Radio Controlled Models - How Does Radio Control (rc) Work?

At first glance, the principles of radio control can seem quite complex. With a little fundamental information about the basics, however, you will quite soon be able to enjoy building and operating your first radio controlled model - be it a car, plane, boat, or perhaps even a radio controlled truck or tank.

Radio controlled models obviously appear quite different in design and size but at their core are the same radio control principles of transmitter, receiver, aerial or antenna, servos, batteries, etc. With some knowledge of how these components fit together, it is much simpler to understand how to build your first radio controlled model. Just the same way that radio stations operate on designated frequencies, the radio frequencies for radio control are allocated by the various authorities from country to country. The frequency bands are carefully allocated to ensure that there is no possibility of interference with frequencies used by public services such as air traffic control or the emergency services.

Futaba Transmitter
Futaba Transmitter
Traxxas Crystal Pair
Traxxas Crystal Pair

Radio Controlled Channels

Several different models can be running at the same time because the allocated frequencies are further sub-divided into channels, or frequency bands, one of which is used by each model according to the crystals that are fitted to transmitter and receiver. The frequencies used by radio control enthusiasts vary from country to country but usually, if buying a model to operate in a particular area, it will come with radio control gear suitable for that country. Some care should be taken though if you import equipment directly, to make sure that it will operate safely in the country where it will be used.

The transmitter is the radio set, or control box, held by the operator to control the model and the receiver is the control box in the model that receives the transmitted signal. Control is nearly always proportional, which means that the further the control lever is pushed on the transmitter, the further the servo on the model moves. So, for example, full control movement on the transmitter will give full rudder movement on a radio controlled model boat.

As mentioned above, at the heart of the transmitter and receiver system is the pair of crystals (Xtals) fitted to the transmitter and receiver. Without getting into to much technical detail, the transmitting crystal vibrates at high frequency when energised and, along with the other circuitry, forms the radio signal that is transmitted through the aerial. The aerial on the model picks up this signal and the reverse happens. The crystal vibrates and with its circuitry generates the electrical current to operate the servos. The transmitter crystal is marked TX and the receiver crystal marked RX, and additionally they are both marked with the channel number - the number has to be the same for the crystals to be a matched pair and for the transmitter to receiver system to work successfully. All crystals can be removed and changed quite easily so that there is no interference between models operating at the same time in the same area. Enthusiasts simply compare crystal markings and if they find that there is a clash between radio controlled models, a pair of crystals can be changed to avoid interference.

Radio Controlled Servos

The servo is a key piece of equipment in the radio control model. It usually is a small black box with a control arm and linkage or pushrod on its top or side. The servo moves on command from the receiver and through the control linkage moves the throttle or control surface, or whatever, on the model. Usually one servo is used for each control function. They come in a very large variety of shapes and sizes depending on the force they need to provide. Electronic speed controllers on electric rc cars are similar to servos but instead of having a control linkage to control the throttle, electronic circuitry is used directly to control the motor speed.

All this electronic equipment needs power to operate. The transmitter does not take that much power and has its own small batteries to power it. The radio control model needs power to operate and the most usual source of power is a rechargeable Ni-Cad battery pack. An electric car will need a sizeable pack to provide a reasonable running time, but a nitro powered model does not need the power to drive an electric motor so battery packs can be smaller.

Much more could be written about radio control but with this basic information you should be able to understand enough about the parts of the system to get started without too much confusion!

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Comments 16 comments

Aidan James profile image

Aidan James 7 years ago from Sydney

Excellent - thumbs up from me! Check out my hub on radio control airplanes for beginners if you get a chance.

bigmikeh profile image

bigmikeh 7 years ago from UK Author

Thanks Aidan.

Bill Coton 6 years ago

Thank you for a great post, there is something to learnt here for the beginner and more advanced RC enthusiast

bigmikeh profile image

bigmikeh 6 years ago from UK Author

Thanks for your comment Bill.

Gerd Schmidt 6 years ago

Great Hub. It's always a pleasure to read a hub of a passionate RC fan that can help the beginner and the expert!

bigmikeh profile image

bigmikeh 6 years ago from UK Author

I'm pleased you enjoyed reading this article, Gerd.

John 5 years ago

Thank you for this post.

Can you explain me why some store sell "mixed pair", for exemple : TX 27.315 Mhz and RX 26.860 MHz ?

I thought it was always used to be a matched pair like you said...

bigmikeh profile image

bigmikeh 5 years ago from UK Author

I don't know, John. But if I find out I'll post a reply.

Neil Larson 5 years ago

Thanks for explaining bigmikeh! Not hard to understand even for beginners! Great hub.

bigmikeh profile image

bigmikeh 5 years ago from UK Author

Pleased I managed to keep it simple for you Neil

Justice 5 years ago

Awesome stuff to learn here as a begginer,so can you please show me all the electronics that are needed to build your own controlling system

bigmikeh profile image

bigmikeh 5 years ago from UK Author

Justice - suggest you do a search on the Internet for such detailed information.

Forrest 5 years ago

Interesting,I've just learned a lot of cool things from this.Thanks! It's simple and easily understud. Now I'll build a radio controlled car.

Great job!

Felix 5 years ago

John wanted to know why the Tx and Rx Xtals for the same channel have different frequencies. This is because the Xtals are used very differently in the two circuits.

In Transmission, the Xtal frequency is used directly to determine the carrier frequency, whereas in Reception the Xtal frequency is *mixed* with the received signal, to produce an intermediate frequency of 455 Khz, which is further processed. Thus the frequency of the Rx Xtal will always be equal to the Tx frequency minus 455 Khz.

Hope that brief intro helps ...

Felix 5 years ago

If I could pre-empt someone asking "ok, but why do this ?" ... well, here's the answer:

Let us suppose that a signal is being transmitted at precisely 27.145 Mhz - and as we wish to receive that signal, and ONLY that signal, at first sight it would seem sensible to employ a reference Xtal within our receiver of exactly the same frequency. But that's not a very good idea - but why ?

Well, a Xtal in a circuit works by oscillating at a precise frequency when stimulated, creating a usable voltage swing of perhaps a few volts. If we were to have such a reference Xtal in our receiver, then it will be creating oscillations of *volts*, while we were trying to detect transmitted oscillations at that exact same frequency but perhaps only in *nano- or even pico-volts*. We'd never be able to detect them, as they'd be swamped by local oscillations many millions of times stronger.

Indeed, we may well be able to detect frequencies on either side of 27.145 but the one frequency we'd never EVER be able to detect is 27.145 itself.

But surely our receiver must have a precise reference frequency ? Yes, indeed - and the way we reconcile this conundrum is to have a reference Xtal in the receiver a known frequency apart from the desired one. And this frequency spacing has been generally agreed upon as 455 Khz.

And so the receiver Xtal for a Tx of 27.145 is 26.690 (as 26.690 + 0.455 = 27.145). And now you also know why one Xtal of a 'matched pair' is marked Tx and the other Rx !

bigmikeh profile image

bigmikeh 5 years ago from UK Author

Thank you Felix!

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