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.
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".
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.
|Segment d||Segment g||Va||Vb||Vc||Vd||Ve||Digit|
|Off||Off||4.7 V||4.7 V||2.6 V||2.3 V||0 V||On|
|Off||On||4.7 V||3.2 V||2.6 V||1.6 V||0 V||On|
|On||Off||3.2 V||4.7 V||1.8 V||2.3 V||5 V||Off|
|On||On||3.2 V||3.2 V||1.8 V||1.6 V||0 V||On|
Also a description for a circuit with n+½ digits.
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