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This year for a Halloween party I wanted something a bit different from the usual pumpkin and something that would let me play with my new microcontroller and nixie bargraphs.

So I present the nixie jacobs ladder pumpkin! This uses two nixie IN-9 bargraphs, cheap nixie power supply, trinket microcontroller from Adafruit and a single RGB led to get a cool electric shock feel!

Step 1: Assemble the Components

To build this pumpkin you only need a few cheap components, all of which I already had. However if you do not have some of the parts, there are many suitable alternatives or if you are interested in electronics they would be useful to own anyway for future projects.

Parts list:
2x IN-9 nixie bargraphs (lot of 10 for $14 on eBay)
1x high voltage nixie tubes power supply module kit ($12 from eBay)
1x Adafruit trinket ($7.95 from Adafruit.com)
2x MJE340 transistors (10 for $10 on eBay)
2x 300 ohm resisters
2x 3.9 Kohm resisters
3x 480 ohm resisters
2x 0.1uF capacitors
1x 10mm diffuse RGB led
1x Pumpkin
1x 5V power supply (I used a mobile phone charge with a mini USB connector)
1x 12V power supply

Tools and other bits:
Soldering iron
breadboard
jumper wires

Step 2: Circuit

The circuit is pretty straight forward. The height of the plasma in the nixie tube is a function of how much current passes through the tube. We have a fixed 140V from the power supply and we use a transistor to control the current flow. The current flowing through the nixie tube is controlled by controlling the current into the transistor base. 

The trinket has 3 outputs that can supply pulse width modulation (PWM) but here we will only use two of the three, outputs 0 and 1 on the trinket. By adding a resister and a capacitor we can filter the fast pulses into a (nearly) DC output. This can be then used to control the height for the nixie bargraph.

To light up the inside of the pumpkin I decided it probably wasn't a good idea to use a burning candle so I wired up a 10mm RGB led to the 5v output from the trinket with some current limiting resistors. This lit up the pumpkin nicely.

Step 3: Carving the Pumpkin

I decided on a simple design for the pumpkin of three lightening strikes around the outside. The two nixie tubes would stick out the top like a Jacobs ladder.

To make the holes for the nixie tubes I used a drill bit slightly smaller than the tubes themselves and manually drilled out the pumpkin. Once I made the holes a pushed the tubes through were a good tight fit.

Step 4: Wire Up Your Pumpkin

I made a small hole near the base of the pumpkin on the back. I then fed the wires for the two tubes and led through this hole. I used a bit of masking tape round the nixie tube wires as I wanted to ensure then wouldn't bend and accidentally touch when closing the pumpkin, afterall there is 140 volts going through them!

Step 5: Wiring Up the Circuit

Since this project was only for one night I wired the circuit up on a small breadboard. The Trinket was part of a separate clock project on a separate breadboard. I left it where it was and just used jumpers to connect to the transistors. As you can see it's not very tidy plus I dont yet have a box for the 140 nixie voltage supply. All this was carefully tidied away behind the pumpkin for the evening. But the tubes are working!

To power to the trinket it requires a 5v supply. Normally I use my laptop but since I wanted this to sit in my dining room the laptop would spoil the appearance. Additionally since we are dealing with 140v I wanted to avoid any chance of frying my computer. So instead I used a phone charger, this has a 5v output and allows for different usb cables so I just swapped over the phone usb (micro usb) for a mini usb cable.

We also need a power supply for the 140v nixie supply. For this I found an old power adapter for some unknown long forgotten bit of tech. This had a 12v output and after snipping off the jack I soldered a couple of wires so it could be plugged straight into the breadboard. Job done!

Step 6: Programming the Trinket

The Trinket is a brilliant tiny microcontroller from Adafruit. It can be programmed from the Arduino IDE however to get it to work reliably is tricky! Fortunately there are lots of very helpful people over there and the answers can be found on the learn website and forums. It is worth the effort!

Obviously if you have an arduino then that would be great to run this project from. In fact I could have used my beaglebone black to run the tubes, all you need is 2 PWM outputs. Where they come from doesn't matter!

Anyway here is the arduino script I used to run the two tubes, as you can see it is very simple!

Script
int led0 = 0;
int led1 = 1;
void setup() {
  pinMode(led0, OUTPUT);
  pinMode(led1, OUTPUT);
  
}
void loop() {
  for (int i=0; i<256; i ) {
    analogWrite(led0, i);
    analogWrite(led1, i);  
    delay(10);}
  
  for (int i=255; i>=0; i--) {
    analogWrite(led0, i);
    analogWrite(led1, i);
    delay(0.1);
  }
}
 

Step 7: Assemble and Enjoy!

Put the pumpkin back together and enjoy the effect!
<p>I just built your circuit for a project and it rocks - but you have a program bug in it...</p><p>you need the double &quot;++&quot; signs to increment the for statement. like this:</p><p>for (int i=0; i&lt;256; i++)</p>
<p>Very Cool idea and blend of old nixie tube technology and the new digital!</p>

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