Five LEDs in buttons for the Elecraft K2

The Elecraft K2 is quite a small transceiver, made for portable use. Therefore it has fewer indicators of status than many other transceivers. There isn't really much space for this on the rather small frontpanel. But I felt for a long time felt that it should be possible to put additional LED indicators on the K2 but I wanted to avoid drilling holes in the front panel. Here is a method based on using small LEDs in the keycaps, see info from W3FPR/KR5L, as you can always get replacement key caps from Elecraft.

On W3FPR’s recommendation I have used 2 mm LEDs of types HLMP6300 (red), HLMP6400 (orange/yellow), and HLMP6500 (green). They can be bought from Digikey as well as from ELFA in Northern Europe. The first two of the five LEDs that I have added can be connected from the front panel board without additional wiring to other boards.

This YouTube video shows the three first LEDs, the Zero-Beat Indicator, the Split Indicator, and the Audio Filter On indicator:

K2 Fixed Level Audio Output

I started by making a very simple modification for a low level audio output with level that was independent of the AF pot setting.

The next step was to modify my front panel board by adding an audio output (mic plug) via a 25 x audio buffer and a zero-beat indicator which uses the same signal. The added circuit is built Manhattan-style on the upper right on the back of the front panel board, see image.

In this way three things are achieved:

1. The amplified audio output is independent of the AF pot setting.
2. The zero-beat indicator is also independent of the audio pot setting.
3. The ‘no-wire’ philosophy of K2 is maintained, as there are no wires that need to be run to any of the other boards.

The schematic of the audio buffer and the updated zero-beat indicator is here. Note that the zero-beat indicator’s component values have some minor changes from the original design, that gave a nicer behavior of the display.

I also have added an optional signal from the sidetone circuit on the K2's control board (requires a plugable strap between the boards). This gives a visual check of the zero-beat indicator when you transmit, and it is also nice for demo purposes as both the received and the transmitted CW are output and can be decoded using e.g. a PC.

QRP Zero-beat Indicator for the Elecraft K2

This is a circuit that builds on KR5L’s zero-beat indicator circuit. He suggested trying a CMOS version of the 567 instead of the standard bipolar version, and here it is: the QRP version of his circuit. It draws only 0.9 mA at 5V compared to about 20 mA/8V for the original circuit.

Compared to the original circuit, C2 and C3 have been divided by about eight, and C1 has been divided by about two, give or take values found in my junk box.