Circuit From the Past

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. 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. This entry was posted on 3 January 2010, 4:30 pm and is filed under Ham Radio. You can follow any responses to this entry through RSS 2. 0. Both comments and pings are currently closed.

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