Sunday 6 March 2016

555 Flyback Driver and Plasma Speaker Part III

So here is the complete Plasma speaker circuit in all it's glory!

It actually creates a significant amount of high voltage and works very well.  I would caution anyone else attempting to replicate this circuit to please be very careful.  I haven't given myself a shock yet but it could happen and will hurt if it does....Exercise sensible precautions please!

Here is the previous post in case people need to catch up:

555 flyback driver and plasma speaker part II

I have found that the 3D printed HV probe holders work quite well.  I also have found that setting the distance between the probes is critical to obtaining a reproducible arc and that the constant re-strike of the arc causing the audio to sound terrible.  From experimentation I have found that the audio signal from my mobile phone is more than enough to drive the 555 modulation pin when it isn't capacitively coupled.  When capacitive coupling is added the audio is barely heard.  The capacitor on the audio input reduces the hissing considerably.  Here is a video showing the current audio output of the plasma sounds pretty terrible but it does work:

I have decided to do two things....improve the HV probes and provide a simple class A audio amplifier to the pin 5 input of the 555.  This should improve the sound and get rid of the horrible hissing!

So to that end I have designed a very simple single transistor class A amplifier using a BC548 transistor.  Here is the schematic:

In designing the circuit I referred to this website...which is rather useful for this kind of thing:

I knew how to design a Class A amplifier well enough but I had forgotten how to select the components values particular I wanted to increase the low frequency response and limit the bandwidth of the amplifier to reduce the high frequency response.

The circuit works fairly simply...An audio signal from a suitable source is presented at the 3.5 mm headphone jack input - only one side of the audio signal is provided - this amplifier is mono. This is then passed to C1 - a 1 uF electrolytic capacitor which is used to remove any dc offset and chosen in such a way as to not overly affect the bass response of the amplifier (more on this later).  The next components in the circuit are R3 and R4 which bias the NPN BC548 transistor into constantly being ON.  These values are set by ohms law.  We need at least 0.7 volts to turn an NPN transistor ON. Lets do the maths just for fun:

Ohms Law; V / R = I

In this case:

V: 12 Volts
Rt: R3 + R4 which is 120 kΩ + 10 kΩ = 130 kΩ

I = V / Rt

I = 12 V / 130 kΩ

I = 9.23076923077 * 10^-5 A or 92.3 µA

The voltage applied to the base of the BC548 transistor can be calculated by = I * R4
therefore the voltage applied to the base of the BC548 transistor:

92.3 *10^-6 A * 10 kΩ

The voltage applied to the base of the BC548 transistor is 0.923 Volts or 923 mV

The circuit has been designed so that 0.923 volts is always applied to the base pin of the transistor to 'bias' the transistor ON.  The audio signal applied will increase this voltage and be amplified.  The next components applied to the collector of the transistor are a 10 kΩ potentiometer and a 100 Ω resistor.  At the emitter of the transistor we have another 10 kΩ  potentiometer and a 10 uF capacitor. All of these components combined set the gain of the amplifier. There are formulae that can be applied to calculate the amount of gain.  I guessed at it...It's not particularly important in this case. When the potentiometers are at maximum (according to my simulations) the input signal is amplified roughly 130 times greater than the input...the amount of gain is controlled both 10 kΩ  potentiometers which can be set by the operator.  The 10 uF electrolytic capacitor C3 is known as the emitter decoupling capacitor and is added to prevent any stray audio signal being present on the emitter pin of the transistor.

Finally at the output of the amplifier we have a 1 nF ceramic capacitor C4 and a 10 uF electrolyitic capacitor C2.  The electrolytic capacitor C2 prevents any dc voltage being passed to the next stage of the circuit, in our case, pin 5 of the 555 timer. C4 is used to limit the bandwidth of the amplifier.  In this case I have set all of the capacitor values to set the amplifier's frequency bandwidth to be between 200 Hz and 20 kHz which is roughly the range of human hearing.

I simulated the circuit in order to check what the output would be like and check the gain would be sufficient and to verify the frequency response.  It was helpfully not clipped and gave a good amplified approximation of what was to be expected.

Here are the results of the simulation...I have placed probes at the more interesting points in the circuit:

Simulation Schematic
Here is the simulated oscilloscope output:

The input signal is shown with the blue trace, the red trace shows the amplified output.  The output is inverted but that won't matter in this case.

The really good thing about simulating circuits is that the frequency bandwidth can be checked without actually building the circuit.  Here is the simulated audio frequency response of the amplifier:

If the capacitor values C1, C3 and C4 are changed for different values the frequency response of the amplifier is significantly affected.  C1's value changes the bass frequency responses, C3 changes the treble response and C4 changes the bandwidth of the amplifier.  In this case I have tweaked the values to try to give the best response between 200 Hz and 20 kHz without losing too much bandwidth.

Because its me I've designed a simple single sided PCB for this circuit.  It could easily be made on veroboard (stripboard) or using some other method.

Top Layer of PCB
Bottom Layer of PCB

Here is a render of the PCB to show how it will look once etched and populated:

Top View of Class A Amplifier Render
ISO view of Class A Amplifier Render
Here is the bill of materials:

Part Value Device Description Vendor Part Number Quantity Cost

12VDC_INPUT N/A M025MM Standard 2-pin 5mm screw terminal Farnell 9632972 1 0.245
AUDIO_OUT N/A M025MM Standard 2-pin 5mm screw terminal Farnell 9632972 1 0.245
C1 1uF CAP_POLPTH1 Electrolytic Capacitor Farnell 1236686 1 0.0464
C2 10uF CAP_POLPTH1 Electrolytic Capacitor Farnell 9451056 1 0.034
C3 10uF CAP_POLPTH1 Electrolytic Capacitor Farnell 9451056 1 0.034
C4 1nF CAPPTH1 Ceramic Capacitor Farnell 1141779 1 0.0758
C5 100uF CAP_POLPTH1 Electrolytic Capacitor Farnell 1902882 1 0.0345
C6 100nF CAPPTH1 Ceramic Capacitor Farnell 1141775 1 0.0721
JP1 N/A AUDIO-JACKPTH 3.5mm Audio Jack Farnell 1608405 1 0.534
R2 100 RESISTORPTH-1/4W ? Watt Carbon Film Resistor Farnell 9342397 1 0.0523
R3 120k RESISTORPTH-1/4W ? Watt Carbon Film Resistor Farnell 9342540 1 0.0492
R4 10k RESISTORPTH-1/4W ? Watt Carbon Film Resistor Farnell 9342419 1 0.0523
RV1 10k POTALPS-KIT PCB Mount Variable Resistor Farnell 1191725 1 1.4
RV2 10k POTALPS-KIT PCB Mount Variable Resistor Farnell 1191725 1 1.4
T1 BC549 BC549-NPN-TO92-CBE BC549 NPN Transistror Farnell 2453797 1 0.238

Total 4.5126

Again I haven't factored in the cost of the PCB or it's manufacture but it would be reasonable to estimate the total cost of the project to be around £6.00

Here is a quick video showing the circuit in operation with the plasma speaker.  The audio is very much improved!

Now I need to get back to putting the HV section and the electronics into some sort of casing.  That's all for now - take care people!

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