Designing LED Matrix Circuit
LED
matrix is the number of LEDs arranged in columns and rows. Those LEDs
are then used to display images or text that usually displayed with
certain animation effects. Therefore, the LED matrix is often referred
to as the Running Text or Moving Sign.
Arrange LEDs as matrix
MXLED
is a simulator of the circuit matrix of LEDs. With this simulator, we
can try to program the LED matrix controllers even without the hardware.
MXLED made LED matrix by arranging the LEDs on the vertical lines and
horizontal lines. We must provide the number of rows of horizontal lines
(we make eight lines). Then, we also make as many vertical lines of
columns. Arrangement of vertical lines and horizontal images are as
follows:
Vertical and Horizontal Line
Lines
of vertical and horizontal are not connected. Then, at each meeting
point between the vertical and horizontal lines, attach a LED by
connecting its anode to the horizontal lines and its cathode to the
vertical lines. Installation of these LEDs are as shown below:
Installation of LEDs
By
installing LEDs as above, the light up LED is the LED where the anode
is connected to the horizontal lines which is high (1) and the cathode
is connected to the vertical lines which is low (0). There is only one
low vertical line at a time, while other line must be kept high. This
low vertical line we refer to as the active column. Unlike the vertical
lines, horizontal lines consisting of eight lines may has high or low
value without having to pay attention to the other horizontal lines.
Horizontal Control
To
give the voltage on this horizontal line, we can not directly connect
it to port. This is caused by the need for large electrical currents.
Therefore, horizontal lines are the powered using a PNP transistor as
shown below:
The power supply for the horizontal lines
Each
horizontal line given a transistor like the picture above. In this way,
to make the horizontal lines can give the current to the LED, then the
base must be low state (0). Meanwhile, if the base was given a high
state, then the line could not provide current to the LED.
Vertikal Control
As
horisontol lines, vertical lines can not be controlled directly using
the port. This is caused by the large amount of current that must be
sunk into the ground. Therefore, we can use the NPN transistor to sink
current from these vertical lines. Way of installation is as shown
below:
Vertical line current sink
Each
vertical line is controlled using a transistor as shown above. If the
base was given a high condition (1), then the line (column) will be an
active line. Conversely, if the base were given the condition of low
(0), then the line becomes inactive column.
As already mentioned
above, there is only one column that should be active, while the other
columns must be inactive. There are many ICs that output works like
that. One of the cheapest is the 4017.
Columns control using IC 4017
Too bad that this IC only has 10 outputs. Thus, we can only make the controls to 10 columns
Relax,
we can use other tricks to extend the control capabilities of this
4017. The trick is to make the block columns. The columns are grouped
into block of columns. Each block columns consists of 10 columns. The
columns in each block columns are numbered from 0 to 9. So, column 0 is
the column 0 of block 0, column 10 is the column 0 of block 1, column 21
is column 1 of block 2, and so on.
Each base of the column with
the same number are combined into one and is controlled by an output of
4017. For example, the base of the column 0 is connected to the base of
the column 10, column 20, column 30, and so on. Then the bases which
have become one, controlled by the Q0 of 4017.
Block Columns
From
the picture above, we can see that although the base of column 0 and
column 10 are controlled concurrently, but the column that can sink
current is only the column with the active block. Seen that way of
controlling the “column” or “block columns” is the same. If the number
of block columns is only a few, then the controlling block of columns
can be done directly using the port of the microcontroller. However, if
the number of block columns is quite a lot, then the block columns can
also be controlled using the other 4017. So, we develop a multilevel
4017, ie 4017 units (control column) and 4017 tens (the controlling
block of columns). And if the number of block columns more than 10, then
we can make the next level, ie 4017 level in the hundreds.
To
cascade 4017 as above, is by connecting the carry output of 4017 units
level as the input clock for the 4017 of tens level. And if there is
4017 of hundreds level, then the carry output of 4017 of tens level is
used as the input clock for the 4017 of hundreds level. And so on. While
the reset input of all of 4017 are combined into one. Thus, the overall
control system only requires two lines for columns control, ie the
input clock (clock input for 4017 of unit level) and the reset input
(composite of all reset of 4017). Thus, the preparation of the LED
matrix is exactly the same as the
MXLED simulation.
Power Current Calculation
From
the explanations above, we can see that there are three kinds of
transistors viewed from the position. First is the row controller
transistor, second is the column (unit) controller transistor, third is
the column block (tens) controller transistor. Of course this
calculation assuming that the number of columns are not more than 100
columns.
Calculation of the row controller transistor current
If
each LED using a current of I, then the total maximum amount of current
through the line control transistor is I x number of columns. For
example, if each of our LEDs designed to use 5mA current and the number
of columns are 30 columns, then the row controller transistor should be
able to drain 5 x 30 = 150mA current.
The next question is, what is the value of collector resistor of the transistor?
To
answer these questions, we should see the path of current from the
power supply to get to the ground. First, the power supply current enter
the PNP transistor through the emitter to the collector. Then passes
through the collector resistor, then continues to enter into LED, enter
into the collector of column controller, then enter the collector of the
block columns controller.
If the power supply used is 5V, how
much is the clamp voltage across the resistor? The voltage across the
resistor is 5V – V on LED – VCE line controller tansistor – VCE of
column controller transistor – VCE of column block controllers
transistor. LED clamp voltage is typically about 1.7 V, but there are
some types of LEDs which have a clamp voltage of up to 3V. While VCE
transistors in saturation state is typically around 0.3 V. We assume
that we use a regular LED with a clamp voltage of 1.7 V. Thus, the
voltage across the resistor pin is 5 – 1.7 – (3 x 0.3) = 2.4 V.
Once
we know the clamp voltage at the collector resistor of the line
controller transistor, to calculate the magnitude of the resistor is R =
V / I = 2.4 V / 150mA = 16 ohms. Too bad that we might be difficult to
obtain this value of 16 ohms. So we can use a slightly lower value, eg
15 ohms, so that the LED current will be slightly higher, or use a
little higher, for example 18 ohms, so that the LED current will be
slightly lower.
Things should be kept on the LED matrix design
like this is, that these calculations is the calculation of the current
if the scanning is running. If scanning is not running, then a current
of 150mA will be entered on one LED alone
And currents of this magnitude would likely destroy the LED. So, we
must take precautions so that the current should only flow if scanning
process has been running. A little good news is, if we use the MCS-51 as
a controller, so when the reset (the scanning process is not running)
port is always in a state of high. So, if the base of line controller
transistor is connected to the MCS-51 port, so it can be sure that the
current will not flow to the LEDs on the reset state. However, you must
ensure that the program will not hang. Because if the program hangs and
stops the scanning process, then it means disaster for your LED matrix.
And if you are not sure that your program can run smoothly, then you can
reduce the LED current so that even if the scanning process is not
running, current flows are still small enough to be able to be borne by
one LED.
To calculate the base resistor value of the line
controller transistor, we only consider the transistor to work as a
transistor switch, which is so current that flows is the saturation
current. With the calculation of the collector current of 150mA, then we
can use the C9012 transistor that has a current gain of about 150. We
can take a value of 100 to make it more secure. With the gain of 100 and
the collector current of 150mA, then the base current should be about
1.5 mA. Clamp voltage at the base resistor is approximately 5V – VBE,
where VBE is about 0.7 V. Thus, the clamp voltage at the base resistor
is about 4.3 V. Thus, the base resistor value is 4.3 V / 1.5 mA = 2K8.
And again that this value is not available values. So we could slightly
reduce the value to the nearest number of available, namely 2k7.
Calculation of the column controller transistor current
Column
controller will sink as much current number of rows in that column. And
we have determined that the number is eight lines. If we have
determined that the LED current is 5mA, then the maximum current to be
sunk by the column controller transistor is 8 x 5mA, which is 40mA. With
the value of this, we simply use the C9013 transistor to control the
column. The Gain of C9013 transistor is also about 150. And as before,
we take saver value to 100 only. With the gain of 100 and the collector
current of 40mA, then the base current in the column controller
transistor is 0.4 mA.
Clamp voltage at the base resistor of the
column controller transistor is the output voltage of 4017 minus 2x the
VBE voltage. If we use a 5V power supply, then the output voltage of
4017 is approximately 4.8 V. Thus, the clamp voltage at the base resitor
is about 4.8 – 2 x 0.7 = 3.4 V. Thus, the base resistor value of the
column controller transistor is 3.4 V / 0.4 mA = 8K5. And again, this
value is not available values. So, just use the 8K2.
Calculation of the block columns controller transistor current
The
block columns controller transistor will sink as much current in the
column controller transistor x 10, which is 10 x 40mA = 400mA. From the
datasheet, C9013 has a maximum collector current of 500mA. So, we can
still use the C9013 to controll block columns.
Clamp voltage at
the base resistor of the block columns controller transistor is the
voltage used to control this transistor reduced by 1x VBE. If we also
use 4017 to control the block column, then the clamp voltage across the
base resistor of the block columns controller transistor is
approximately 4.8 – 0.7 = 4.1 V. Thus, the value of the base resistor of
the block columns controller transistor is 4.1 V / 4mA = 1K.
That’s it. Happy trying
Versi B.indonesia
