Fluorescent Display Digital Clock – Built 1973

Built from scrounged parts.

No schematic was ever drawn, just some notes that are long ago lost.

I built this when I was 16 years old. It's a pretty straightforward design using 7400-series integrated circuits, popular at the time. 7490 decade counters, 7447 BCD-to-7-segment decoder drivers, a 555 timer (used for time set), and a 7404 hex inverter (along with a zener diode to clip and buffer the 60 Hz line frequency for time keeping).

The display tubes are fluorescent. They have a filament like a radio or TV tube had. Each segment has its own transistor driver circuit to provide the 24 volts DC to light it up.

The cabinet was once a 4-track tape player. These predated the 8-tracks that some people may remember from the 1970's.

I think the only parts I really bought were the display tubes, their sockets and the 24-volt transformer (used for the display). The rest of the stuff I found in the myriad of part drawers in our basement, left over from my dad and big brother's jobs.

I don't remember how much time I put into building the thing. It was a project for when I didn't have anything else to do. I know it was a few months from start to finish.

Front view

Display tubes inverted to keep them close together

Tubes with all segments lit (lamp test)

A peek inside

Counter stages

Chip position documentation

BCD-to-7-segment decoder drivers

Cabinet from automobile 4-track cartridge tape player

Knob is display brightness control
3-position toggle switch (Set, Stop, Run)
Rotary switch selects the time set speed

LM7805 5-volt regulator mounted on bottom