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  • Author Author: tariq.ahmad
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Charlieplexing Buttons and LEDs at the Same Time - How Hard Can It Be? -- Episode 604

So many buttons so little IO. When you want to drive a large number of LEDs or read buttons the obvious choice is multiplexing them. But there is a method called charlieplexing that lets you use even more LEDs and buttons with even less IO pins. Clem wants to interface a large amount of buttons and LEDs with a microcontroller but has a limited number of pins.   He's also trying to save money on parts. Could you just replace expensive signal diodes with cheap LEDs? Could you use the same pins to read buttons and drive LEDs in the same matrix? Could you also achieve true n-key rollover without having a diode on every single button? How hard can it be?

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Bill of Material:

Product Name Manufacturer Quantity Buy Kit
LED, Super Red, Through Hole, T-1 3/4 (5mm), 12 mA, 5 V, 640 nm Multicomp 9 Buy Now
Arduino Mega 2560, ATmega2560, 54 I/O, 256KB Flash, 8KB SRAM, 4KB EEPROM Arduino 1 Buy Now
Jumper Wire Kit, Multicolour, 2 mm - 125 mm, 22 AWG, 140 Piece Multicomp 1 Buy Now
Breadboard, Mounted, 1580 Pin, 10mm, x 135mm Multicomp 1 Buy Now
 

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  •   Mike, this is a very sore subject with me.  I have considered using Charileplexing my Navigation Display Unit to refresh your minds this beast has 13 seven-segment displays and 4 sixteen-segment displays. I was first introduced to it when I was looking for an IC to drive this mess. I came up with a MAX6955 the problem was two-fold

    1. my displays are Pinlite and they are incandescent
    2. Not only it is plain in the rear to wire but try and debug it. I double dare you.

    came up with my own solution which which needed 3 seven-segment decoders driven from an Arduino Mega I also used an MCP23016 to drive the four 16-segment displays. The word drive is a misnomer it should have been select. This requires 7 selects (7-seg) and 16 selects (16-seg) which is (16x4) + (7x13) ::= 64+ 105 = 169 each driven by a transistor. 

    Then you need 17 more selects that drive one transistor per select.  so that 17 + 169  transistors moral of the story? Why in gods name do we have to make our crap harder than it has to be? Yes, it works in production, when pennies count. Never in experimental electronics. It should have been buried a long time ago. Some folks like Mike like to flagellate themselves. Also, don't ever obfuscate something.  As Mr T. said "pitty the fool!" the problem is you end up being the fool when you have to figure out why the creation aka Frankenstein does not work.  

    I have probably said too much. ~~ Cris H. 

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  •   Mike, this is a very sore subject with me.  I have considered using Charileplexing my Navigation Display Unit to refresh your minds this beast has 13 seven-segment displays and 4 sixteen-segment displays. I was first introduced to it when I was looking for an IC to drive this mess. I came up with a MAX6955 the problem was two-fold

    1. my displays are Pinlite and they are incandescent
    2. Not only it is plain in the rear to wire but try and debug it. I double dare you.

    came up with my own solution which which needed 3 seven-segment decoders driven from an Arduino Mega I also used an MCP23016 to drive the four 16-segment displays. The word drive is a misnomer it should have been select. This requires 7 selects (7-seg) and 16 selects (16-seg) which is (16x4) + (7x13) ::= 64+ 105 = 169 each driven by a transistor. 

    Then you need 17 more selects that drive one transistor per select.  so that 17 + 169  transistors moral of the story? Why in gods name do we have to make our crap harder than it has to be? Yes, it works in production, when pennies count. Never in experimental electronics. It should have been buried a long time ago. Some folks like Mike like to flagellate themselves. Also, don't ever obfuscate something.  As Mr T. said "pitty the fool!" the problem is you end up being the fool when you have to figure out why the creation aka Frankenstein does not work.  

    I have probably said too much. ~~ Cris H. 

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