Read the text and translate it: IR 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 widespread use the required components are quite cheap, thus making it ideal for us hobbyists to use IR control for our own projects.

Infra-Red actually is normal light with a particular colour. We humans can't see this colour because its wave length of 950nm is below the visible spectrum. That's one of the reasons why IR is chosen for remote control purposes, we want to use it but we're not interested in seeing it. Another reason is because IR LEDs are quite easy to make, and therefore can be very cheap. Although we humans can't see the Infra-Red light emitted from a remote control doesn't mean we can't make it visible. A video camera or digital photo camera can "see" the Infra-Red light. Unfortunately for us there are many more sources of Infra-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. Therefore we have to take some precautions to guarantee that our IR message gets across to the receiver without errors.

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 everything else. You can think of this blinking as attracting the receiver's attention. We humans also notice the blinking of yellow lights at construction sites instantly, even in bright daylight. 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 light 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 receiver's output. A 'mark' is then automatically represented by a low level.

The transmitter usually is a battery 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 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 processor 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 shocks. 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 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 parameters can be found in the LED's 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 Ohm's law. Remember that the nominal voltage drop over an 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 series 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.

Post-text exercises:

1. Put the words in the right order to make a sentence:

1. Infra-Red, light, particular, actually, is, normal, a, with, colour. 2. The, the, a, key, wakes, when, up, to, transmit, is, appropriate, IR, command, only, processor, pressed. 3. A, or, video, digital, can, "see", the, Infra-Red, camera, camera, photo, light. 4. A, be, transistor, the, circuit, used, can, drive, to, simple, LED. 5. Power, are, very, low, transmitters, microcontrollers, used, IR, in. 6. A, in, of, this, a, across, pulses, constant, resistor, magnitude, constant, resistor, amplitude, current, constant, results. 7. The, will, as, drops, the, well, battery, as, current, LED, voltage, decrease, through. 8. Power, of, maximum, the, average, the, though, dissipation, LED, value, not, the, should, exceed.

2. Fill in the blanks with the following words:

Light, modulation, battery, power, chips, camera, sheet, resistor, command, crystals

1. The processor wakes up to transmit the appropriate IR … only when a key is pressed. 2. Infra-Red actually is normal …with a particular colour. 3. The transmitter usually is a … powered handset. 4. A video … or digital photo camera can "see" the Infra-Red light. 5. Very low … microcontrollers are used in IR transmitters. 6. This constant amplitude across a constant … results in current pulses of a constant magnitude. 7. All these parameters can be found in the LED’s data …. 8. Quartz … are seldom used in such handsets. 9. … is the answer to make our signal stand out above the noise. 10. Many … are designed to be used as IR transmitters.

3. Complete the following sentences using active vocabulary:

1. The cheapest way to remotely control a device … 2. With modulation we make the IR light source blink in … 3. LED currents can be that high because … 4. … only when a key is pressed. 5. Quartz crystals are seldom used in … 6. A transistor with a suitable HFE and switching speed should be …. 7.... quite cheap, thus making it ideal for us hobbyists to use IR control for our own projects. 8. Although we humans can't see the Infra-Red light emitted from a remote control doesn't …. 9. … are commonly used in consumer electronics. 10. Ceramic resonators are much more suitable here, because they ….

4. Say if the following statements are true or false:

1. Humans can see the Infra-Red light emitted from a remote control. 2. Wave length of 950nm is above the visible spectrum. 3. Humans can see a particular colour.4. A video camera or digital photo camera can’t "see" the Infra-Red light. 5. There is not source of Infra-Red light. 6. Everything that radiates ice, also radiates Infra-Red light. 7. A simple transistor circuit can’t be used to drive the LED. 8. Average power dissipation of the LED should not exceed the maximum value though. 9. A trade-off should be made between LED parameters, battery lifetime and maximum control distance. 10. A 'mark' is then automatically represented by a high level.

5. Answer the following questions:

1. What is normal light with a particular colour? 2. What device can be used to drive the LED? 3. How many diodes will limit the pulses on the base of the transistor? 4. Does the processor wake up to transmit the appropriate IR command only when a key is pressed? 5. What is the default signal, which is the off state in the transmitter case?

6. Why can't humans see a particular colour? 7. What radiates Infra-Red light? 8. Is 1.1V or 1.3V nominal voltage drop over an IR LED? 9. Can the resistor values simply be calculated using Ohm's law? 10. What is the answer to make our signal stand out above the noise?

6. Match the first part (1-7) of the sentence with the second part (a-g):

1. Ceramic resonators are …

2. A resonator is…

3. The older chips were …

4. Quartz crystals are …

5. A simple transistor circuit can …

6. A 'mark' is…

7. The 'space' is…

a) … be used to drive the LED.

b) …. seldom used in such handsets.

c) … a device or system that exhibits resonance or resonant behavior.

d) … dedicated to only one of the many protocols that were invented.

e) … is the default signal, which is the off state in the transmitter case.

f) … is then automatically represented by a low level.

g) … much more suitable here, because they can withstand larger physical shocks.