TubeTime

Before I put together a long term solution, I wanted a quick way to transfer software from my PC to my Commodore PET. There are other circuits, but I wanted something that used just one part.

The PET input is TTL level, so I needed something to go from audio level to TTL. Ideally a comparator circuit with plenty of gain. Well, I couldn’t find a decent comparator in my junk box, but I did find a CA3140 op-amp. This is way faster than I needed, but should work OK as a comparator. If you build this circuit, try to find a comparator or an op-amp with a rail-to-rail output.

Here is the circuit:

And here’s what it looks like:

It works quite well. The circuit is not very sensitive to the output audio level from the PC–I set mine halfway (line level) and it was fine.