Reliable auto-detect for keyer for Elecraft K2

The Elecraft K2 has an autodetect feature for the keyer input. When enabled, and when the DASH and the DOT lines are keyed at exactly the same instant in time, the K2 will detect that this is a hand key or an external keyer. I and several other K2 owners have however experienced erratic action of the autodetect with seemingly random keying.

My problem only occurred when keying was done from an external keyer. This could happen when keying from e.g. the WinKeyer.

The recommended circuit in the K2 manual is shown in the first figure. Two diodes make sure that the DOT and DASH lines are keyed in sync.

The keyer input from J1, the K2 key jack, goes through a low pass filter (220 ohm, 0.001 uF) for debouncing and then on to the /DASH and /DOT-PTT lines which end up as inputs for the 18C452 MCU.

The datasheet for the PIC processor states: "Input low voltage 0.8V for 4.5V < VDD < 5.5V"

This value is rather high and could be the explanation for the erratic behavior. Use of the common 1N4148 silicon diodes in the autodetect circuit will make the K2 input no longer 0, but say 0.7 Volts when keyed. When I tried to key the K2 using autodetect and the diodes from my PC via an optocoupler I got into trouble. The saturation voltage of the optocoupler was not small enough and when added to the 0.7 Volts, the total was not low enough for the K2 to reliably sense a key-down.

The optocoupler (4N31) had a voltage of 0.65 to 1.8 V in saturation. I lowered it by increasing the drive current in the LED, but still had almost 1.4 V (opto 0.65 + Si diode 0.7).

The remedy was to replace the autodetect diodes with Shottky diodes. One of the suggested diodes in the top figure, 1N5817, is actually a Shottky diode, but the importance of choosing such a diode rather than the common 1N4148 is not spelled out there. I have several hundreds of PRLL5818 in my shack so I used one of them, but the type is not critical at all. The data for that diode says Vf < 0.33 V at I = 100 mA, so the forward voltage with the low current here is even smaller than that.

[This post is an expansion of a posting on the Elecraft mailing list some time ago. All figures are from the Elecraft K2 manual.]

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