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This
article will show you how to use the components in our infrared data
communications parts KIT.
The PIC Circuit:
Figure 1: PIC12C508 connections.
The small 8-pin PIC12C508 is
pre-programmed to generate our 38KHz carrier frequency by simply pulsing
I/O-pin GP1 (shown above).
The PIC will generate either 38KHz
or 40KHz, depending on the state of GP3 when power is first applied. If you
connect GP3 to ground, then apply power to the circuit, the frequency will be
40KHz. This was implemented for people using the older 40KHz style infrared
detector modules. The default is 38KHz with pin #4 (GP3) left floating or (not
connected to anything),,,
Pin #7 (GP0) is to inhibit (stop)
the carrier output. When GP0 is connected to ground, the PIC will halt
generation of the carrier frequency. This is an optional control feature for
power saving, and in most cases, un-necessary, but it's there if you need to
use it. Ground pin #7 to inhibit the carrier frequency. Release this pin from
ground to start the carrier frequency again.
Connect GP1 (pin #6) of the
PIC12C508 to pin #1 of the two-input NAND schmitt trigger as shown in figure
2. The data signal pin can be any I/O-pin from the BASIC Stamp or other
microcontroller.
Figure 2: Transmission circuit.
By referring to the NAND truth
table below, and the schematic above, you can see how the PNP transistor is
held OFF by the logic "1" output of the NAND on pin #3 until both
inputs #1 & #2 are logic "1"
The 38KHz carrier generated by
the tiny 8-pin PIC pulses from logic "1" to logic "0" at a
rate of approximately 38,000 cycles per second, hence 38KHz. The serial data
signal is 1200 or 2400 bps (bits per second), and considerably slower than the
38KHz carrier frequency (as shown below).
| NAND
SCHMITT TRIGGER TRUTH TABLE |
| 1 |
2 |
3 |
| L |
L |
H |
| L |
H |
H |
| H |
L |
H |
| H |
H |
L |
A = Serial data applied to NAND (pin # 1)
B = 38KHz PWM applied to NAND (pin # 2)
A = NAND output (pin # 3) applied to base of PNP
B = 38KHz PWM applied to NAND (pin # 2)
You'll need a power supply that
can deliver 5-volts @500mA for the circuit configuration shown in Figure 2.
The Lumex OED-EL-8L, High-Power
180mW, 940nm Infrared LEDs will drop approximately 1.6V each. To figure total (peak pulsed
current): 5V - ( 1.6V + 1.6V ) / 5.1-ohms = 352mA peak pulsed current.
A simple +5-volt regulator circuit
as shown in Figure # 3 will do fine.
Figure 3: Power supply circuit.
Here is a detailed schematic
showing connections for using Holtek remote control ICs, and the BASIC Stamp
or PIC with these circuits.
Click HERE
to download the (three page) schematics in .pdf format.
For further details on infrared
remote control applications visit the following links. These are articles that go into greater detail on infrared
remote control......
