The proposed Icom remote tuner controller must accomplish the following tasks:

• Supply power to the antenna tuner,
  
• Send request to initiate a tuneup action,
• Key the non-Icom transmitter when requested by the tuner,
• Send request to go into tuner pass-through mode,
• Display success/failure of antenna tune-up action,
• Display in-line/pass-through tuner status,
  
• Display an error when the tuner is not present,
• Allow controller bypassing, restoring "regular" tuner control with an Icom radio.
  

Electronically speaking, and in a simplified way, the proposed controller must accomplish the following signal handshake with the tuner. There are two control signals interfacing with the tuner, namely "START" and "KEY". A low-level, START pulse is sent to the tuner to initiate a tuneup process. The tuner then replies with a low-level KEY signal, which is a request to go into transmit and send RF. The tuner then controls the KEY signal release to indicate how successful the tuner was in tuning the antenna. This process is very well described with timing figures in Chris-K9EK's document. In addition to the START and KEY lines, a Vcc (+13.6 VDC, 2 Amp max.) supply line and a ground make the 4-wire tuner interface.

To supplement the basic tuner interfacing above, the controller provides the following features:

• A ground-active PTT output line to control the sending of RF power from the non-Icom radio at the proper moment during the tuning process. This line could typically be wired up to the CW key,
• A tuner presence detection by sensing the current sent onto the tuner Vcc DC supply line,
• A single push-button to control the tuner,
  
• LEDs to provide tuner and controller status,
• A toggle switch to select either this controller or an Icom radio as the control host.
  

CIRCUIT DESIGN

The proposed controller circuit has the following characteristics:

• It is based on the RP2040-Zero, a physically reduced version of the Raspberry Pi Pico development board. Its powerful dual-core CPU, low current consumption and ability to run Micropython or C language make it a good choice for most micro-controller projects.
• For interfacing with the Tuner's control lines and the PTT line, it uses Panasonic AQV212 Solid State Relays (SSR). They can offer galvanic isolation, can switch up to 60 Volts AC/DC at up to 550 mA and are insensitive to polarity. Moreover, when purchased from AliExpress, they cost less than $1 US each.
• The controller detects tuner presence by sensing the current sent onto the Vcc tuner supply line, using a mini-board hosting an INA226 I²C current/voltage sensor.
• A bi-color status LED informs the user of the tuner status and the tuning process. Three colors are produced: green, red and amber (a blend of green and red).
  
• A PTT LED indicates when a PTT signal is sent to the transceiver.
• A push-button provides control of the tuner actions.
  
• A front faceplate CONTROLLER/RADIO four-pole toggle switch allows to either take control of the tuner, or reconnect it directly to an Icom radio, restoring "regular" operation and disable the controller.
• A 10-pin "Phoenix" type connector with detachable screw-in wire block provides easy hookup to the tuner, the Icom radio and +13.8V power.
• The PCB dimensions allow it to neatly slide into a standard 88(W) x 38(H) x 100(D) mm extruded aluminum enclosure, eliminating any mounting hardware.

Once you know the above, the circuit diagram is relatively straightforward to understand. Here it is (click on it to zoom in):

The circuit is powered using a +13.6V external supply. A 78L05 voltage regulator feeds +5V to the RP2040-Zero micro-controller board through a Schottky diode, preventing any back-feed from the USB port's +5V line. That USB port is located on the micro-controller board, so is not shown on the schematic. The rest of the circuit uses the micro-controller board's +3.3V supply coming from its builtin voltage regulator.

S1, the four-pole dual-throw (4PDT) toggle switch provides full transfer, including the Vcc supply line, between the controller and an existing Icom transceiver. It also turns off the controller when the latter selection is made.

Instead of having to design in the INA226 current/voltage sensor (a fine-pitch SMD chip) on to the controller PCB, it was much simpler to integrate a small Chinese-made INA226 board, which also contains the proper measurement shunt resistor. The shunt resistor in connected in series on the "High-Side" of the +13.6V tuner supply line, which allows to measure the tuner current consumption, and thus detect its presence. The current is read over a two-wire I2C bus fed by the micro-controller.

The selected INA226 board

The author has had previous success with the Panasonic AQV series of Solid State Relays; he used them often when interfacing with the outside world. They are cheap, simple, reliable, and their outputs do not care about signal polarity or common-mode voltage. They can also provide galvanic isolation between the input and output sides. Their implementation is simple compared to using power transistors, which require accompanying bias and pull-up resistors. The selected AQV212 SSR is a good compromise, as it can switch up to 60 Volts AC or DC, at up to 550 mA, which is suitable for many ham applications. Other AQV models with different specifications are available.

The AQV212 in DIP-6 package

D1, a bi-color (green/red) LED is lit by opposing High and Low logic levels on the GP14 and GP15 outputs, depending on which color is required. The amber color is produced by quickly toggling the two control outputs between red and green. To produce a decent amber color, the duration ratio between red and green may not be 50%; With the cheap no-name LED used by the author, it is necessary to dwell much longer on the red color. This is done by the firmware (software). Depending on which model of LED used, it may be necessary to change the dwell time ratio in the firmware to get suitable amber color.

The two LEDs have a 220-Ohms series resistor to limit the DC current. A 5mA current limit is recommended. If LED brightness is judged inappropriate, the R1 and R2 resistor values can be changed, as long as a maximum current of 5mA is met. Note that resistor replacement may affect the amber color tone.

The PCB assembly process is relatively easy, with only through-hole components to install and solder. No surface-mounted devices are used.

The author mounted the AQV212 SSRs on "machined" DIP sockets to facilitate replacement in the unlikely case a device is blown up. Machined style sockets are more reliable than cheaper bent strip style sockets.

The same can be said about the RP2040-Zero micro-controller board. The author cut two 9-pin pieces from a single in-line male-male "machined" 40-pin strip and soldered them to the micro-controller board to act as a DIP pins. Corresponding machined female-male header socket strips are used on the PCB to host the micro-controller board. This allows for a quick RP2040-Zero removal / replacement if necessary. Note that the perpendicular row of contacts (opposite the USB connector) is not used on micro-controller board, and no pins must be installed on that row.

The type of "Machined" SIP header referred to by the author

The 78L05 voltage regulator does not require any heat sinking under normal conditions.

The push-button and the toggle switch should be mounted to the front faceplate using the provided hardware.

The two LEDs should be mounted to the front faceplate using 5mm LED black plastic sockets (holders).

Wiring up the four-pole toggle switch needs attention. A series of 11 wires must be made to link the S1 contacts on the PCB to the actual toggle switch contacts. All wires should be soldered at both ends. The S1 PCB layout is arranged in one row of contacts, numbered from 1 to 12, as shown to the right. The corresponding toggle switch contact numbering is also shown to the right. The wiring should follow that numbering pattern, PCB-1 to Switch-1, PCB-2 to Switch-2, and so on. Contacts 4, 5 and 6 should be run using a bigger gauge wire. AWG-20 stranded wire is suggested, as the DC current could reach 2 Amps. Other contacts can use a smaller gauge, such as AWG-24 stranded. Contact 10 is not used, so there is no need to wire it up. Proposed individual wire length is 9 cm, which provides just enough slack to easily reach the front faceplate, while still easily fit inside the enclosure.

S1 - Correspondence of PCB numbering vs. toggle switch numbering

The proposed 88(W) x 38(H) x 100(D) mm extruded aluminum enclosure is commonly available on AliExpress (as of 2026). Searching for "38 x 88 x 100mm Aluminum Case" should provide a list of enclosures that match the one used by the author. Expect to pay less than $10 US for one unit. The PCB outline was chosen to fit inside the proposed aluminum enclosure, without the need for mounting standoffs and screws. The PCB simply slides into the enclosure side rails. Four mounting holes have also be included for other integration approaches. Some glue may be needed to maintain the rear PCB edge against the rear faceplate. A drop of glue at the interfaces of the PCB and both rails should suffice.

The selected extruded enclosure

In addition to the circuit PCB, the author designed faceplate PCBs to replace the blank faceplates provided with the enclosure. When ordered with black solder resist and white silkscreen, they color-match the black aluminum enclosure and give a more professional look to the project. The bottom side of the faceplate PCBs has a continuous copper plane, which helps reduce signal radiation. As the faceplates have dimensions smaller than 100 x 100 mm, they also qualify for "special price" PCB manufacturing. Note that the two faceplates must be ordered as separate PCBs, hence the separate gerber zip files provided. Note that depending on the connectors, LEDs and switches used, the faceplate hole sizes may have to be manually adjusted. Because the provided M3 size screws have a chamfered (conical) head, they do not work well with the PCB faceplates unless a chamfer is created in the holes. The author has instead elected to use black M3-8mm narrow hexagonal head screws, which give a neat look. The replacement M3 narrow hexagonal head screws

Front FacePlate V2 PCB - Gerber files Rear FacePlate V2 PCB - Gerber files