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A project log for Twin 'T' Morse Code Practice Oscillator

A pleasant sounding Morse Practice Oscillator for self training or providing practice for others.

Steve SmithSteve Smith 08/11/2018 at 17:530 Comments

I first of all prototyped his circuit up on breadboard, and although very pleasing, the two transistor amplifier Mike used seemed a little under powered for my liking. I modified the circuit by substituting the two transistor amplifier with a venerable LM386. This produced the kind of volume I was expecting and could easily drive a pair of headphones or a moderate speaker.

Once I had proved the circuit with my added LM386, I worked up a design in KiCad, in a little bit of an excited rush it has to be said. I didn't really consider where the controls were positioned on the board since I knew I was going to house the board in a project box and the controls and key socket would be mounted on the front panel.

Once the boards arrived from the fabricator, I built one up and was pleasently surprised when it worked first time. I did notice the attack time of the waveform was a little high. To find a better value for the resistor in question, I temporaily wired a potentiometer in the resistors place and assertained a value which would give me a more agressive attack but still avoid any noise. I then measured this and chose the closest value I could find. This turned out to be perfect for me.

Having done this, I housed the board in a project case along with a 9V PP3 type battery holder and mounted the speaker on the top panel after drilling sound holes out.

The volume level is easily adequate for practice and the note is nice and clean and easy to listen to.

Design and fabrication files can be found on ProjectAVR.com

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