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Universal Serial Infrared Receiver.

It uses a PIC microcontroller to characterize the demodulated infrared signal and then transmit it as a serial data stream to the PC.  

Software running on the PC recognizes data stream for each key press (in fact the PC software has to learn the data stream for each key press) and activates the program (or event) that you specify.  Since the software can learn each key press you can use it with any remote control  e.g. TV, video, DVD, satellite etc.  

It's even good with multiple remotes so you could use any one of a set of remotes to control the PC

Specification for Serial Infrared receiver project.

Baud Rate	57600 Baud
Clock	Internal 8Mhz
Remove Control type	Any (Point any control at it and let the PC software learn its codes).

Note: Some remote controls need two key presses (if you press the same key) as they output an inverted key sequence for a repeated key.  If you press a different key then it works as a normal remote.

Serial Infrared receiver : Project Details

Serial Infrared Receiver Project
Compiler	Mikrolectronika MikroC  Free!
Target	16F88 (retargetable to other PICs that have a built in USART).
Software level	Easy.
Software notes	No special notes.
Hardware level	Easy.
Hardware notes	No special notes.
Project version	1.01
Project files	Enter your details to get the Download Link and get the microcontroller newsletter: (Note: Your email is safe it will never be sold or rented). You will get All theC source code and hex file. Note: Check your email for the project code download link.

How the Serial Infrared receiver works

This project uses a standard Infrared detector module (one of those 3 pin devices) as the main input.

It's not worth making your own circuit up as it will not match the performance of a three pin demodulator module.

I had a go using some circuits I found on the web and but these use a PIN diode and a high impedance amplifier e.g. LM3140 - but the circuits are actually light meters and saturate when a light shines on them  (even desk lamp) - so they become insensitive to the IR light - the PIN diode reacts to any light source.  You can get some use out of them up to about six feet but a lot of engineering has gone into the three pin modules so it really is worth using them and they are very cheap.

Inside the module is a PIN diode, AGC circuit, band pass filter, control logic and output detector.  The datasheet specifies use from 15m (45ft) up to possibly 30m (90ft) max!. These are very good detectors that are also insensitive to sunlight (has a physical filter over the PIN diode and the electronic bandpass filter).

What to do if you only have a USB port

Todays PCs only have USB ports so you need some adaptor hardware to allow serial RS232 operation. There are two options:

• A USB Digital Serial Interface
• A USB RS232 adaptor

Both of these are very cheap and useful. You can learn more about usb adaptors in the link.

Serial Infrared receiver

How the Serial Infrared receiver works.

This project took longer than it should have and I have ended up with a far smaller source code than my initial attempts!  First off I used the capture facility in the CCP and the Timer1 timer to accurately measure each period of high and low decoded IR.  This turned out to be too accurate!

IR signal codes decoding accuracy

You don't need accuracy in this project - IR codes are designed not to need accurate measurement and if you attempt it slight variations in the edge positions or measuring accuracy cause the output numbers to change.  Since the decoder software in the PC relies on a repeatable data stream (with slight variation) this method does not work.

IR signals are designed so that you can sample them but this requires that you know the code sequence (or encoding e.g. RC5) in advance so that you can time from the start sequence to the exact middle of a bit position.

Since this project allows you to use any remote control unit, the encoding sequence can not be known in advance, so the the solution is to sample the remote control sequence at a high frequency.  

The problem is that if you start decoding from the first rising edge then errors accumulate due to slight variations in the remote control output (or the sampling edge occurs at the same time as an input transition) causing a random data stream output which can not reliably be decoded by the PC software.

IR decode solution

The solution is to sample the input data starting from each rising and falling edge where a bit measurement period counter (period counter) is reset.  At each sample point (here it is about 50us) the period counter is incremented. This gives a number that represents the period of each high or low part of the signal. 

The 50us sample period is about ten times the expected signal period (IR codes generally use 500us minimum period) so this gives a good period measurement.

Note that the period counter is 8 bits long so that for long input sequences (high or low) the counter wraps around - this does not matter since the period counter will always produce the same value for the same input sequence and this is what the PC decoder software is looking for.

RS232 Output

At every signal transition the period counter value is sent to the RS232 interface so the value represents the number of 50us periods in the previous frame.  The value sent is a raw binary number and you can not view it on a terminal such as Tera Term as you won't get a readable value.

To stop the sequence from generating RS232 data forever the counter value is monitored until it goes above 200. If at the same time the value of the input is zero then output stops and control is returned to waiting for the next interrupt i.e. the next remote control key press.

Note: The internal USART takes about 30us to set up and it then goes off and transmits the byte by itself which at 57600 baud takes about 170us so there is plenty of time before the next transmission (500us IR code period). You could use a lower baud rate e.g 38400 which would still fit.  

If you used a baud rate such as 9600 then a byte would take ~1ms to transmit which would mean lost period data - it probably works as you would still get an indication of code periods but may not always work.

PC Software

The main infrared control program that runs on the PC is WinLIRC which allows control of the PC via a serial port input (or a USB digital serial port).

Testing

I have tested the infrared receiver and PC Remote Control with the following remotes:

• JVC
• Thomson
• Philips
• Samsung
• Grundig
• Medion

These all worked fine - some you have to press the button twice as the code alternated for the same key.  It expect it to work with just about any remote control as these represent a wide range of differing encoding schemes.

Serial Infrared Receiver Hardware

The universal infrared receiver uses an IR decoder module (Note that there is no standard pinout and you have to check your version's pinout using its data sheet).

There is nothing difficult about the circuit and it just consists of a PIC micro, an ICSP interface and an RS232 level translator.

Serial Infrared receiver Software

The software is contained in two files:

Infrared Receiver Source Files

16F88-infrared-decoder.c
bit.h

bit.h contains compiler independent bit manipulation macros.

Infrared Receiver Project Files

16F88_Infrared_decoder.mcppi

Infrared Receiver Output files:

16F88_Infrared_decoder.hex

For a tutorial on compiling these files click here.

You can recompile the serial Infrared receiver files to examine the operation of the code (using the built in simulator) or change the source code. The hex file is contained within the download.

You can program the PIC in circuit through the ICSP connector.

16F88_Infrared_decoder.c

The only use for main( ) is to initialize the ports and interrupts - it then enters an infinite loop waiting for an interrupt.

Input pin RB0 is used as an interrupt source and then polled.  An interrupt for RB0 is setup to trigger when a rising edge occurs (probably should be falling edge as the decoded output is inverted - but it works as is!)

When the interrupt occurs the routine sample_bits is called  - Note this is within the interrupt so sample_bits becomes part of the interrupt routine itself.

sample_bits() takes control and polls the RB0 input as described previously.  It also returns control to the interrupt routine at an appropriate point; waiting for the next remote control key press.

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