‰  NOW 30 WPM  ‰  TEXT IS FROM JANUARY 2017 QST  PAGE 47‰

OF EITHER 8 INCHES OR 6 INCHES OF MAST TUBING OVERLAP AT MAXIMUM EXTENSION
BY OPTIONALLY SHORTING PINS 6 AND 7 ON THE CONTROL CABLE.  THERE IS A
SINGLE LIMIT SWITCH AT THE BOTTOM.  A 4PDT SWITCH S1A AND S1B REVERSES
POLARITY TO THE DC MOTOR FOR UP/DOWN CONTROL.  S1C SWITCHES IN THE CORRECT
SET OF LIMIT SENSORS, WHILE S1D LETS THE RASPBERRY PI KNOW WHETHER THE MAST
IS GOING UP OR DOWN.  IN A REDESIGN, I WOULD REPLACE S1 WITH A HEAVY DUTY
4PDT RELAY.  THERE IS QUITE A BIT OF CURRENT FLOWING THROUGH THE FRONT
PANEL MOUNTED SWITCH IN THE PROTOTYPE.  ELECTRICAL TRANSIENTS GENERATED
WHEN THE MOTOR IS STARTED AND STOPPED ALSO CAUSED MANY PROBLEMS.  THE
RASPBERRY PI WOULD FREQUENTLY CRASH IN EARLY VERSIONS.  IN ADDITION TO
IMPROVED BYPASS FILTERING, A SOFT START CIRCUIT USING A HEXFET POWER
SEMICONDUCTOR WOULD BE A WORTHY ADDITION, AND PROBABLY EXTEND THE LIFE OF
THE MECHANICAL ASSEMBLY AS WELL.  MOST OF THE ELECTRONICS ARE ISOLATED FROM
THE MOTOR DRIVE CIRCUITS.  THE USE OF A PHOTO COUPLER ISOLATES THE SENSING
AND CONTROL CIRCUITS FROM THE POWER SWITCHING RELAY.  WHEN SWITCH S3A IS
CLOSED IN THE RUN POSITION K1, POWER RELAY COIL CURRENT FLOWS THROUGH THE
‰  END OF 30 WPM TEXT  ‰  QST DE W1AW  ƒ