Led Bar Demo Board ManualFunctional description.
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| Led Bar Demo Board Manual Functional description. Although this site is as up-to-date as possible, we can not be held accountable for possible damages as a result of use of information and/or software obtained from this site. |
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The powersupply for the Led Bar Demo Board is
quite simple. You can use any DC supply from 8V through 16V, as a mains
adapter or a car battery. D1 prevents you from blowing up the Led Bar
Demo Board when you accidentally reverse the polarity on TP6 and TP7.
C1 and C3 will do some basic suppression and equalisation on the
voltage going into IC1, our voltage regulator. The LM317 can be set to
a specific output voltage using a combination of two resistrorvalues,
R4 and R5. There is a formula: R5 equals R4 times the wanted
voltageminus 1.25 divided by 1.25. If we want a voltage of 5V, and we
have a R4 of 330E, we will get 330 * (5-1.25)/1.25 equals 990E, so
close to 1k. C5 and C6 do some additional suppression. As you see the
elco's are "only" 100u, which by a rule of thumb means that our
powersupply can only deliver about 100mA. This however is more than
enough since due to the design of the Led Bar Demo Board it is
impossible to draw more than the current of one led (!) plus the few
milliamps the microcontroller uses. Maximum current drawn is about 25mA.
As I said before, we can only drive one led at
a time. To be able to drive 12 different leds with only 4 outputs, we
have to make some kind of matrix. A very important thing about the
Microchip microcontrollers, is that you can change functionality for
every I/O pin on- the- fly. You can make a pin a high output, a low
output or a HighZ input. To the left of the picture on the right, you
will see the resistors (R7-R10) going to the microcontroller. Those are
connected to I/O pins. If we want to illuminate LED1, we have to make
the pin connected to R7 a high output, the pin connected to R8 a low
output, and the other two HighZ inputs. If we want LED2 to glow, we
have to keep inputs and outputs the same, but we have to reverse
polarity on R7 and R8, singe LED1 and LED2 are connected in anti-
parallel. With four I/O pins we can drive 12 different leds, but not at the same time.
Try to get LED1 and LED2 both to illuminate at exactly the same time,
it's just not possible. Note. If you decide to use the Led Bar Demo
Board to be built in some cabinet, you could mount the leds on the
solderside of the PCB. LED1 will still be LED1, but logically in your
software you could call it LED12, that's the beauty of software. The
maximum current that can be drawn by a led is about 3 volts (led drops
about 2V) accross 2x330E, so about 5 mA. You can change R7-R10 to
whatever value you like, but don't go below 68E (max 20mA).
The
circuit on the left will let us talk about inputs. As I mentioned
before, we use four pins to drive the leds. These are connected to GP1,
GP2, GP4 and GP5. We're not going to talk about those in this section.
An 8 pin microcontroller of the PIC12F675 type has five I/O pins and
one input- only pin. That pin (GP3) can never be used as an output. The
combination R3/C2 on its input will debounce any spikes on TP4. We have
to prevent any voltage coming into GP3, so we will use D2 for that
purpose. The R3 resistor is a pullup resistor, so when not activated,
GP3 will see a high input. When you short TP4 and TP5 (so to ground)
the GP3 becomes low. This will be our active state for GP3. There is
another pin we can use as an input, GP0. Well technically we could
program it as an output, but the hardware behind it is designed to
accomodate various inputs. Warning!
Never use TP2 as an
input! Always use TP1.
If you leave TP1 and TP2 allone, you can set an analog DC value with
POT1, to anywhere between 0V and 5V. Via R2 and R6 this voltage is fed
to GP0. If the microcontroller is configured to handle A/D signals on
GP0, you can react to the voltage, set to with POT1, so you can
illuminate different leds. When you provide a voltage on TP1,
the
measured voltage will be divided between that voltage and the voltage
set with POT1. With POT1 set to 0V, a voltage of 8V on TP2 will result
in a voltage of 4V at GP0. There is practically no current flowing
through R6, so voltages at both ends of R6 are equal. R6 also prevents
the voltage on GP0 to exceed 5V. C7
keeps the voltage at GP0 stable. What if you want to measure higher
voltages? Well, the simplest thing to do is, conenct a known resistor
in series to TP1. Let's say you wanted to measure your car battery
voltage, connect a 10k resistor between the battery and TP1. A voltage
of 13.8V will result in a voltage on GP0 of 4.6V (again when POT1 is
set at 0V).| WARNING! This site is constantly under construction. |
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