Auto ranging 4-1/2 Digit Digital Voltmeter
Auto Ranging for ICL7107 had to be done with
window comparators. In ICL7135 Digital Outputs are
available for autoranging. Here in this circuit is
a close approximation of what i may have designed
The below Details from Datasheet - These pins are
used in this circuit to Autorange the DPM with
some peripheral CMOS Logic.
"OVERRANGE (Pin 27)
This pin goes positive when the input signal
exceeds the range (20,000) of the converter. The
output F/F is set at the end of BUSY and is
reset to zero at the beginning of reference
integrate in the next measurement cycle.
UNDERRANGE (Pin 28)
This pin goes positive when the reading is 9%
of range or less. The output F/F is set at the
end of BUSY (if the new reading is 1800 or less)
and is reset at the beginning of signal
integrate of the next reading."
The Range is Defined by QB and QC of 4029, there
are four ranges. The two bit Code from this can
control the decadic range of an Attenuator or
The Overrange and Underrange are in a OR formation
with R26-27. Clock from either will Inc/Dec 4029.
The Underrange Pulses are used by D2
-C8 to form a Steady Logic signal on Up/Down
Control of 4029. This works in a closed loop
manner, to Change the Range to match input Signal.
When there are no Pulses from Either UR or OR,
then that range is held.
This may be Slow Responding, There may be minor
errors in the circuit documentation, but it is a
product that worked and was used in some
Here you can see ICL7135
Printer port Interface
This circuit can be easily modified into a Process
Indicator, Field programmable with a uC Design. You
can replace the logic and controls with the PIC.
Retain ICL7135 as a A/D.