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EDN, May 26, 2011

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Figure 2 This modified version of the circuit in Figure 1 produces current using negative voltages. 10 μF C1 220 R1 −950V 100k R2 ZR431LF01 D2 BZV55C12 D1 330k R3 330k R4 330k R5 BUZ50 Q1 OUT− 10 μF C2 + + 10M R6 10M R7 Arrange LEDs as seven-segment displays Charaf Laissoub, Valeo Engine and Electrical Systems, Créteil, France ↘ When you need to drive three 5V 1 VDD R1 1k 100 nF C1 8 VSS PIC12F508 4 10k R2 GP3 GP0 GP1 GP2 GP4 GP5 7 6 5 3 2 7 6 5 3 2 Figure 1 An eight-pin microcontroller can drive three seven-segment LED displays. 100 R5 R4 100 R6 100 R7 100 R8 100 1N4148 1N4148 1N4148 1N4148 1N4148 G LED21 470 R3 5V HUNDREDS TENS B A F G E D C E A F G B C D E UNITS A F G B C D NOTES: R1 IS OPTIONAL BECAUSE GP3 IS AN INPUT-ONLY PIN. OTHERWISE, IT IS SAFER TO PUT R1 WHEN USING A TRUE I/O LINE AS INPUT. USE 21 SUPERBRIGHT FLAT LEDs, ARRANGED AS A THREE-DIGIT PSEUDO-SEVEN-SEGMENT DISPLAY. 19 G 20 G 91011121314 EF CD DC 15 16 17 18 EF FE 12 34 56 78 AB CD AB BA seven-segment LED displays, you typically need 10 I/O lines—and that's without a decimal point. You might think that you cannot accomplish that task without a binary-to-seven-segment EDN 110526DI5142 Figure 2 DIANE decoder or a serial-to-parallel shift reg- ister (Reference 1). Many previous Design Ideas have shown how to maxi- mize the number of LEDs you drive with a minimum number of I/O lines (references 2 through 5). This Design Idea shows how you can build a circuit that drives 21 LEDs, thus forming three pseudo-seven-segment displays. MAY 26, 2011 | EDN 55 EDN 110512DI5135 FIGURE 1 DIANE

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