I came across this nice little Nikko NT-790 tuner. No high end aspirations here. The tuning indicator is done with three LEDs. A signal meter is not there. Nikko put just a single LED on the front panel to indicate the presence of a signal but it is pretty useless. I decided to widen it a bit:
A piece of prototyping board has its holes spaced 0.1" (2.54 mm) apart. Perfect to put 2.5x5.0 mm LEDs in. Initially I had 13 LEDs but I discovered there was place for one more, so 14 LEDs is what I made.
The code is really straightforward. Just a bunch of comparisons that run over and over again against the signal meter output (pin #13) from the IF chip LA1231. Using a signal generator I made a calibration table for the input level to LED illumination. I chose 5 dB signal level change per LED. In order to make the transition between LEDs smooth I also included a brightness change per 1 dB, giving a total of five steps per LED. At first I made it a simple 20-40-60-80-100% duty cycle change but the brightness change once a LED lit up was too small. Doubling the brightness every step solved this. Now it is 1/16, 1/8, 1/4, 1/2 and FULL. The glued together scope images nicely show this:
The brightness change was easily implemented by including a few delays in the code (asm). The short zero spikes in the image above are the actual compare statements in the code where the LED is first turned off and only back on when the compare matches. The OFF trace is not included because it is just that, off. As nice cherry on top of the signal meter cake I also made a quick Lamp Test at powerup:
So that was the software.
Putting the LED bar into the front panel of the tuner was relatively easy. I took care not to damage the soft aluminium by using tape during the jigsawing of the slit:
Mounting the LEDs on the LED board of the tuner was a bit more work:
The last image shows the PIC board dangling between flatcables. Now I have a nice signal meter a bit like the magic eyes of tube radios!
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Date: 19 February 2021
This software is licensed under the CC-GNU GPL.