The IR Signal The IR detector is only looking for infrared thats flashing on and off 38,500 times per second. It has built-in optical filters that allow very little light except the 980 nm infrared. It also has an electronic filter that only allows signals around 38.5 kHz to pass through. This is the type of signal produced by the remote control. This prevents IR interference from common sources such as sunlight and indoor lighting.
Important Concepts Pulse width modulation (PWM): Pulse durations are used in many applications, a few of which are motor control, and communication. Since the IR detector sends low pulses that can be measured to determine what information the IR remote is sending, it's an example of using PWM for communication. Carrier signal: The IR remote uses a 38.5 kHz "carrier signal" to transmit the pulse durations from the remote to the IR detector. Communication protocol: A communication protocol is a set of rules for devices that have to exchange electronic messages. Protocols tend to have rules for voltages, the amount of time signals last, carrier signal frequencies and/or wavelengths, and much more. When two or more devices follow the rules of a given protocol, they should be able to communicate and exchange information.
The TV Remote Control ( 4 Function Universal Remote ) You must configure your universal remote so that it sends PWM messages to a television set using the SONY protocol. TV remote setup Press and release the TV key. Press and hold the SET key until the indicator LED on the remote turns on and stays on. Use the digit keys to enter 0001. The LED may turn off briefly as you press each digit. VCR remote setup Press and release the VCR key. Press and hold the SET key until the indicator LED on the remote turns on and stays on. Use the digit keys to enter 1028. The LED may turn off briefly as you press each digit.
The TV Remote Control (SYSTEMLINK 3 RCA) You must configure your universal remote so that it sends PWM messages using the SONY protocol. TV remote setup Press and hold the CODE SEARCH button until the indicator LED lights, then release the CODE SEARCH button. Press and release the TV button (the indicator LED will blink and then remain lit). Use the digit keys to enter the code 002. After your code is entered, the indicator LED will turn off. VCR remote setup Press and hold the CODE SEARCH button until the indicator LED lights, then release the CODE SEARCH button. Press and release the VCR button (the indicator LED will blink and then remain lit). Use the digit keys to enter the code 004. After your code is entered, the indicator LED will turn off.
Protocol Details This message consists of thirteen negative pulses that the BASIC Stamp can easily measure. 1: the start pulse, which lasts for 2.4 ms. 2-13: will either last for 1.2 ms (binary-1) or 0.6 ms (binary-0). 2-8: indicates which key is pressed. 9-13: indicate if the message is being sent to a TV, VCR, CD, DVD player, etc. Pulses are transmitted in least significant bit first fashion. the first data pulse is bit-0. the next data pulse is bit-1 Etc. If you press and hold a key on the remote, the same message will be sent over and over again with a 20 to 30 ms rest between messages.
How the IR Detector Works The Boe-Bot IR receiver is the same detector found in many TVs and VCRs. This detector sends a low signal whenever it detects IR flashing on/off at 38.5 kHz and a high signal the rest of the time. When the IR detector sends low signals, the processor inside a TV or VCR measures how long each of the low signals lasts. Then, it uses these measurements to figure out which key was pressed on the remote. Like the processor inside a TV, the Basic Stamp can be programmed to detect, measure, store, and interpret the sequence of low pulses it receives from the IR detector.
PULSIN Command Complementary command to PULSOUT The syntax for the PULSIN command is PULSIN Pin, State, Variable Pin: the I/O pin for measuring the pulse. State is used to determine whether the pulse is a high pulse (1) or a low pulse (0). Variable stores the pulse duration measured in units of 2 micro-seconds. The IR pulses are inverted (i.e., low). To measure them with the IR detector circuit, you will have to use the command: PULSIN pin#, 0, variableName pin# is the pin connected to the IR detector; pin 9 in the previous slide.
Using PULSIN time VAR Word counter VAR Word DO PULSIN 9, 1, time IF (time > 1000) THEN counter = counter + 1
Storing Pulse Width: Arrays This array declaration creates twelve different word variables, each of which can store its own value: time(0), time(1), time(2), and so on, up through time(11). Each of the elements in the time array can store a different value between 0 and 65535.
Reading IR Messages time VAR Word(12) ' SONY TV remote variables. index VAR Nib DEBUG "time ARRAY PWM MEASUREMENTS", CR, "Element Duration, 2-us", CR, "------- ----------" DO ' Beginning of main loop. DO ' Wait for rest between messages. PULSIN 9, 1, time(0) LOOP UNTIL time(0) > 1000 PULSIN 9, 0, time(0) ' Measure/store data pulses. PULSIN 9, 0, time(1) PULSIN 9, 0, time(2) PULSIN 9, 0, time(3) PULSIN 9, 0, time(4) PULSIN 9, 0, time(5) PULSIN 9, 0, time(6) PULSIN 9, 0, time(7) PULSIN 9, 0, time(8) PULSIN 9, 0, time(9) PULSIN 9, 0, time(10) PULSIN 9, 0, time(11) FOR index = 0 TO 11 ' Display 12 pulse measurements in units of 2 micro-secs. DEBUG CRSRXY, 0, 4 + index, "time(", DEC index, ")", CRSRXY, 9, 4 + index, DEC time(index) NEXT LOOP ' Repeat main loop.
Interpreting the IR Message The Idea: represent the pulse sequence as a bit sequence. The IR message consists of thirteen pulses with the following format: 1: the start pulse lasts for 2.4 ms. 2-13: will either last for 1.2 ms (binary-1) or 0.6 ms (binary-0). Map the duration of pulses 2-8 to their corresponding binary value Use PULSIN and RCTIME to measure the pulse duration Use the.BIT modifier to create the corresponding binary representation
How It Works RCTIME is used because the IF…THEN statement does not finish until after the next data pulse has already begun. Since the start of the pulse (its negative edge) is missed while the IF…THEN statement is executing, PULSIN cannot properly detect the beginning of the pulse. remoteCode VAR Byte remoteCode = 0
Code Sample time VAR Word remoteCode VAR Byte remoteCode = 0 DO RCTIME 9, 1, time LOOP UNTIL time > 1000 PULSIN 9, 0, time IF time > 500 THEN remoteCode.BIT0 = 1 RCTIME 9, 0, time IF time > 300 THEN remoteCode.BIT1 = 1 RCTIME 9, 0, time IF time > 300 THEN remoteCode.BIT2 = 1 … RCTIME 9, 0, time IF time > 300 THEN remoteCode.BIT5 = 1 RCTIME 9, 0, time IF time > 300 THEN remoteCode.BIT6 = 1 DEBUG CRSRXY, 4, 3, BIN8 remoteCode, CRSRXY, 14, 3, DEC2 remoteCode