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Trying to understand multiplexing

Former Member

I think I understand the idea behind the schematic shown below.

 

 

image

The way I understand it is,....

 

If P1 is positive and P2 is negative, then LED3 will be turned on.

If that is reversed and P1 is made negative and P2 is made positive, then LED1 will be turned on.

 

By the same token, if P2 is positive and P3 is negative, then LED2 will be turned on.

Reversing the polarity of these two pins making P2 negative and P3 is positive, then LED4 will be turned on.

Same idea applies to P1 and P3 with LED5 and LED6.

 

What is baffling me is, when wiring these components and connecting to the pins on the Arduino, how do you reverse the polarity?

If pin1 is set HIGH (positive) and pin2 is connected to the ground pin, how can you reverse the polarity to switch LED3 off and turn on LED1?

This is probably something that an Arduino guru would know but as a newbie, it certainly has me baffled!

Of course, maybe I am just not really understanding the idea of multiplexing!

 

John

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  • John,

     

    The circuit you have is commonly referred as a charlie plexing.

     

    You have the basics of pin 1 set high and pin 2 as low. However, to operate properly, P3 must be tri-stated. That is, it is neither 1 nor 0. This is accomplished different ways for different microcontrollers. I am not that big of an expert on Arduino, so I will not go into that.

     

    For Charlie Plexing to operate correctly, the microcontroller must have truly tri-state able ports.


    Steve

  • in reply to LuckyMrH

    There's an article on Charlieplexing at Wikipedia, with a section on the circuit you've shown.

  • in reply to johnbeetem

    I agree with John you should read the Wki help but here is another quick quide that should help explain it.

     

    using three pins and six LEDs.

    Lets's say you have three LED control lines.  Call them A, B, and C.  Connect a pair of back-to-back LEDs between A and B, between B and C, and between A and C.

     

    Now start by setting pins A and B as outputs, and C as an input.

    Turn on LED AB1 by setting high on pin A and a low on pin B.  Turn on LED AB2 by reversing the polarities of A and B.

    Set pin B as an input and leave A as an output.  Set pin C as an output.

    Turn on LED AC1 by setting high on A and a low on C.  Turn on LED AC2 by reversing the polarities of pins A and C.

    Set pin A as an input, and B as an output.  Leave pin C as an output.

    Turn on LED BC1 by setting high on B and a low on C.  Turn on LED BC2 by reversing the polarities of pins B and C.

  • in reply to johnbeetem

    Thanks John,

     

    I will read the wiki article and see if it helps.

     

    John

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