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Tuesday, September 1, 2009

Multiplexing

Now I plan to connect another alpha-numeric segment led. But I have only one output pin for multiplexing. Do you know what is multiplexing? Well, when we need to use many segment indicators on one micro-controller with not that much pins, we have to use multiplexing. Here how it works: we turn on first indicator and show segments, then after 0.1 seconds we turn off first - and turn on second indicator and show segments again and so on and so on. Our eyes can't see any flickering but we can see all indicators and segments like if they turned on at the same time. Maybe my explanation too bad, you can always google it :)

In many articles about segment leds multiplexing I saw authors used one pin for one indicator. So if I have 2 indicators I should use 2 pins, but it's wasting of pins! I have only 1 pin for showing one of two indicators at a time. And know what? I have made a little schematics which can let me do it! I have used a NOT logic gate made of transistors.

Schematics:

How it works:
When switch (SW1) at lower position Q1 (PNP) turns on and let current flow to SL1 and SL1 lights up. When switch at higher position Q2 (NPN) turns on and work as NOT gate for Q3 (PNP) which gets low signal and turns on, as Q1 when switch at lower position, and SL2 lights up. For my project I will put segment leds indicators in place of leds on picture and instead of switch I will connect wire to a pin on micro-controller.

That's all for today. Next I will write a program to control multiplexing and show digits and letters on alpha-numeric segment led indicators. Stay tuned.

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