Auto VTVM Picaxe Basic Program


rem "AutoVTVM.BAS" 8/17/2009

rem An ancient Volt-Ohmmeter was refurbished using a PICAXE instead of RTL logic.

rem It has 8 voltage and 16 resistance 5-15 ranges, selected by relays.

rem There is a single control, a voltage-ohm switch.

rem It uses a rectifier in the meter amplifier feedback to measure voltages

rem of either polarity or AC without switching. AC gain is modified automatically

rem to correct for average-RMS of a sine-wave.

rem Indicator lamps show the range in use and the mode: Positive, Negative or AC

rem voltage, or Kilo or Megohm resistance.

rem There are three inputs to the PICAXE:

rem Pin 0 is high in voltage mode, connected to the ohms relay coil

rem Pin 1 is analog meter current from an opto-isolator and is low-pass filtered

rem Pin 2 is the low-pass filtered output of the meter amplifier. 

rem Due to the bridge rectifier and the opto-isolator diode there is

rem at least a 2.5 volt drop in the feedback path when in range.

rem On AC the mode voltage averages out to zero.

rem A bias results in a 0.5 to 4V span, neg high as the amplifier inverts.

rem There are 8 outputs.

rem Pins 0-2 are the range. They go to ancient diode NAND decoders to illuminate

rem a single range lamp and the coils of three reed relays which control the dividers.

rem Pin 3 is the gate of a 74259. This chip has a demux and 8 D registers, which can be

rem individually set. In this circuit, they can only be all cleared at once.

rem Pins 4-6 select the 74259 register to set. 0-4 are the mode lamps, 4 is the megohm

rem lamp and also the megohm relay. Output 6 is the meter enable. 5 & 7 not used

rem Pin 7 is the 74259 clear.



#Com 1

rem Bit variables - uses bytes 0 and 1

symbol NewRange        = bit1    'Set temporally when range changes

symbol Ohms            = bit2    'Set in ohms mode

rem word variables - uses bytes 2 - 5

symbol Current        = w1        'The current through the meter via on opto-isolator

symbol ModeSense        = w2        'The voltage at the output of the meter amplifier

rem byte variables - uses bytes 8 - 11

symbol Range        = b8        '0-7 corresponds to ranges 0.015 through 50

symbol Allouts        = b9        'This sets all of the output pins at once

symbol Mode            = b10        '0=Pos.1=AC,2=Neg,3=Kohm,4=Megohm,>4=all off        

rem constants

symbol Pos            = 300        'ModeSense less than this indicated a positive voltage

symbol Neg            = 600        'ModeSense greater than this indicated a negative voltage

symbol GoDown        = 30        'Current less than this causes a lower range

symbol GoUp            = 400        'Current greater than this causes a higher range

Init and Main



LOW 3                'Ensures the idle state of the 74259 gate

LET Range = 4        'Start in middle

IF pin0 = 0 THEN        'Test for ohms-volts selection

    LET Mode = 3

    LET Ohms = 1


    LET Mode = 0

    LET Ohms = 0


LET NewRange = 1        'Ensures entering the range into the pins

PAUSE 200            'Settle the circuits

READADC10 1, Current    'Initial meter current reading


rem Test for in-range and increment/decrement if needed

IF Current > GoUp THEN

    IF Range < 7 THEN

        LET Range = Range + 1

    ELSE            'Go to megohms if high on 50 range

        IF Ohms = 1 THEN 

            LET Mode = 4



    LET NewRange = 1

ELSEIF Current < GoDown THEN

    IF Range > 0 THEN

        LET Range = Range - 1

    ELSE            'Go to Kohms if low on 0.015 range

        IF Ohms = 1 THEN 

            LET Mode = 3



    LET NewRange = 1


rem since the PICAXE must set all 8 pins at once, must also set the mode lamp

IF NewRange = 1 THEN    'Enter range into pins to illuminate a lamp and energise relays

    LOW 7            'Clear all 74259 outputs, lamps and meter enable

    LET Allouts = Range & 7    

    LET Allouts = 16 * Mode + allouts + 128 'lift 74259 clear and set one mode lamp

    LET pins = Allouts

    PULSOUT 3,1        'Pulse the 74259 gate

    LET NewRange = 0


PAUSE 200            'Wait for the current and range low-pass filters to settle

rem Only read the mode and enable the meter if the current is withing the limits

READADC10 1, Current

IF Current > GoDown AND Current < GoUP THEN

    READADC10 2, ModeSense

    LOW 7            'Clear all 74259 outputs

    IF Ohms = 0 THEN    'Voltage mode only    

        IF ModeSense > Neg THEN

            LET Mode = 2

        ELSEIF ModeSense < Pos THEN

            LET Mode = 0


            LET Mode = 1    'AC by default



    LET Allouts = Range & 7        'Keep curent range    

    LET Allouts = 16 * Mode + allouts + 128

    LET pins = Allouts

    IF Mode < 5 THEN

        PULSOUT 3,1        'Pulse the 74259 gate


    LET Allouts = Range & 7    

    LET Allouts = allouts + 208    'Set meter enable now

    LET pins = Allouts

    PULSOUT 3,1            'Pulse the 74259 gate    


rem check if ohms switch has changed

IF pin0 = 0 AND Ohms = 0 THEN Init

IF pin0 = 1 AND Ohms = 1 THEN Init

rem loop while in range to avoid cycling lamps and meter enable


    READADC10 1, Current

LOOP WHILE Current > GoDown AND Current < GoUP

GOTO main