Building a Spectrum Analyzer From Seven IN-13 Linear Nixie Tubes
This unique audio spectrum analyzer uses a series of linear nixie tubes to display frequencies in a fun way.
The idea
Audio spectrum analyzers are amazing devices that allow for a user to easily see which frequencies are strongest in the sound being inputted. Typically, they consist of either an LED matrix or a screen that shows vertical and/or horizontal bars which correspond to the frequency being monitored, whereas the extent is the amplitude. But for Yannick Turcotte, he wanted to get a bit more creative and build a spectrum analyzer that houses a series of uncommon IN-13 linear Nixie tubes.
What is the IN-13?
Unlike the Nixie tubes almost everyone are familiar with, the IN-13 is an analog and linear display, where the amount of distance illuminated by the glowing gas within is proportional to the current flowing through the device. This means that just like other Nixie tubes, IN-13s require a voltage source of 140V DC, except they also need an analog input to control them rather than a strictly digital one.
Getting frequency data
Turcotte designed his frequency analyzer to capture data via a single microphone element or, optionally, a line-in microphone via a pair of RCA plugs. From here, the signal pass through a low-noise filter that eliminates any stray sounds in the lower frequencies for cleaner audio, and it then gets passed into an MSGEQ7. This chip's job is to take in sound and perform a Fourier transform on it, thus splitting the signal into seven distinct frequency bands. Because there is only a single output pin, the IC contains an internal multiplexer that is advanced by sending a pulse to the STROBE pin, which is performed by an ATmega328p microcontroller.
PCB design and assembly
Coming up with the schematic and testing each component individually took several weeks of hard work, but once he had the design, Turcotte was able to create a board design that could house everything in one compact area. Due to the abundance of small SMD components, a stencil was used to apply a thing layer of solder paste, after which the components were carefully placed with tweezers and finally had hot air applied to secure them to the surface.
He also made a small stand that props up seven holding tubes for the IN-13 gauges by 3D printing a base and then gluing in acrylic tubing.
Driving the display
When the spectrum analyzer is switched on, the microcontroller is initialized to multiplex the first tube, which also matches with the MSGEQ7 spectrum analyzer chip. When it is time to display the next updated frequency, the MCU toggles the MSGEQ7's STROBE pin and then quickly switches the multiplexer to the next tube, where the output of the IC dictates the amount of current the Nixie tube receives through a simple and adjustable driver. There is also a large potentiometer on the front that can change the incoming audio signal's strength to ensure the correct range is shown.
To read more about this project, you can visit Turcotte's write-up here on Instructables.