A blast from the past! While going through old stuff, I found this device, which I designed and assembled maybe 25 years ago. I never used it. I think it didnâ€™t work on the first try, and I got distracted by other shiny things. Story of my life. I thoughtfully left my future self a schematic diagram.
Click to embiggen.
I donâ€™t recall getting this out of a magazine. I came up with the design on my own, though I wouldnâ€™t be surprised if either of these specialized ICs have a usage note with a similar circuit. Itâ€™s a pretty straightforward design. Thereâ€™s a simple power supply, a JFET op amp input stage, a seven decade LED driver, a timebase, and a good old 555 timer driving the multiplexing.
The construction of the circuit is nothing to be proudÂ of. (notice no pictures here!) It nicely fits on one of those pre-made circuit boards the same size and shape as a wireless breadboard.
Any electronics folks care to comment on this design? Iâ€™m more than tempted to fire it up and see if I can get it working. -m
Update: the more I look, more more odd the 555 section seems.Â There are two pin 3s marked on the 7208. Iâ€™m pretty sure thatâ€™s not how MUX is supposed to work. Time to pore over some old data sheets…
Update2: I removed the 555 circuit, and instead fed the existing MUX signal from pin 12 of the 7207a to pin 19 (MUX1) of the 7208. And wouldnâ€™t you know, the thing works. I now have a fully functioning frequency counter.