‰  NOW 40 WPM  ‰  TEXT IS FROM MAY 2015  QST  PAGE 32 ‰

PROJECT.  STEVE ULRICH, NW0C, DESIGNED A PRINTED CIRCUIT BOARD SO THAT
MEMBERS OF THE MINNESOTA AMATEUR RADIO TECHNICAL SOCIETY MN ARTS COULD
BUILD THE MONITOR AT OUR ANNUAL BUILDERS DAY MEETING.  THE POWER MONITOR
THE POWER MONITOR PROGRAM SQUARES THE INPUT VOLTAGE TO DISPLAY POWER.  IF
THE ANALOG INPUT TO THE PICAXE IS A SINE WAVE ACROSS A 50 LOAD LIKE RL, THE
POWER IS THE SQUARE OF THE PEAK VOLTAGE DIVIDED BY 100.  THE PICAXE CAN DO
THAT WITHIN ITS LIMITS OF INTEGER CALCUL

ON THE MONITOR.  PICAXE PROVIDES A CABLE WITH A SERIAL CONNECTOR OR A USB
CONNECTOR, AND A 1/8 INCH STEREO AUDIO PLUG.  TYPE THE PROGRAM INTO A TEXT
FILE AND PASTE INTO THE PICAXE EDITOR TO PROGRAM THE PICAXE 18M2.  THE
SAVED PICAXE PROGRAM BECOMES A BAS FILE.  SWITCH S1 SELECTS EITHER AN
INTERNAL 9 V BATTERY OR THE EXTERNAL BATTERY THAT THE CIRCUIT MONITORS.
THE WATTMETER FUNCTION REQUIRES THE INTERNAL BATTERY.  WHEN THE VOLTMETER
USES INTERNAL POWER ITS INPUT RESISTANCE IS MORE THAN 15 K.  SELECT THE
VOLTMETER OR WATTMETER FUNCTION WITH SWITCH S2.  THE WATTMETER REQUIRES A
PEAK VOLTAGE FROM AN RF DETECTOR AT ITS INPUT.  FIGURE 2 SHOWS THE RF
DETECTOR AND ITS CALIBRATING POTENTIOMETER R11.  THE VOLTMETER AND
WATTMETER FUNCTIONS USE TWO DIFFERENT PROGRAMS.  WHEN YOU SELECT THE
WATTMETER, S2 ALSO PULLS DOWN THE NORMALLY HIGH PIN C2 OF THE PICAXE TO
SELECT THE WATTMETER PROGRAM.  MY MONITOR CIRCUIT FIGURE 1 HAS AN INPUT
VOLTAGE DIVIDER TO SCALE DOWN THE INPUT TO THE 0 5 V LEVEL REQUIRED AT THE
ADC INPUT PIN C1.  THE PICAXE 18M2 PROVIDES A 10 BIT ADC USING THE 5 V
SUPPLY AS A REFERENCE, SO 5 V INPUT BECOMES DECIMAL 1024 WHEN CONVERTED.  A

A VOLTAGE OR POWER MONITOR THIS VERSATILE MICROCONTROLLER BASED CIRCUIT
PROVIDES CRITICAL INFORMATION AT A GLANCE.  MERT NELLIS, W0UFO  I
PREVIOUSLY WROTE A BATTERY MONITOR ARTICLE FOR QST THAT USED AN LM339
COMPARATOR, BUT THIS ONE, BASED ON THE PICAXE 18M2 MICROCONTROLLER, IS
EASIER TO BUILD, TO CALIBRATE, AND CAN HAVE MANY MORE MONITOR LEVELS.  I
USE ONE ANALOG TO DIGITAL ADC INPUT AND 11 OUTPUTS.  THUS IT PROVIDES 20
LEVELS OF VOLTAGE MONITORING IN TWO DECADES, OR 11 LEVELS OF RF POWER
MONITORING.  FOR FEWER OUTPUTS, YOU CAN USE THE LOWER COST PICAXE 08M2,
WHICH CAN PROVIDE THREE MONITOR POINTS IN AN EIGHT PIN PACKAGE.  MY PICAXE
BASED CIRCUIT FIGURE 1 INDICATES EITHER VOLTAGE OR RF POWER BY LIGHTING
ELEVEN LEDS.  I PROGRAMMED MINE FOR 5 TO 14R5 V IN 1/2 V STEPS AND 1 TO 11
W IN 1R0 W STEPS.  I COMBINED BOTH VOLTAGE AND RF POWER MONITORS INTO ONE
UNIT, BUT YOUR PURPOSES MIGHT BE BETTER SERVED BY SEPARATE DEDICATED
MONITORS.  THE MICROCONTROLLER SYSTEM THE PICAXE SYSTEM USES BASIC FOR
PROGRAMMING.  YOU CAN DOWNLOAD THE EDITOR, SIMULATOR, AND PROGRAMMER
SOFTWARE FOR FREE FROM THE PICAXE WEBSITE.  THIS CIRCUIT AND EXAMPLE CODE
ARE JUST ONE POSSIBILITY.  YOU CAN EASILY CHANGE THE PROGRAM TO MONITOR ANY
FUNCTION THAT YOU WANT.  FOR PROGRAM DOWNLOADING AND EDITING, CONNECT A
SERIAL PROGRAMMING CABLE FROM THE COMPUTER TO THE 1/8 INCH CIRCUIT JACK J1
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ