Monday, 15 May 2017

DL4YHF CW Keyer - Taming the speed control?

Ive had a quick chat with Wolfgang DL4YHF by email regarding the speed range of the keyer. As suspected, the speed control range cannot be easily changed in the code, so the best plan of attack is to modify the resistance range of the control.

Yesterday I measured the resistance at my preferred slowest speed (just before the control becomes unresponsive) as 47k, and that at my preferred maximum speed (probably about 45wpm) as 18k.

The fast speed is the easy part to deal with. By assuming the pot to be at zero, then its a case of replacing the 1k input series resistor with an 18k unit. But the slow limit is more difficult. The resistance difference range needed from the pot is 29k - of course no such pots exist.

Rather than replace the 100k pot that is in place, the answer is to parallel it with another fixed resistor to modify the range. In this case, a maximum value of 29k can be achieved by paralleling the potentiometer with a fixed resistor of 39k.

I will try this tonight and see if I get an acceptable control range. Replacing the 1k resistor would entail removing a number of wire connections to get the PCB out, so instead I will probably keep it in circuit and add the 18k in series with it. I doubt the extra 1k will make much difference to the speed range.

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