全面了解红外遥控器.doc

《全面了解红外遥控器.doc》由会员分享，可在线阅读，更多相关《全面了解红外遥控器.doc（18页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、【精品文档】如有侵权，请联系网站删除，仅供学习与交流全面了解红外遥控器.精品文档.全面了解红外遥控作者：San BergmansIR Remote Control Theory The cheapest way to remotely control a device within a visible range is via Infra-Red light. Almost all audio and video equipment can be controlled this way nowadays. Due to this wide spread use the required com

2、ponents are quite cheap. Thus making it ideal for us hobbyists to use for our own projects. This part of my knowledge base will explain the theory of operation of IR remote control, and some of the protocols that are in use in consumer electronics. Infra-Red Light Infra-Red actually is normal light

3、with a particular colour. Us humans cant see this colour because its wave length of 950nm is below the visible spectrum. Thats one of the reasons why IR is chosen for remote control purposes, we want to use it but were not interested in seeing it. Another reason is because IR LEDs are quite easy to

4、make, and therefor can be very cheap. Although we humans cant see the Infra-Red light emitted from a remote control doesnt mean we cant make it visible. A video camera or digital camera can see the Infra-Red light as you can see in this picture. Unfortunately for us there are many more sources of In

5、fra-Red light. The sun is the brightest source of all, but there are many others, like: light bulbs, candles, central heating system, and even our body radiates Infra-Red light. In fact everything that radiates heat, also radiates Infra-Red light. Therefor we have to take some precautions to guarant

6、ee that our IR message gets across to the receiver without errors. Modulation Modulation is the answer to make our signal stand out above the noise. With modulation we make the IR light source blink in a particular frequency. The IR receiver will be tuned to that frequency, so it can ignore everythi

7、ng else. You can think of this blinking as attracting the receivers attention. We humans also notice the blinking of yellow lights at construction sites instantly, even in brought daylight. In the picture above you can see a modulated signal driving the IR LED of the transmitter on the left side. Th

8、e detected signal is coming out of the receiver at the other side. In serial communication we usually speak of marks and spaces. The space is the default signal, which is the off state in the transmitter case. No light is emitted during the space state. During the mark state of the signal the IR lig

9、ht is pulsed on and off at a particular frequency. Frequencies between 30kHz and 60kHz are commonly used in consumer electronics. At the receiver side a space is represented by a high level of the receivers output. A mark is then automatically represented by a low level. Please note that the marks a

10、nd spaces are not the 1-s and 0-s we want to transmit. The real relationship between the marks and spaces and the 1-s and 0-s depends on the protocol thats being used. More information about that can be found on the pages that describe the protocols. The Transmitter The transmitter usually is a batt

11、ery powered handset. It should consume as little power as possible, and the IR signal should also be as strong as possible to achieve an acceptable control distance. Preferably it should be shock proof as well. Many chips are designed to be used as IR transmitters. The older chips were dedicated to

12、only one of the many protocols that were invented. Nowadays very low power microcontrollers are used in IR transmitters for the simple reason that they are more flexible in their use. When no button is pressed they are in a very low power sleep mode, in which hardly any current is consumed. The proc

13、essor wakes up to transmit the appropriate IR command only when a key is pressed. Quartz crystals are seldom used in such handsets. They are very fragile and tend to break easily when the handset is dropped. Ceramic resonators are much more suitable here, because they can withstand larger physical s

14、hocks. The fact that they are a little less accurate is not important. The current through the LED (or LEDs) can vary from 100mA to well over 1A! In order to get an acceptable control distance the LED currents have to be as high as possible. A trade-off should be made between LED parameters, battery

15、 lifetime and maximum control distance. LED currents can be that high because the pulses driving the LEDs are very short. Average power dissipation of the LED should not exceed the maximum value though. You should also see to it that the maximum peek current for the LED is not exceeded. All these pa

16、rameters can be found in the LEDs data sheet. A simple transistor circuit can be used to drive the LED. A transistor with a suitable HFE and switching speed should be selected for this purpose. The resistor values can simply be calculated using Ohms law. Remember that the nominal voltage drop over a

17、n IR LED is approximately 1.1V. The normal driver, described above, has one disadvantage. As the battery voltage drops, the current through the LED will decrease as well. This will result in a shorter control distance that can be covered. An emitter follower circuit can avoid this. The 2 diodes in s

18、eries will limit the pulses on the base of the transistor to 1.2V. The base-emitter voltage of the transistor subtracts 0.6V from that, resulting in a constant amplitude of 0.6V at the emitter. This constant amplitude across a constant resistor results in current pulses of a constant magnitude. Calc

19、ulating the current through the LED is simply applying Ohms law again. The Receiver Many different receiver circuits exist on the market. The most important selection criteria are the modulation frequency used and the availability in you region. In the picture above you can see a typical block diagr

20、am of such an IR receiver. Dont be alarmed if you dont understand this part of the description, for everything is built into one single electronic component. The received IR signal is picked up by the IR detection diode on the left side of the diagram. This signal is amplified and limited by the fir

21、st 2 stages. The limiter acts as an AGC circuit to get a constant pulse level, regardless of the distance to the handset. As you can see only the AC signal is sent to the Band Pass Filter. The Band Pass Filter is tuned to the modulation frequency of the handset unit. Common frequencies range from 30

22、kHz to 60kHz in consumer electronics. The next stages are a detector, integrator and comparator. The purpose of these three blocks is to detect the presence of the modulation frequency. If this modulation frequency is present the output of the comparator will be low. As I said before, all these bloc

23、ks are integrated into a single electronic component. There are many different manufacturers of these components on the market. And most devices are available in several versions each of which are tuned to a particular modulation frequency. Please note that the amplifier is set to a very high gain.

24、Therefore the system tends to start oscillating very easily. Placing a large capacitor of at least 22F close to the receivers power connections is mandatory to decouple the power lines. Some data sheets recommend a resistor of 330 Ohms in series with the power supply to further decouple the power su

25、pply from the rest of the circuit. There are several manufacturers of IR receivers on the market. Siemens, Vishay and Telefunken are the main suppliers here in Europe. Siemens has its SFH506-xx series, where xx denotes the modulation frequency of 30, 33, 36, 38, 40 or 56kHz. Telefunken had its TFMS5

26、xx0 and TK18xx series, where xx again indicates the modulation frequency the device is tuned to. It appears that these parts have now become obsolete. They are replaced by the Vishay TSOP12xx, TSOP48xx and TSOP62xx product series. Sharp, Xiamen Hualian and Japanese Electric are 3 Asian IR receiver p

27、roducing companies. Sharp has devices with very cryptic ID names, like: GP1UD26xK, GP1UD27xK and GP1UD28xK, where x is related to the modulation frequency. Hualian has its HRMxx00 series, like the HRM3700 and HRM3800. Japanese Electric has a series of devices that dont include the modulation frequen

28、cy in the parts ID. The PIC-12042LM is tuned to 36.7kHz, and the PIC12043LM is tuned to 37.9kHz. The End? This concludes the theory of operation for IR remote control systems intended for use in consumer electronics. I realise that other ways exist to implement IR control, but I will limit myself to

29、 the description above. One of the issues not covered here is security. Security is of no importance if I want to control my VCR or TV set. But when it comes to opening doors or cars it literally becomes a key feature! Maybe I will cover this issue later, but not for now. I also realise that my smal

30、l list of manufacturers is far from being complete. It is hardly possible to list every manufacturer here. This page only described the basic theory of operation of IR remote control. It did not describe the protocols that are involved in communication between transmitter and receiver. Many protocol

31、s are designed by different manufacturers. You can find the protocols of some manufacturers in the link section at the top of this page. ITT Protocol The ITT IR protocol is a very old one. It differs from other protocols in that it does not use a modulated carrier frequency to send the IR messages.

32、A single command is transmitted by a total of 14 pulses with a width of 10s each. The command is encoded by the distance between the pulses. This protocol is very reliable and consumes very little power ensuring long battery life. Many consumer electronics brands used this protocol in Europe. Among

33、them were: ITT, Greatz, Schaub-Lorenz, Finlux, Luxor, Salora, Oceanic and later also Nokia, to name but a few. Features Only 14 very short IR pulses per message Pulse distance encoding Long battery life 4 bit address, 6 bit command length Self calibrating timing, allowing only simple RC oscillator i

34、n the transmitter Fast communication, a message takes from 1.7ms to 2.7ms to transmit Manufacturer Intermetal, now Micronas Protocol An IR message is transmitted by sending 14 pulses. Each pulse is 10s long. Three different time intervals are used to get the message across: 100s for a logic 0, 200s

35、for a logic 1 and 300s for the lead-in and lead-out. The preliminary pulse is used by the receiver to set the gain of the amplifier. Then follows a lead-in delay of 300s, after which the starting pulse is given. The first bit sent is always logic 0, which has a delay duration of 100s. This start bit

36、 can be used to calibrate the timing of the receiver. After the start bit follow 4 bits (MSB first) that represent the address of the message. After that a total of 6 bits (MSB first) for the command are transmitted. A trailing pulse should follow this command. Finally another 300s delay follows bef

37、ore the very last pulse, functioning as a lead-out. There are a few things the receiving software can check to verify the validity of the received message. The lead-out interval should be 3 times longer than the start bit time, which has a duration of 100s. Bit times should not be off by more than 2

38、0% of the start bit length for logic 0s, or 2x the start bit length for logic 1s. Dont keep waiting for pulses after 360s after the last received pulse. Its very likely that the transmission is interrupted or no transmission took place at all if you have to wait longer than that. The preliminary pul

39、se serves only AGC purposes and may be ignored by the receiving software. Decoding of the message should start at the Start pulse. Address and Command A control message is divided into two groups, an address of 4 bits and a command of 6 bits. Addresses range from 1 to 16, and commands range from 1 t

40、o 64. Before the address and command are sent, 1 is subtracted from both values to get them in the range 0 to 15 and 0 to 63. Addresses are used in pairs. A pair of addresses is a value of 1 to 8 (0 to 7 actually), and its inverted counter part 16 to 9 (15 to 8 actually). The lower value address is

41、transmitted the first time a key is pressed. The address value of all subsequent messages will be the inverted value of this first address until the key is released. This enables the receiver to interpret repeat codes properly. Messages are repeated every 130ms as long as the key remains pressed. Tr

42、ansmitter Intermetall has developed a few transmitter ICs for use in handsets. Later microcontrollers were used to facilitate the combination of TV, VCR and SAT remote control in one handset. The SAA1250 was the first IR controller IC to be released. It can be set to generate 3 different address pai

43、rs. A fourth option is transmitting any of the 16 addresses. That option is rarely used, for it requires a manual setup procedure every time the power is lost. The second generation of IR controller ICs are the IRT1250 and IRT1260. These chips are identical in operation and differ only in the operat

44、ing voltage. The IRT1250 is intended for 9V operation, whilst the IRT1260 is designed for 3V. The footprint of the IRT12x0 is the same as that of the SAA1250. The devices differ in addressing capability and current drive capacity for the output stage. Two address pins are available to set the addres

45、s pair used. A1A2AdressesHH1 & 16LH3 & 14HL7 & 10LL4 & 13Addresses 1 and 16 are always used to control TV sets. Other address pairs are not always uniquely linked to a particular equipment family. Receiver The ITT protocol makes no use of a modulated carier, so the previously mentioned IR receivers

46、wont work for this protocol. Intermetall has created the TBA2800 for use with this protocol. It is a highly senisive IR detection circuit and should be shielded completely inside a metal box that is connected to ground, leaving only a small hole just in front of the IR diode. There is actually not m

47、uch more to be told about this IC. Just connect it as shown in the diagram as shown above, and it should work. You can chose between a normal high going output, and an inverted low going output. It depends on the rest of your circuitry which one you should use. In case of excessive interference you

48、could reduce the sensitivity a little by grounding pin 6 via a 10k resistor. Pre-defined Commands Some of the 64 possible commands are predefined. But unfortunately the definition of the commands is not as clear as with RC-5. You can find most of the pre-defined commands for TV purposes in the following table. TV commands use the address pair 1 and 16. CommandFunction12Stand-by3TV4Ideal5Up6Down7Mute8P+9P-10Left / Bilingual11Right121314Last15161711821932042152262372482592602728Zoom