I'd been looking for a way of practicing my Morse sending without using a radio. I searched around and looked at several circuits but eventually settled on a Twin 'T' oscillator designed by Mike Maynard K4ICY. The Twin 'T' produces a really nice sine wave note which is particularly pleasant to listen to and much better than a raspy square wave. Mike also added wave shaping to eliminate clicks and other noise.
Having successfully built the oscillator and housed it in a nice project case, I showed my friend Phil G3MGQ. I help Phil teach students in a local school with a view to them passing their radio licences (We had four passes last term!). When Phil saw the oscillator, he enquired whether I would further develop the circuit to include an on board LED for silent cross-classroom comms and a built in micro switch Morse key. This would mean that the board could be used without the need to case it.
I set about shuffling the components around to make it usable as an uncased unit. The first iteration turned out to have a few issues.
I added a base plate to protect the soldered connections from being shorted on the surface it was being used on. This is fixed with M3 nuts and bolts which can also stand in as feet (although I added stick on rubber feet on my one). Although obscured in the above image, the nut in the bottom left could not been fixed since the pitch potentiometer is too close.
The micro switch Morse Key is very close to the volume potentiometer and this makes it hard to use. Whilst on the subject of the mocro switch, I managed to get the footprint wrong, making the outer connections .5mm too close to the center. I managed to force a micro switch in to test the above circuit but I have since corrected the footprint.
This is the next version of the board and will be fabricated shortly. As you can see, I have moved the components around again. The pitch potentiometer now gives enough space for the nut to be fixed. The micro switch Morse key is on the far right, for ease of use although this entailed moving the key socket to the rear.
Design and fabrication files for this version of the board will be uploaded once proved to be accurate.
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.