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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.

ICL7135 4 and Half A to D converter with BCD Output

The below Details from Datasheet - These pins are used in this circuit to Autorange the DPM with some peripheral CMOS Logic.


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.


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.

Auto ranging Digital

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.

Here you can see ICL7135 to 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.

PNG-Schematic   -   PDF-Circuit   -   DSN Source

Auto ranging 4-1/2 Digit Digital Voltmeter - del20003

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.

Use Only Low Leakage Multilayer Plastic Capacitors for A-D convertors of Intersil like 7107, 7109, 7106, 7135. Use MFR 1% for all Resistors, 33E means 33 ohms, 22K means 22 kilo ohms and 4K7 is 4.7 K.

474 CD means 47 with 4 zeros pF, 470000 pF, remove 3 zeros 470 nF, shift decimal 3 places again 0.47uF.

The circuits that this can control are here ..
Here is another circuit -  Interface 80C51-80C31-89C51 to ICL7135

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20th Mar 2020


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