Sunday 7 February 2016

555 Flyback Driver and Plasma Speaker

I haven't really had much electronics inspiration at the moment.  It can be like that sometimes...So I decided to fill my time with a display project.  I'm going to build a simple plasma speaker!  These are essentially just a high voltage arc being modulated with audio to produce sound.  They aren't particularly good at producing sound and are quite dangerous so they aren't used apart from for effect.

WARNING - This is a High Voltage circuit!  Using Flyback transformers without due care and attention is DANGEROUS. Lethal voltage and current is present when operating this circuit.  The author is not responsible for anything which occurs by constructing or operating this circuit!

There are lots of tutorials and videos on YouTube and Instructables about this subject.  I used the site below as my inspiration:

Here is the schematic diagram for the circuit:

The circuit is fairly simple in operation.  Power is supplied via a standard 12 Vdc 5 amp power supply via the DC barrel socket or via the 5 mm screw terminal JP1.  The 12 volt supply is smoothed by the 100 uF and 100 nF capacitors.

The main part of the circuit is made up of a 555 timer in astable mode.  Astable means there will be a constantly repeating 12 volt peak square present at pin 3.  The frequency of the square wave is set by the 50 k potentiometer RV1.  The mark space ratio of the square wave (the width of each pulse and the gap between each pulse) is set by the 50 k potentiometer RV2.  The output at pin 3 is used to drive two bipolar transistors which in turn drive a high current, high voltage N-type MOSFET.  The MOSFET will drive a flyback transformer which will have it's output at the secondary spaced so as to draw a high voltage arc.  The flyback transformer will be connected externally via the 5 mm screw terminal JP3.

The audio signal for the plasma speaker will be coupled to the circuit via the 5 mm screw terminal JP2.  This will take in a standard audio signal either from an audio amplifier or directly from an audio source such as an MP3 player or a signal generator - I haven't decided yet!

To make things easy for me and to ensure this circuit works as intended I simulated the circuit first. It works perfectly well.  The voltage generated by flyback transformer at the secondary should be around 1.7 kV assuming I have guessed at the turns ratio of the flyback transformer correctly.

I then designed a printed circuit board for the circuit.  I find it much easier to lay circuit boards out than to use stripboard to create circuits however stripboard would work perfectly well.

Here is the PCB layout:

Plasma Speaker Top Layer
Plasma Speaker Bottom Layer
Both layers with dimensions
In designing this layout I was trying to make the circuit as small as possible but still use through hole components.  I find it much easier to work with through hole components when prototyping.  If I was going to make more of these circuits I would design with surface mount components and reduce the size to less than 50 mm x 50 mm.  This way I can get PCBS made for a reasonable price in China by Elecrow.

Just for fun I've rendered the circuit in 3D using Sketchup so that I can visualise how the circuit will look once it is complete.  It also means I can spot any potential construction and layout issues before I etch and populate the PCB.

Isometric Render of populated Plasma Speaker PCB
Top View of Plasma Speaker PCB
In order to populate the PCB the following components will be required:

Part Value Description Vendor Part Number Cost (£)

C1 10 nF Ceramic Capacitor Farnell 1141772 0.0851
C2 100 nF Ceramic Capacitor Farnell 1141775 0.0721
C3 220 nF Ceramic Capacitor Farnell 2395774 0.132
C4 100 nF Ceramic Capacitor Farnell 1141775 0.0721
C5 100 uF Electrolytic Capacitor Farnell 2346578 0.1178
D1 UF4007 High Speed Diode Farnell 4085310 0.372
IC1 ICM7555 CMOS 555 timer Farnell 9488243 0.528
J1 n/a 2.5mm DC barrel Jack Farnell 1737246 0.469
JP1 n/a 5mm Screw terminal Farnell 2493614 0.16
JP2 n/a 5mm Screw terminal Farnell 2493614 0.16
JP3 n/a 5mm Screw terminal Farnell 2493614 0.16
JP4 Jumper 2 pin header Farnell 3418285 0.27
KK1 SK104 Heatsink TO247 Heatsink Farnell 1892329 1.06
Q1 BC549 TO92 NPN Transistor Farnell 2453797 0.232
Q2 BC559 TO92 PNP Transistor Farnell 2453808 0.232
Q3 IRFP250 TO247 High Power MOSFET Farnell 8649260 1.26
R1 270 Ohms ¼ Watt Carbon film Resistor Farnell 9339353 0.0356
R2 22 Ohms 1 Watt Carbon Film Resistor Farnell 1565366 0.0664
R3 150 Ohms 1 Watt Carbon Film Resistor Farnell 1565346 0.0664
RV1 100 k-Ohms ALPS PCB mount Potentiometer Farnell 1191742 1.28
RV2 100 k-Ohms ALPS PCB mount Potentiometer Farnell 1191742 1.28

The total cost of components, not including the PCB or flyback transformer will be:


Flyback transformers can be very easily sourced from old televisions, junk shops and everyone's favourite online auction site:

Ebay - Flyback Transformer

They are currently on sale for £7.81 - I remember them being cheaper but they are becoming more rare!

I'm guessing at the cost of making and etching a PCB for this project at £3.00

That brings the total cost to £18.93

Not bad I suppose...I'll probably etch and populate a PCB and test the circuit in the next post.  That's all for now

Take care people - Langster!

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