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How Remote Controls Work

Remote controls: We use them for garage door openers, car alarms, TV sets, model airplanes, you name it! With all of the other licensed radio services that use the radio spectrum, how is it that these devices don't need a license? Why does your garage door open when you push the button, but not half of your neighbors'? Why do some remote controls work better and farther than others, while some barely make it across the room?

It turns out that there are two major types of wireless remote controls. The type commonly used for televisions uses infrared transmission, while garage doors, car alarms and model airplanes use radio. There are some old types of remotes that used ultrasonic sound, but you don't see them too much any more... Let's have a look at the differences.

Wireless Remote Controls

Garage doors, alarms and such

Garage door remote controls and alarm remotes are quite similar. Most of them use a simple tuned transmitter and receiver to send the signal via radio. The typical frequency is around 400 MHz, but can vary widely, depending upon the unit involved. These frequencies are covered under Part 15 of the FCC rules, which authorize low power unlicensed radio systems.

A special digital coding system is used to separate one system from the next. For example, if you have a garage door opener, you'll most likely find a series of small switches inside which you can use to set the "code." This code is actually a binary number that differentiates your garage door system from your neighbors'. If you accidentally set the code to the same as your neighbor, and both of your systems are on the same frequency, both of your doors will operate at the same time!

How does this code work? It's actually fairly simple. A digital circuit in the transmitter creates a serial pattern of ones and zeros which are sent by the transmitter repeatedly to the receiver at the far end. The receiver recovers the pattern of ones and zeros, which are sent to a decoder circuit. When the decoder circuit gets a pattern that matches what has been set by the user (the little switches you set at each end) it provides a control signal to whatever the gadget is that you want to control, be it a motor to open the garage door, or a switch to turn off the alarm in your car.

Remote Model Controls

Remote controls used for models are a bit more sophisticated than garage door openers. Instead of simply turning something on or off, they must control remotely a range of movement, such as turning the steering wheel on a remote control car, or the rudder on an model airplane. In addition, these remote devices usually have to perform many control tasks at once. In a model airplane, there can be many functions controlled all at once, such as rudder, ailerons, flaps, landing gear, throttle, and other ancillary functions.

In these types of remote controls, information from the position of the control levers (or joysticks) is converted to a series of digital pulses. The system may be as simple as changing the timing of the pulses, or as complex as actually sending the numerical value of the angle of the control via a binary number. Information from all of the channels is then combined into one continuous stream of data, which is then transmitted to the remotely controlled device via radio.

At the remote end, a receiver picks up the signal and recovers the original data stream. The data stream is fed into special decoding circuitry that separates the various channels and then recovers the position information which is sent on to the motors that do the actual mechanical controlling of the model.

Infra-red Remote Controls

Infra-red remote controls work in much the same way as radio remotes, except that instead of transmitting the signal over radio waves, the signal is transmitted using pulses of infrared light. Infra-red light is just below the red portion of the visible spectrum, and so is invisible to the human eye.

In order to keep the various remotes from interfering with each other, a special set of codes has been worked out that identify the make and model of television or stereo (or whatever) that is being controlled. This keeps your TV remote from starting up your VCR. Universal remotes work by allowing multiple different codes to be sent so that you can use one device to control all of your components. Some remotes even "learn" by recording the pattern from all of your different, existing remotes, and then using the recorded pattern to control each different device.

Infra-red remotes have one major difference from radio units: They use light, and so cannot penetrate walls and work around corners very well. This can be an advantage as well as a disadvantage. Imagine if your kids kept changing the channel on your TV from their room! Remote control wars! What a scary thought!!!

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