What is 'Charlieplexing'? It is driving lots of LEDs with only a few pins. In case you're wondering Charlieplexing is named after Charles Allen at Maxim who developed the technique.
This can be useful for lots of things. You may need to display status information on a small microcontroller, but only have a few pins spare. You may want to show a fancy dot matrix or clock display but don't want to use lots of components.
Some other projects demonstrating charlieplexing you may want to look at are:
How to drive a lot of LEDs from a few microcontroller pins.
by Westfw :- http://www.instructables.com/id/ED0NCY0UVWEP287ISO/
And a couple of my own projects,
The Microdot watch:- http://www.instructables.com/id/EWM2OIT78OERWHR38Z/
The Minidot 2 clock:- http://www.instructables.com/id/E11GKKELKAEZ7BFZAK/
Another cool example of the use of charlieplexing is at:
The Minidot 2 clock introduces an advanced charlieplexing scheme for fading/dimming which won't be discussed here.
UPDATE 19 August 2008 : I've added a zip file with a circuit that may be able to exploit the matrix charliplexing for high power LEDs discussed (at length :) ) in the comments section. It has a pushbutton + position encoder to do a user interface, plus circuitry for either USB or RS232 computer control. Each of the high side voltage rails can be set to one of two voltages, say 2.2V for RED LEDs and 3.4V for green/blue/white. The voltage for the high side rails can be set by trimpot. I'd envisage that a 20wire IDC ribbon cable be plugged into the board, and 20pin IDC connectors added along the length of the ribbon, each LED board having links to whatever wires in the matrix are desired. The circuit is in Eagle Cad and rendered in the sub image below. The high side circuit is implemented by using optocouplers which I think might be suitable.
I haven't actually tested this circuit nor written any software because of lack of time, but have put it up for comment, I'm particularly interested in the optocoupler implementation. Anyone brave enough to give it a go...please post your results.
UPDATE 27th August 2008: For those not using EagleCad....added below is a pdf of the schematic
Step 1: Some LED theory
Firstly what happens when you connect an LED to electricity.
The main diagram below shows what is called the If v Vf curve of a typical 5mm low power LED.
If stands for 'forward current'
Vf stands for 'forward voltage'
The vertical axis in otherwords shows the current that will flow through an LED if you put the horizontal axis voltage across it's terminals. It works the other way around as well, if you measure that the current is of some value, you can look across to the horizontal axis and see the voltage the LED will present across it's terminals.
The second diagram shows a schematic representation of an LED with If and Vf labelled.
From the main diagram I've also labelled areas of the graph that are of interest.
- The first area is where the LED is 'off'. More accurately the LED is emitting light so dimly you won't be able to see it unless you had some sort of super-duper image intensifier.
- The second area has the LED just slightly emitting a dim glow.
- The third area is where an LED is usually operated and is emitting light at the manufacturers rating.
- The forth area is where an LED is operated beyond it's operating limits, is probably glowing very brightly but alas for only a short time before the magic smoke inside escapes and it won't operate again......ie in this area it burns out because too much current flows through it.
Note that the If/Vf curve or operating curve of the LED is a 'non-linear' curve. That is, it is not a straight line...it has a bend or kink in it.
Lastly this diagram is for a typical 5mm red LED designed to operate at 20mA. Different LEDs from different manufacturers have different operating curves. For example in this diagram at 20mA the forward voltage of the LED will be approximately 1.9V. For a blue 5mm LED at 20mA the forward voltage might be 3.4V. For a high power white luxeon LED at 350mA the forward voltage might be around 3.2V. Some LEDs packages might be several LEDs in series or in parallel, changing the Vf/If curve again.
Typically a manufactuer will specify an operating current which is safe to use the LED at, and the forward voltage at that current. Usually (but not always) you get a graph similiar to below in the datasheet. You need to look at the datasheet for the LED to determine what the forward voltage is at different operating currents.
Why is this graph so important? Because it shows that when a voltage is across the LED, the current that will flow will be according to the graph. Lower the voltage and less current will flow.....and the LED will be 'off'. This is part of the theory of charlieplexing, which we'll get to in the next step.