microcontroller driving 4x7 segment display pic 2

  
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I`m designing a circuit to drive a 4x7 segment display using a PIC. I`ve done this in the past without any drama. I would just use four pins to control each of the four digits (one each). I`d switch one digit on, wait a bit, turn it off, then switch the second digit on etc. I was hoping that I could get away with using just one pin from the PIC an
microcontroller driving 4x7 segment display pic 2 - schematic

d pulse it (square wave) through some interfacing electronics which would have four output pins that toggle on and off in a sequence, one at a time. (IE 1, 2, 3, 4, 1, 2, 3, 4 etc) Or even better a 4511 BCD-7 segment driver chip, that even has high current capacity to feed the display - it`s often used commercially to save pins when multiplexing 7 segment displays. Another option to consider is to use an 8-bit shift register (like the 74HC595) to connect to the 8 segments, and drive the common cathode/anode pin using a transistor. This will require 6 pins, 3 for the shift register and 4 for the digits. If board space is tight and you don`t have room for an extra chip, you might consider Charlieplexing the display. The "up side" is you only need eight (8) pins and you should get the same brightness and performance you`re getting now (with direct segment drive). The "down side" is the funky display wiring. The last project I used a 4x7seg display and switched each digit on/off with NPN transistors and drove each segment directly (via resistors) from the PIC. On a side note, I used PIC16F88. See image below. This is an older one I did some time ago. As you can see, the board`s getting pretty cramped. Although I was fairly happy with the current usage (overall about 70mA including running the sensor on the board). And it was quite an effective little unit. It`s a small display. Actually, the board size limitations are a mere...



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