Picture of How to drive a lot of LEDs from a few microcontroller pins.
Using the fact that many microcontroller pins have three states (+V, GND, or
"high impedence", you can drive N*(N-1) LEDs from N pins. So the little 8
pin microcontroller like a PIC12Fxxx or an ATtiny11 can drive 20 LEDs on
its five available output pins, and still have one pin left for some kind of input.

See also http://www.instructables.com/id/Charlieplexing-LEDs--The-theory/

Step 1: 20 LEDs on 5 pins

Picture of 20 LEDs on 5 pins
The current crop of low pin-count microcontrollers (6 pins to 20 pins on
the whole package) are attractively priced and 'cute', but the question
arrises as to how you can make the best use of those pins for common
applications such as driving LEDs.

A direct-connect approach to driving LEDs consumes one pin for each
LED. A traditional multiplexing scheme where rows of LED anodes are
driven by one set of N pins and each row's common cathode is driven by
another set of M pins manages to light N*M LEDs with N+M pins.
However, on a processor with only 5 or fewer outputs (as is the case
with most 8-pin microcontrollers), this barely gets you any more
outputs than direct drive.
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dedward6 years ago
Hiya, I'm really interested in how you've done this, and I would like to use the technique on a project I'm working on at the moment, which is a working model theatre. Does anyone know whether it is possible to have more than one LED on at once if they are connected through to the same pins?? Thank you!!! Dedward
You Can, Almost all Large LED displays use this Multiplexing Theory. Its in the code to have many lights on in the same row and column on at the same time. Sadly, I'm still working on learning that Part. but there are many resources out there that can help with the code
westfw (author)  Freeztech6 years ago
(after nearly a year of deep thought,) I think the problem with having more than one LED on at a time in a charlieplexed environment stems from using the microcontroller pins as both the current source and current sink for the LEDs. Most microcontrollers (old-school ones, anyway, like PICs and AVRs) can easilly source/sink enough current for ONE led (10-40mA), but if you start talking about a pin sinking the current for SEVERAL LEDs, you quickly run the risk of exceeding the maximum current specs.

In a traditional multiplexed environment it is easy to make the anodes and/or cathodes each have their own high-current drivers as needed, but it's much more difficult to do this with a connection that can be anode, OR cathode, OR off depending on circumstances (at least, without reaching a circuit complexity level that is the opposite of what we're trying to achieve by charlieplexing.)

This is a long time later, but still an interesting problem. You can turn on more than one LED by using PWM to light the LEDs. e.g.You can multiplex through turning on each LED (if it's LED is meant to be on) and keep it on for up to a set amount of time depending on how bright it is meant to be (lets say 1ms is the brightest). You ignore the LEDs that are meant to be off and continue on to the next LED and the next and so on until you reach the last. If the last LED is done, you then start at the beginning again. Now the problem with this is that you will pass through all the LEDs in max 20ms. The eye perceives flashing if the changes happen slower than 24mS, so we are under that threshold, so it should work. The max brightness for any LED will be 1/20 of full brightness (if all LEDs are on). However, if typically only a small number of LEDs are on, leds say 3 or 4, then they'll be on for 1/3 or 1/4 max brightness. If someone has this circuit, maybe they can experiment and see how well this works....the LEDs might be too dim if all on to be able to see any one LED being only on for 1/20 of the time.

Well, that's pretty impressive for all passive hardware!
It puts the programming a bit beyond a lot of people though! LOL!
jalke1163 years ago
good day!!! i just want to know of what are the software used for making schematics like in this instructable...

hoping for your answers...
Eagle is commercial product but there is free edition which has some restrictions of course (board size, number of layers etc.). There are open source alternatives like KiCad.
westfw (author)  jalke1163 years ago
These were done using EAGLE from Cadsoft. http://www.cadsoftusa.com
bpark10003 years ago
You mention the trick of re-arranging the bits to make the circuit board simpler. Good plan! The way to do it easily is to first write the software as if the bits didn't need switching. Instead of writing the output byte directly to the port, you first write it to the index register. Then you use a "scramble table" to re-arrange the bits. You index into the table, then write that byte to the port. The advantage of this scheme is if you need to change the bit order, no change is needed to the code; only the table.

(I'm not familiar with this processor. My comments are based on the ability to do indexed addressing mode.  Index into a table. For example, for the "noadjust" routine, you would have:
TABLE: .byte 0b00100, 0b00010, 0b00001, 0b10000, 0b01000

copy TEMP to index register
load accumulator TABLE,index
copy accumulator to ANODE
You can have separate tables for the port and the direction register. The tables can be large (256 bytes for an 8-bit scramble function) without needing a compare for each value possible. The code becomes compact and easy to debug. You can stack tables. For example, one set to give the port/direction drives, and one to do the bit scrambling (used twice: once to scramble the port bits, again to scramble the direction bits)).

Table lookup is the key to fast processing!
weeladalah4 years ago
can i use Attiny13A instead of Attiny11...?? thanks :)
westfw (author)  weeladalah4 years ago
Yes; the same principles would apply with any 6-IO cpu...
awidarto5 years ago
This might be of interest :
the additional transistor should be able to source additional LED in series, at some additional cost though...
akatsuki6666 years ago
can you download it on the computer if so what websight?
westfw (author)  akatsuki6666 years ago
What operating system are you using? And what AVR hardware? Did you actually build the PCB that was downloadable? While this code is a sample of how to do "Charlieplexing", it's certainly not an "easy to understand" thing for a beginner. You might want to look at This Instructable, which uses 6 outputs from an Arduino to drive 27 LEDs...
Hi, i'm also kind of new to the arduino and multiplexing i know hom it works but the only thing i need help with is how i can send "GND" signals through the pins.? What do i need to write in the code to make it do that?
westfw (author)  MultiViews6 years ago
In Arduino, "digitalWrite(pin, LOW);" outputs GND signals through that pin, for all practical purposes. (digitalWrite(pin, HIGH); sends +5, and "pinMode(pin, INPUT)" turns the pin off, mostly.)
Thank you :D
geeklord7 years ago
Hey im a 13 year old kid, and it looks like using a microcontroller would be a fun thing. Where the heck did most of u guys learn this stuff? Even if i knew how to just set up a microcontroller and just copy&pasted code would be nice
Zdawg geeklord6 years ago
Boe Bot kitwhats a microcontrolerhi, im 13 also and i use microcontrolers for a ton of fun things, a great way to get a start is with a parallax, whats a microcontroler kit(my first kit), or a parralax Boe bot, which im still working on improving. for the most fun i woudl recoment getting the boe bot
I agree, Parallax Does an exellent job of teaching these kinds of techtronics. I started with the BASIC Stamp 2 Board Of Education kit and I can honestly say that every penny was worth it. I would definatly recommend buying the Boe-Bot kit if you are a fairly quick learner. All of Parallax's starter kits are definatly high-quality and the included textbook is really well-written (i.e, something that you can actually read without letting the ZzZzZs turn your book into a pillow.) One Kit can pretty much teach you all of the basic -level stuff, and from there you can buy other kits, or you can put some thought into design and make just about anythhing you can come up with. But i would recommend taking a computer science class to become more familiar with programming computers. Also, if you are unfamiliar with Java, computer sciences can help, if you are a seasoned veteran to programming, we have our own subspecies! Here it is:

Homo sapiens programmiens- A solitary, red-eyed mammal which often talks to inanimate objects.
True skills.JPG
westfw (author)  Zdawg6 years ago
See also http://www.instructables.com/id/How-to-choose-a-MicroController/
which discusses a number of strategies for starting with microcontrollers in some detail. It does include mention of the Parallax Basic Stamp, Arduino, and other "entry level" alternatives...
Zdawg Zdawg6 years ago
they both use a BASIC Stamp microcontroler,
Zdawg Zdawg6 years ago
and they both come with guides on how to program them with examples, and if u follow it u will be programing them in no time! there are also online codes and help and a discussion frum on the parralax website
thanks for all the comments. I'll probably get an arduino...
Buy an Arduino, and get the free software with example codes. Also, on their main website there are many tutorials and guides that help you learn.
westfw (author)  geeklord7 years ago
Take a look at the How to Choose a Microcontroller instructable. Be sure to read the comments; there's a lot of good data there as well as in the instructable itself.
akatsuki6666 years ago
umm i tryied to download the file to my arduino microcontroller and it has a error for about everything u typed. why does it do that??????
westfw (author)  akatsuki6666 years ago
Um. How exactly are you trying to "download" it? It's an assembler program rather the C or Arduino sketch, so it has an entirely different way that it needs to be built (and a different set of tools.)
akatsuki6666 years ago
i kno im getting annoying.. but how do you ground the microcantroller?????????????
westfw (author)  akatsuki6666 years ago
"GND" on circuits like this usually just means a common connection to the negative side of the power supply. (VCC or VDD being the positive side...)
man you own lol. ty
akatsuki6666 years ago
how u kno what resistors to use agin? umm
(total - led voltage)/ma = ohms??? if that is pls say or tell me the right 1
westfw (author)  akatsuki6666 years ago
That looks right to me. There are lots of Intsructables/Tutorials/calculators about computing the LED resistances...
akatsuki6666 years ago
hi its my first time too and im just wondering... what is the ma of a led???
westfw (author)  akatsuki6666 years ago
Most common LEDs are spec'ed at up to 20mA or so (continuous.) In an arrangement like this, where each LED is only on for a fraction of the time, you can get away with higher currents, but the absolute max depends on the exact LED. There are "high power" LEDs that will take over 1A.
Modarius6 years ago
Kind of a newbie question but how do you open the eagle files in eagle?
westfw (author)  Modarius6 years ago
Depending on your OS, you should be able to unzip to .brd and .sch files, and then just double-click them. Or use the "open" menu of eagle.
Modarius westfw6 years ago
thanks very much it worked like a charm
DT sleper7 years ago
Geeklord visit sparkfun.com and look at the picaxe microchips they are cheap, the software is free and they are easy to learn. Its a good place to start or look at parallax.com at their basic stamp it is also easy but more expensive. I haven't upgraded myself to more powerful chips yet because I am happy with the picaxe.
aiden1200008 years ago
i wish i understood what this is talking about
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