am in no way responsible if you mess up with this circuit. If you mess up, receive an electrical shock or burn your house down whilst making this circuit you have no-one to blame but yourself. By following this guide in order to make this circuit you agree to accepting all liability if something were to go wrong.

This project requires some knowledge on how to read circuit schematics. It is basically a 555 timer operating in astable mode, pulsing a MOSFET with a frequency and duty cycle set via the potentiometers.

I recommend using the first (BEST) schematic. You can use the second one if you do not have any BJT's (Bi-polar junction transistors) to hand.

I do not recommend the third schematic as it uses the same power supply for both the 555 timer and flyback and this can cause problems.

I also added a real world view graphical diagram showing how the 555 timer is connected.

Using some magnet wire, 26AWG or thicker, wind about 8-12 turns onto the core.

Do not use thin insulated wire as it will melt (I found out the smoky way!).

This circuit requires a 12v supply that can deliver at-least a few amps, which means that most types of battery's are useless for this except for larger lead acid battery's or a power supply that can deliver the current required.

AA cells will run down quite quickly. They can how-ever be used for powering the 555 chip if you are using separate power supply's. If you really have no other choice then use them, but the arc will not be very big.

9V /PP3 Battery are a definite no-no.

Lantern battery's Should last a good while, providing you limit the current draw by upping the primary coil turns.

12V Lead acid battery's are perfect for this type of thing.

A Power supply is good.

Once you have the circuit made you can now test it and see if you get any arc from the flyback. Tweak with the potentiometers until you get the best output (You might also have to switch the polarity of the primary coil).

The MOSFET can get VERY hot even with a good heatsink!

Simply connect up your audio source (such as a phone) as shown in the schematics and play an audio track of your liking, you will have to tweak the potentiometers to get the best sounding output.

You will have to find a way of connecting your audio source to the circuit, I used an old headphone cable. I cut it in half and soldered some conductive contacts onto the ends so I can quickly clip my alligator leads onto it and be sure a good connection is made.

See the third picture on this page to see my cable.

In headphone cables gold is usually ground and red and blue are separate audio channels. Just use one of the channels and leave the other unconnected.

An easy way to remove the enamel insulation from the audio cable is to burn it off with a flame.

Note: the audio level from the arc will not be as loud as a regular speaker, that is just the nature of plasma tweeters.

Disclaimer: Whilst I have not personally had any problems using this circuit with my phone, I take NO responsibility if you somehow manage to brake your phone using this circuit. So it might be a good idea to use an older device to provide the audio signal so in the event of something going wrong only it would not be such a loss.

The 555 timer generates a square wave of a frequency and duty cycle determined by the potentiometers and capacitor across pins 8-7 and 7-6,2. This square wave is applied to the gate of the MOSFET turning it off and on many times a second (tens of thousands) thus allowing pulsing current through the primary coil of the flyback at a frequency generally in the 30-50khz range.

When an audio source is fed into pin 5 of the 555 chip it will actively change the duty cycle and frequency of the square wave to the sound waves of the audio source. This is audible in the arc via the changes in air pressure surrounding the arc, and as it is an audio signal altering this pressure we can hear it as sound.

If you have an oscilloscope then you can view this square-wave by connecting your scope probe to pin 3 of the 555 timer. Connect the scope probes ground clip to circuit ground.

Q: It does not work
A: Check to make sure that all connections are secure (soldered) and there are no shorts anywhere.

Q: The arc is small.
A: Adjust the potentiometers trying all combinations, there should be a "sweet spot" at some point. If not then reverse the wires going to the flyback transformer primary coil. Most Flyback transformers are polarized and will only work best one way (because of the built in diode).

Q: There is no audio.
A: Check for continuity of your audio cable, most likely that there is no continuity there as these wires are very fiddly to get right thanks to the very small conductive strand and thick enamel coating.

Q: It worked but now it does not work.
A: Check the MOSFET has not blown. If you have used a lower voltage MOSFET (below VDS400v ) then chances are that a high voltage spike on the primary has caused it to break down. I originally used an IRF540 that only has VDS100V and it did not last too long before a spike on the primary killed it.

Q: The MOSFET gets hot or shorts.
A: This is one of the major shortfalls of this design, the leakage inductance of the primary coil causes the voltage on the primary coil to rise to a level much higher than the input voltage every-time the MOSFET switches. Avalanche rated MOSFET's can survive for a while, dissipating this energy as heat but after a while it can cause the MOSFET to short.

One way of fixing this is by the use of an "RCD snubber". This is a resistor, capacitor and diode in parallel with the primary coil and it keeps the peak voltage seen on the drain of the MOSFET to a level lower than the breakdown voltage of the device.

Choosing the correct values is not easy and putting anything across the primary coil is going to reduce the final output voltage, since it is this "flyback kick" on the primary coil that is creating such a high output voltage on the secondary coil (in this topology anyway).

As a starting point use a 150 ohm 2w resistor, UF4007 diode and 220nF capacitor.

Also make sure that the duty cycle is not too high, a tell-tale sign of this is a high current draw. This may give the impression of the circuit being tuned in resonance with the flyback but what is actually happening is that the flybacks core never fully resets due to a too high duty cycle, and in worst case scenario the core saturates and the MOSFET goes pop. This is where an oscilloscope comes in useful.

Want some more volume? Try this little pre-amplifier modification.

Special thanks to Atomicrox.