Project under development

Arduino based Johnson KW Matchbox Autotuner

I **LOVE** the old Johnson KW Matchbox.  It is a great balanced tuner.  It is a great bandpass filter.   It will tune about anything.  The problem is that they are fairly high-Q and requires tuning even with small frequency changes,  especially on 80 meters.   80 meters is a large band in terms of percentage bandwidth, and the small value of capacitors in the matchbox yields very high Q.

So... I thought,  what about putting stepper motors on one and controlling with the Icom CI-V interface.

Stepper motor on
      Matching Cap

Top View

(Hint:  for more detail download the images.)

I currently have the Matchbox installed and operating in what I call pure "Table Mode" where the tuner has table and the stepper motors tune to a position based on the table.  This is done by reading Icom CI-V frequency messages and going to a look up table to determine where to set the knobs.  It is only a accurate as the table.  So far, weather conditions have been stable, and so has the tuning.  The Arduino uses an Atmel Mega processor with a built in A/D converter,  the though is to tune based upon the table and then "tweak" for the lowest refelected power,  using the A/D converter to measure SWR.  This feature has not yet been written.

The problem with a table based approach is that if the feeder is wet,  tuning will not be the same as when it is raining,  but I am waiting for rainy season to see how much of a problem that is.

When the tuner is not tuning,  the operator has full control over the tuner by turning the knobs, so it still operates as a normal Johnson KW Matchbox.

Watch a video showing how it works

The motor drivers are done through an I2C interface, using an 8 bit I/O expander,  PCF8575N.  For each motor, there are two lines that define the phase, and one line that is an enathe motor .  The other three lines are used to enable power to the LED in the two opto-couplers and two sense transistors are brought back to the controller to mark where home is located.   The N channel FETS for the motor drivers must easily be able to handle the current of the motors.  NOR gates should have enough drive current to operate the power FETs,  suggest HC or HCT parts.

Rough schematic

The current code can be downloaded in the following zip file.  This does not include the the LCD library.  This software has a modified version of the CI-V router CI-V software, where all frequency messages are routed to a global routine, for further processing.

Current Matchbox Source Software

If you have any questions contact  jim at k9jm dot com