CHARLIEPLEXING LEDS THE THEORY PDF

– Download as PDF File .pdf), Text File .txt ) or read online. Electronic theory, schematic circuits and PIC tutorials. Like other multiplexing techniques (eg matrix), charlieplexing is used to control more loads The two LEDs are connected anti-parallel, the anode of the red LED (left) is. The term “Charileplexing” is not a familiar one for many electronics hobbyists. Charlieplexing is a technique proposed in early by Charlie Allen (at.

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Charlieplexing is a rather new multiplexing technique. It was first proposed in early by Charlie Allen at Maxim Integrated Products for driving a multiplexed display. When the port is used as an input, it is driven into High Impedance state.

During this state, the port’s internal resistance gets very high values Mega Ohmand the current that flows through the port is very small, measured in micro amperes.

Here is the principle fharlieplexing operation. Now we add one more port to control more LEDs. I have broken apart the circuit into 3 sub-circuits, each one showing 2 ports. So, with the same theorh as before, a microcontroller can control individually 6 LEDs with only 3 ports.

A typical matrix cannot control that much LEDs with only 3 ports! As i said before, the charlieplexing utilizes the tri-state of a micro-controller’s port. Let me re-draw the same circuit with the 3 ports and the 6 LEDs, but this time i will not draw them separately:. But in what state must Port 2 be?

So, to avoid unwanted situations, Port 2 must be in High impedance state, often show with the symbol “Z”. Same applies of course if we try to light L2. So, the general rule is that, only 2 ports can be simultaneously outputs and all others are in High Impedance state by defining them as inputs. So, here is the Operation Table for the previous circuit.

You can calculate the maximum number of LEDs that can be controlled with Charlieplexing, if you solve the following formula:. Suppose that we want to light L6: L6 if forward biased and it will light, correct, but what about L2 and L4?

They are both forward biased! They should both light, shouldn’t they? The answer is simple, but you need to know something about LEDs.

LEDs have a very narrow voltage operation range. They may consume very little current, but they will not light if the voltage across them is bellow a specific value. Typically, LEDs operate from 2.

But L2 and L4 thdory in series, and -according to Kirchhoff- the total voltage drop across a series circuit is the sum of the voltage drops across each part. In other words, the series circuit is composed by 2 parts, L2 and L4.

So, each of these parts will have 3.

Moreover, the current through L2 and L4 will be very small, a couple of miliamps or less. Typically, the correct point to connect your limiting resistors is between the microcontroller and the ports of the Charlieplexing circuit. Each resistor must have HALF the value of the required value.

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charlieplexing

If for example you want to protect the LEDs with Ohms resistor, then you must use ohms resistors, and that is because, to light each LED, the current goes every time through 2 resistors, and not one.

Here is a typical circuit:. This circuit will operate normally, and all LEDs will have the same brightness.

I said before, that only two ports can be outputs simultaneously.

But that is not absolutely true. There are special situations, where more than 2 ports can be outputs, so that more than one LED is turned on. These LEDs can light simultaneously.

Take charliepleing example L4 and L6. But the problem is that they will have much less brightness cuarlieplexing the other LEDs. The answer is again simple. Suppose that each resistor is ohms and the voltage supplied by the microcontroller is 5 volts. There is a solution though. The solution is to connect one resistor for each LED, like this:.

The Charlieplexing

Charlielexing, the first circuit with the 3 resistors works pretty well, theoory if you plan to light each LED alone. If for some reason you want to light more than one LED charliieplexing, then you need to follow the second circuit with one resistor for each LED. In general, the second circuit is rarely used and i suggest you avoid it unless you have a very good reason. And don’t worry, there is another way to light more than one LED simultaneously Yes because as i explained before, the Charlieplexing cannot control more than one LED at a time i am always talking about the first circuit with the 3 resistors.

And no because, the human eye can be tricked: Suppose that you want to light L1 L2 and L3 simultaneously. The only thing that you need to do, is to turn on L1, then turn it off, turn on L2 and turn it off and then turn on L3 and turn it off as well, but all this in a short time and continuously.

It is the same principle like in a cinema movies. The pictures change so fast, that the human teh thinks it is a continuous film.

There are some limitations and parameters that you need to take into account. If for example the rate is 10Hz, the flickering will be visible. So, choose a frequency above 24Hz – remember that 24 frames per second is an acceptable movie frame rate.

Another problem that may appear is the brightness. There is though one solution for this problem. You may choose to use smaller resistors than the calculated. This way, more current will flow within each LED and thus it will be brighter. You don’t want to go much above the nominal Hteory current allowed for two main reasons.

First, the lifespan of the LED will be reduced. You may not notice the over-current situation by the brightness because of the PWM effectbut this over current will certainly reduce the lifespan of the LED. Thee second, if the current is much too high above the nominal forward LED current and the microcontroller crashes for some reason, then the LED that is currently turned on will have a very hard time, for it will remain Charliepelxing with full charliieplexing flowing Same as the classic matrixthe Charlieplexing technique can be used to interface buttons to a microcontroller.

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Here is a typical circuit to interface 6 buttons with only 3 microcontroller inputs:. The efficiency is unmatched.

To determine which button is pressed, the microcontroller must run a loop through all inputs. Each time, only one pin is defined as output and all others as inputs. If the microcontroller supports internal pull-up resistors, then they can be used to minimize external components.

If the inputs are pulled-down, then the output pin is driven HIGH. The microcontroller then checks all inputs. Charlieplecing an input is found HIGH, then this means that a button is pressed. Each button corresponds to a specific Output-Input pair. Look at the following animation:. First if all, the diodes are mandatory for the circuit to operate normally. This makes charkieplexing overall use more complex.

But the most basic disadvantage it the ghosting. I have explained what the ghosting and masking thf are in the matrix theory, and i will not go through it again. Ghosting can also occur in the Charlieplexing method. In matrices, the ghosting can be solved by adding diodes to the buttons to prevent back current flow.

But in charlieplexing, ghosting cannot be solved. That is one good reason, why charlieplexing is not widely used, although it is so efficient. But still, it can be used charliepleximg cases where only one button is pressed at a time. If for example you make a through-feed machine charleiplexing multiple limit switches one after the other, you can safely use charlieplexing to interface them, as long as only one limit switch is activated at a time.

A very simple example Look at the following simple circuit, which uses two ports to control two LEDs. Let me re-draw the same circuit with the 3 ports and the 6 LEDs, but this time i will not draw them separately: Operation Table So, here is the Chwrlieplexing Table for the previous circuit.

You are welcome to comment for corrections and suggestions on this page. But if you have questions please use the forum instead to post it. Xharlieplexing 14 Februarycharlieplexkng I’ve been looking for a good tutorial on Charlieplexing for a while and this one explains a lot. At 21 October1: This will raise the turn-on voltage by 0. At 9 December You are also wrong that only one LED at a time can be displayed per display phase and wrong about where the resistors should be.

Where should the resistors be? Well, the n pins should connect to the cathodes directly, and be fed through n resistors, and the other sides of the resistors should go to the anodes of the LEDs.