ChronodeVFD: A Cyberpunk Wristwatch
The ChronodeVFD is a personal project I’ve been working on for a couple of months. It’s a wristwatch built around the IVL2-7/5 VFD display tube. I originally purchased a few of these tubes to build a standard desk clock, but after playing around with them, I realized I could probably build a wristwatch too. The tube has a number of features which make it more suited than most Soviet-surplus VFDs for this purpose.
- nominal 60mA filament current @ 2.4V, but still works with ~35mA @ 1.2V.
- It’s small — only 1.25 x 2.25″
- It’s flat, as opposed to the round tubes like the IV-18, which would be much clunkier in a watch design.
- can operate from a relatively low grid voltage of 12-13V (up to 24V)
- pulls only about 2.5mA/segment from the grid rail @ 12.5V. (“8″ = 20mA)
One other feature that I like about this device is that unlike nearly every other VFD tube, the IVL2-7/5 has no opaque or diffuse backing behind the digits. It’s completely transparent front to back, which means that if you put it on top of a circuit board, you can (with a bit of backlighting) see the PCB below.
The single biggest constraint on a project like this is the power supply. Because this is a costume piece that I’d only wear occasionally, I decided I wouldn’t mind if the battery only lasted 6-10 hours. However, I didn’t want it to be uncomfortable to wear or excessively bulky, so my options for battery power were limited. Coin cells were out because the internal resistance was too high to meet the current requirements, so I was left with AA and AAA single cells. I decided to go with alkaline cells, since the lower nominal voltage of NiMH rechargables would mean an even lower efficiency for the boost converters, and less current for the filament. The finished project can be used with either 1xAA or 1xAAA alkalines (with the appropriate clips), however AAAs only last about 2 hours, so I’m sticking with AAs for now.
The circuit is pretty straightforward. The core is an Atmel ATMega88 AVR, and the real-time clock is a Maxim DS3231. The DS3231 is nice because it’s an all-in-one solution: it has a 32kHz temperature-compensated crystal and capacitors built in, so the only external component required is the backup battery. However, the DS3231 is pretty expensive. I happen to have a tube of them left over from another project, so I decided to use it in this design. There are other, much less expensive RTCs available which would work quite as well (BYO crystal, though).
The VFD display is driven by a Maxim MAX6920 — a 12-bit shift register with high-voltage (up to 76V) outputs. The 12-bit field is designed specifically for driving 4-digit clock displays (4 digit channels and 8 segment channels). It’s easy to use and very reliable and compact. It really is the only chip of its kind, so it’s a tad expensive. It’s also possible to drive the VFDs using a bunch of discrete components, but that was impractical here due to space constraints. If you’re interested in that, you should check out Riad Wahby’s excellent inGrid clock build, which describes his discrete HV driver circuit in some detail.
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