Nixie tubes were originally used in the 1950's and 1960's, before LED and LCD segmented displays were developed, to display numerical information from equipment such as volt meters and early computers. Nixie tubes operate by having 10 different shaped cathodes and an anode cage in a neon atmosphere. With a large positive voltage applied to the cage, and one of the cathodes grounded, electrons will be ejected from the cathode and ignite the neon immediately surrounding the specific cathode, which are formed in the shape of the digits 0-9.
Nixie tubes went out of use quickly once it was possible to replace them with LEDs and LCDs, due to their requirement for high voltage, their fragility, and relatively high power requirement. The steampunk movement has brought back their use as a novelty, which has unfortunately driven up the cost of larger tubes on the second market (Obviously, production of nixie tubes halted decades ago). The IN-16 tubes used for my thermometer are about as big as you can get them while still being affordable, being about $3 a piece on eBay.
The circuit is composed of two major parts: The digital control on the front of the circuit board, and the high voltage power supply behind the tubes.
The jumper next to the AVR controls whether the temperature is displayed in Celsius or Fehrenheit.
The Nixie tubes have a junction voltage of about 120V, meaning the current through the display is determined by the remainder of the voltage across the current limiting resistors: (180V-120V) / 22k = 2.7mA.
Parts list for control circuitry:
- 1x ATTiny2313 AVR
- 1x DS1631
- 3x IN-16 Nixie tubes
- 3x K155 or 74141 Nixie decoder
- 3x 22kΩ resistor
- 3x 4.7kΩ resistor
- 1x MPSA42 300V NPN signal transistor (Digikey)
The 330pF capacitor controls the speed the boost converter runs at, and is mostly a function of how big the power inductor and voltage ratio are. I just used a value close to that in the schematic, but ideally you would calculate this using the tables from the datasheet.
Not shown in the schematic is the very standard 7805 linear regulator used to step the 12V down to the 5V required for all of the digital control.
Parts list for power supply:
- 1x MC34063 boost converter (I managed to smoke the first one with 180V pretty quickly, so having it in a socket and having spares is a good idea) (Digikey)
- 1x 7805 5V linear regulator
- 1x IRF820 MOSFET
- 1x 500μH power inductor (Digikey)
- 1x 1N4937 600V fast-recovery diode
- 2x 100μF 100V capacitors (or a single cap rated for >200V)
- 1x 820k resistor
- 1x 5.6k resistor
- 1x 150Ω resistor
- 2x 47μF capacitor (25V)
- >2x 0.1μF capacitors (25V) - Apply liberally throughout the circuit