Circuit From the Past

I was trying to provide some semblance of order in our basement, which is the storage area for my extensive collection of great radio and computer treasures (sometimes referred to as “junk”). I ended up wading through piles of stuff that I didn’t even know I had. It was like Christmas all over again!

I came across this little gem built into a RadioShack enclosure:

I guessed that it was a Code Practice Oscillator and that I must have built it some time ago. I had a vague recollection of the project. It certainly looked like something I might have built. There was a straight key in the same box that plugged right into the key jack.

I opened the inside and found this circuit board populated with through-hole parts. Replacing the 9V battery with a fresh one, the circuit came to life. A little action on the key and the sound of Morse Code filled the room.

I was curious about the design. There is one integrated circuit on the board but it was labeled only with a proprietary part number. What would I have used to built a code practice oscillator? Probably a good old 555 timer, so I pulled up an online datasheet for that part and got its pin out.

Then I started to do a some lite reverse engineering of the circuit and this partial schematic diagram appeared:

The circuit is not complete — I just sketched out enough to confirm that the design was based on the 555 timer.  There is a lowpass RC filter on the 555 output to round out the square wave before it is fed to the speaker. It’s interesting that the key connects to the reset pin on the IC. I think this was to ensure a clean start up of the oscillator, to eliminate any frequency chirp.

So that’s a little diversion I had this weekend. Now back to cleaning out the basement.

73, Bob K0NR