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Leading Zero Digit Blanking.



This circuit was designed to blank leading-zero digits for an ICL7107 voltmeter chip.
It may be useful for other chips, but it only works with non-multiplexed displays.

Leading Zero Digit Blanking Circuit.

7-segmant number display.
Fig.1: 7-segment number display.

From Fig.1 it can be seen that if we check for segment d off or segment g on, we know if the character is a "0".
This does not work with numbers above "9" as the display drivers I know of use upper-case "C".

Leading Zero Digit Blanking Circuit schematic.
Fig.2: Leading Zero Digit Blanking Circuit.

RA and RB makes sure that there is a measurable voltage swing across the LED segments.
RC..RF Divides the segment voltages so they are within the common mode range of UA.
UA is a dual ( LM393 ) or quad ( LM339 ) comparator with open-collector output.
DA prevents non-leading zeros turning off the digit.

Table 1: Circuit voltages with 5 V supply and 1.8 V LED forward voltage.
Segment dSegment gVaVbVcVdVeDigit
OffOff4.7 V4.7 V2.6 V2.3 V0 VOn
OffOn4.7 V3.2 V2.6 V1.6 V0 VOn
OnOff3.2 V4.7 V1.8 V2.3 V5 VOff
OnOn3.2 V3.2 V1.8 V1.6 V0 VOn


Circuit Calculations.
Also a description for a circuit with n+½ digits.

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