Fred Niell Upgrades a Hewlett Packard 3478A Multimeter with Glowing Nixie tube Display
What do you do when your multimeter's display has a poor viewing angle? Upgrade it with bright Nixie tubes, if you're Fred Niell.
Upgrading the display on test equipment is a common hack, but Fred Niell's approach to improving the visibility of his Hewlett Packard 3478A bench multimeter goes a step beyond most: gone is the transreflective LCD, and in its place are warmly glowing Nixie tubes.
"Like everyone else, I hated the transreflective display on my HP3478A. Right after I got it, I immediately did a backlight conversion. Just like everyone else," Niell writes. "But the viewing angle was crap, especially since I had mine about 9" above eye-level. So the old girl sat."
Inspiration struck while browsing the EEVblog Forum, in the form of a post detailing a conversion of a HP 3457 multimeter to a bright LED display — but Niell wasn't satisfied with LEDs. "The HP 3457A is just the big brother of this little guy," he explains, "so I thought how hard could it be to redo this with Nixies?"
Named for the backronym Numeric Indicator eXperimental No. 1 by its inventors at Burroughs, Nixie tubes are cold cathode display devices which offer a warm and iconic glow — and, key to Niell's project, impressive viewing angle.
"I used the previous LED conversion project as a starting point. I used XORs and flipflops to make a SPI-ish interface just like the LED approach," Niell explains. "I had SOIC '74 and '00 chips available, so did an XOR out of NAND gates. The next bit was to make some code that decoded the serial stream. This was the hardest part of the entire project, getting the old HP verb-noun and communication framing decoded."
The resulting display replacement is a custom PCB based on a PIC18F4550 microcontroller, which drives the Nixies plus miniature neons for decimal points, rectangular LEDs for positive and negative symbols, and a Broadcom HDLO-1414 dot-matrix display for the text portion.
"It worked," Niell writes. "I still need to add a filter so the glare, colour balance, and intensity difference isn't so obvious. There is no noticeable change in accuracy or noise in the instrument. The display is just as fast (lol) as the old one."
Niell's full write-up is available on the EEVblog Forum.