Super simple tune-button
When using the ICOM IC-705 with an external auto-tuner without communication with the radio, switching to e.g. AM or FM and starting transmission every time you need to re-tune can be somewhat tedious. Yukiji 7L1WCX has designed a brilliantly simple circuitry for a button that immediately brings the radio into tuning mode, putting out a constant carrier. Just like when ICOM’s own AH-705 is connected, it transmits at maximum output power. (Original blog post.)
Yukiji’s schematics together with photos of my own design can be found below. Since the voltage drop across the diode approaches zero when the capacitor becomes fully charged, I decided to use a cheap 1N4148 silicon diode instead of a somewhat hard to find Schottky diod. Also, since the radio only samples the START voltage when KEY goes low, the size of the capacitor is almost irrelevant. I used 1uF which is physically small and cheap. The voltage in the circuit is less than 4V so a 15V capacitor has good safety margin.
If the button is not recognized by the radio when connected, just press the “Function” key and touch the “Tuner” button on the screen.
Interface for Elecraft T1 tuner
Yukiji 7L1WCX has also designed a direct interface for the super compact Elecraft T1 tuner. (Orginal blog post.) The interface allows the radio and the tuner to communicate, making it possible to to initiate a tuning cycle from either. It even shows the battery status of the interface at power down.
Yukiji’s schematics together my own design can be found below.