WARNING: This blog involves high voltages around sensitive low-voltage devices. Take great care if you decide to try this for yourself.


Recently, I received a swag pack that included an UNO and a Blue Man so I thought I'd do a very short and simple experimenting-type blog that could, maybe, lead on to something seasonal.


Here they are, the BM and UNO, along with some of the components I'm going to use: a 2k7 resistor; an MPS42A transistor; a 10mH inductor; and a neon bulb.



I'd previously found a bag of neon bulbs that I must have bought for something or other and then never used.



To get the neon to light up, I'm going to need to generate a fairly high voltage. Although I was going to try it with just the components shown above, I then chickened out and added a bit more protection. I didn't know how long it would take the neon to strike, so there was the possibility the voltage might damage the transistor and, once that was gone, might even go on to cause damage to the input of the 'scope. I didn't care about a single, dead transistor, but I do care about my oscilloscope. So I thought of adding a zener to clamp the voltage but didn't have anything suitable so I fell back to using a MOSFET (IRF510) as a zener.


Here's the circuit I ended up with.



[Transistor type should be MPS42A.]


The circuit is a very crude boost converter. The UNO turns the transistor on for about 60us and that allows the coil current to steadily ramp up. When the transistor turns off, the coil generates an emf to keep the current flowing so that it can 'discharge' the energy that is stored in its magnetic field.


Of course, it then dawned on me that I might as well have used the MOSFET as the switch and not bothered with the BJT at all but I left it as it was.


Here are a couple of shots of it 'bird's nested' above the UNO.




Here is the waveform as it starts off



The spikes are the points where the transistor turns off and the voltage leaps up. The first two times the neon doesn't strike and the voltage is being clamped by the zener [MOSFET]. On the third cycle, the neon strikes and, from then on, the warm, ionized gas strikes every time. I put the small capacitor in the circuit to give the neon a helping hand at the start. It isn't enough to power it through a cycle but it keeps the voltage up at the start and allows the gas to ionize - without it, the neon doesn't ever get started.


This is more detail of one of the later cycles





Here's what it looks like when it's running



Although it's non too clear from the video, because I'm driving it with dc the glow is only around the negative electrode (the lefthand one in the picture below):



If it's of any interest, this is the code I was using:


int outPin = 9;   // output pin to use  
// the setup routine runs once when you press reset:
void setup() {
  // set output pin:
  pinMode(outPin, OUTPUT);
// the loop routine runs over and over again forever:
void loop() {
word onPeriod = 0;
  // neon on 
  for(onPeriod=0;onPeriod<6000;onPeriod++) {    // repeat for about a second
    digitalWrite(outPin, HIGH);   // pin is high
    delayMicroseconds(100);        // for 100 microseconds
    digitalWrite(outPin, LOW);    // then low
    delayMicroseconds(60);        // for 60 microseconds
  // neon off
  delay(1000);   // off for a second


That was good fun. A nice flickering effect. Must be able to use that for something.


Update 18th December


Here's something a bit more seasonal. I've used five neons in the shape of a star (each neon has a 100k in series: there was a tendency for one or two to strike and hold off the rest otherwise). Quite by accident, I've contrived a situation where it's all a bit 'touch and go' and the behaviour of the resulting plasma in the bulb is fairly chaotic, giving the nice flickering effect.



Glad I don't have to try and get this through an EMC conformance test.


Update 27th December


Finally, the Blue Man waving a glowing 'torch'. It was falling apart as I was videoing it, but it will do for now.



If you found this interesting and would like to see more blogs I've written, a list can be found here: jc2048 Blog Index