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.
#PICAXE 18X
#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:
SETFREQ m4
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
ELSE
LET Mode = 0
LET Ohms = 0
ENDIF
LET NewRange = 1 'Ensures entering the range into the pins
PAUSE 200 'Settle the circuits
READADC10 1, Current 'Initial meter current reading
Main:
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
ENDIF
ENDIF
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
ENDIF
ENDIF
LET NewRange = 1
ENDIF
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
ENDIF
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
ELSE
LET Mode = 1 'AC by default
ENDIF
ENDIF
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
ENDIF
LET Allouts = Range & 7
LET Allouts = allouts + 208 'Set meter enable now
LET pins = Allouts
PULSOUT 3,1 'Pulse the 74259 gate
ENDIF
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
DO
READADC10 1, Current
LOOP WHILE Current > GoDown AND Current < GoUP
GOTO main