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 once.

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 Amplifier.

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 applications.

This circuit can be easily modified into a Process Indicator, Field programmable with one PIC Design. You can replace the logic and controls with the PIC. Retain ICL7135 as a A/D.

ICL7135 Pin 11 +5V, Pin 1 -5V, Pin 24, digital GND, add a decoupling 104 CD, cap from +5 to GND and -5 to GND.

• Precision Digital Attenuator
• Precision Amplifier

Related Resources -

• ICL7135 4 1/2 Digit BCD Output A/D Converter
• MAX133 MAX134 9V 3 3/4 Digit DMM Circuit
• Galvanometer as a DC Multimeter - Wolfram