TubeTime

Here are some photos of my latest clock. It uses large Panaplex-style neon-filled displays. I do not know the part numbers or the brand: these appear to be factory rejects, and each of the digits appears not to meet mechanical tolerances.

Under each digit is a reed switch that you can trigger using a magnet to change the number of that digit. The circuit board sits behind a regular picture frame where the glass has been painted black from behind. I masked off rectangular regions to allow the displays to show through, and I hand painted the “P” for the PM indication.

This is what the circuit board looks like:

The 180V power supply is on the lower right of the circuit board. The microcontroller timekeeping circuit is located underneath the leftmost digit. It is a PIC18F2420. I am using the onboard 32KHz oscillator with an external watch crystal, but I left room for a DS3231 timekeeping chip. It supports a battery backup, and you can see the place where it would go on the board right underneath the hour digits.

The left edge of the board has a row of header pins that I use to check the voltages, program the PIC in circuit, and probe the cathode voltages with a scope. The neon numerals have very interesting electrical characteristics, and eventually I will post an article about that.

Panaplex displays must be multiplexed to prevent damage. They are slightly more finicky than Nixie tubes, but the driving circuit is quite similar. I had to add a set of clamping diodes to limit the voltage swing on the cathodes (each segment is a single cathode).

This design was very fast and straightforward. I spent two evenings prototyping the circuit and getting the multiplexing working, another two evenings to enter the schematic and lay out the PC board, and two days to assemble, test, and finish the clock software.