Remote control
by Chris Woodford. Last updated: June 24, 2012.
The way we love our remote controls, you'd think our living rooms were the size of Texas! It's an awfully long way to go, isn't it, to change the television channel or pump up the volume on the stereo? Remote controls are a perfect indulgence for couch potatoes everywhere—but have you ever stopped to wonder how they work? How come your TV remote doesn't trigger the video? Why do you have to point it directly at the TV? And when you aim your remote at a neighbor's house, does their TV channel change too? Let's take a closer look at the mysteries of remote control!
Photo: A typical TV remote control unit. Early TV remotes did little more than change the channel. With a modern remote control, you can control virtually every aspect of how a TV works. This remote can also control a DVD player hooked up to the TV set.
How remote controls use infrared beams
The first thing you notice about a remote control unit is that it has no wires, so it has to send signals to whatever it's operating using electromagnetic waves. Light, x-rays, radio waves, and microwaves are all examples of electromagnetic waves: vibrating packets of electrical and magnetic energy that travel through the air at the speed of light. Most remote controls send signals using infrared radiation (which is a kind of invisible red light that hot objects give off and halogen hobs use to cook with), though some use radio waves instead.
Photo: The small infrared LED on the top of a typical remote control.If you look at the top of your remote control unit, you'll see there's a small plastic light-emitting diode (LED) where the infrared radiation comes out. Now take a look at your TV or video. Somewhere on the front, there's a very small infrared light detector. When you press the remote control, a beam of infrared radiation travels from the remote to your TV at the speed of light and the detector picks it up.
Human eyes can't detect infrared, so even if you press the buttons on your remote and stare at the LED you won't see anything happening. Some animals, including rattlesnakes, can detect infrared. Rattlesnakes have tiny infrared detectors buried in pits near their eyes, which work a bit like the infrared detectors on your TV. By homing in on infrared heat, snakes can locate prey at night when there's no ordinary light to see by. What would happen if you pointed a TV remote control at a snake and pressed the buttons? Maybe it would think you were a mouse and slither over to eat you. It's unlikely you could control a rattlesnake with a TV remote—and we don't recommend you try!
Photo: Rattlesnakes "speak" infrared, just like TV sets. Picture by courtesy of US Fish & Wildlife Service.
Remote control codes
It's no good the remote control just sending out a burst of random infrared. Clearly if your remote control has 20 or more buttons on it, it must have a way of sending out at least this many signals—each one different enough for your TV to be able to decode and understand it. When you press one of the buttons, the remote generates a systematic series of on/off infrared pulses that signal a binary code (a way of representing any kind of information using only zeros and ones, which computers use). So a short pulse of infrared could signal a 1 and no pulse could signal a 0. Sending many infrared pulses, one after another, allows your remote to send whole strings of zeros and ones. One code (maybe it's 101101) might mean "volume up", while another (perhaps 11110111) could mean "mute sound."
As well as sending out pulses that tell the TV what you want it to do, the remote also sends a short code that identifies the product you're trying to control (for example, a specific make and model of TV). That ensures your remote operates only the TV, not the video, and not any other TVs that happen to be nearby. Generally, this means each remote control unit can operate only one appliance made by only one manufacturer. Of course if you could discover the codes that different TVs and videos understand, you could build a remote control that operated any appliance. This is how universal remotes work. They let you control any TV or video and, instead of sending out only signals specific to one brand of equipment, they can send out codes that any make or model can understand. One inventor has even gone so far as to develop a remote called TV-B-Gone that systematically sends out a "switch TV off" signal using every possible manufacturer's code. It's designed to allow TV haters to switch off annoying TVs covertly as they wander through shopping malls and department stores!
Can I switch on my neighbor's TV set?
Infrared remote controls are relatively low powered and will send signals only about 10–20m (35–70ft). Like visible light, infrared is relatively low-energy electromagnetic radiation and, unlike radio waves, cannot penetrate through solid walls. That means there's no chance your ordinary infrared remote will switch on your neighbor's TV or DVD recorder by accident!
If you do want to operate equipment in other rooms, it's possible to buy radio-frequency remote controls (and adapters for conventional, infrared remotes) to extend their range. A typical RF remote might operate in the frequency range 400–450 MHz, where an ordinary infrared remote would operate at perhaps 300 THz (3 × 1014 Hz = 300 million million hertz)—roughly a million times higher frequency. With equipment like this, it is possible to receive signals from a neighbor's home, much like you can receive their wireless Internet (Wi-Fi) signals.
Radio control
Infrared remotes can operate TVs and videos only over quite short distances. The infrared LED is quite small and low-powered and the receiver on the TV or video is small too. This is why you generally have to point the remote directly at the appliance you're trying to control. Some remotes are more tolerant and it is sometimes possible to bounce the infrared beam off a wall, mirror, or picture and still change channel. Infrared remotes are no good for controlling things over distances greater than a few meters (feet); the infrared energy is too easily soaked up and dissipated along its journey.
To control things over greater distances, you need to use a different kind of system called radio control. You operate radio-controlled cars, trucks, boats, airplanes, and robots using a handheld radio transmitter box that sends signals from an antenna on the top to a matching antenna on whatever you're interested in controlling. Radio signals can travel much further than infrared ones without interference, especially if the transmitters and antennas are large and powerful.
Photo: A typical radio-controlled helicopter. Inside, the "passenger compartment" is packed with radio-receiver electronics, a motor that spins the main rotor, and servo motors that control the steering (rotor pitch) in exactly the same way as a real helicopter's. This is an exhibit at the Think Tank science museum in Birmingham, England.
Types of radio control
There are two broad types of radio control known as single channel and multi-channel.
Single-channel radio control is effectively an on-off switch operated at a distance by radio wave. So, to give a very trivial example, you could use a single-channel control to switch a lamp on or off at the bottom of your garden. The sender unit would consist of a low-powered radio transmitter, while the lamp would need a radio receiver and a relay (to convert the low-powered, incoming radio signal into a higher-powered electric current big enough to operate the lamp). Single-channel radio control can only switch things completely on or completely off; it can't turn them up or down by degrees.
Multi-channel radio control is used to transmit more complex and useful signals to a piece of remote equipment—for example, to make a radio-controlled car speed up, slow down, or steer from side to side. Instead of just sending a basic on/off signal, it transmits a series of coded analog or digital pulses that are decoded by the receiver and used to produce specific actions. For example, turning a steering wheel on a radio-control transmitter will send a series of pulses that make an electric motor rotate by a corresponding amount to steer a radio-controlled car one way or the other. Motors that work this way are known as servo motors. Unlike normal electric motors, which rotate an arbritary number of times according to how long they receive an electric current, servo motors are much more controllable and have built-in electronic feedback mechanisms (based on potentiometers—similar to variable resistors), which enable you to make them rotate by reasonably precise amounts.
What is radio control used for?
Radio control isn't just used in toys; many of the latest wireless gadgets use similar technology, including Bluetooth® (a convenient way of operating computer-based equipment without wires), RFID (radio frequency identification) technology used in anti-shoplifting systems, and Wi-Fi, used in wireless Internet. Even cellphones communicate using a wireless system not unlike radio control. So maybe remote control isn't just for couch potatoes after all?
