29 April 2011

FT-817 external tuning knob experiment

Time to share a new development in my accessories for FT-817.

Following the suggestion of a keypad owner I investigated a way to implement an external tuning knob that would overcome the limitations of the small radio main dial.

An optical rotary encoder would be the best component for this task, but they tend to be expensive for an experiment and not readily available. (Yes, I know they can be built from stepper motors, but this wanted to be a quick check.)

In my junk box I located a central-zero potentiometer that was in a VCR jog dial control. Left/right with a spring taking it back to midpoint. I wired it to the analog-to-digital converter of an ATmega168 and successfully controlled the VFO frequency as if I was turning slow/fast the main dial.
Implemented transfer function, with a central no-action range

If one was to further develop this accessory, the most interesting part would be to define a transfer function between the pot position and the VFO change: both step and update delay can be computed to implement a slow-to-fast tuning. For this first experiment I simply used a linear transfer function giving steps in the 130-1300 Hz range.

This kind of interface requires the microcontroller to have an analog-to-digital converter onboard, so one of the ATmega series used in my Frequency Readers. The ATtiny2313 is not suitable for this option unless I get to use a proper quadrature optical encoder (not planning to invest on it).
If you have a spare central-zero potentiometer and ATmega48/88/168 laying around I can send over a diagram and the compiled firmware.