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About Servos

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How It Works

 Safety Info

Is Your Servo Going?

The other day I was asked if there is any way to tell when a servo is going bad. That's like asking a mechanic to tell you exactly when your car will stop running. But, just like your car, servos about to fail will give you some warning signs before deciding they've had enough. For example, you may notice slow movement, increased current drain, black residue inside the case under the motor, squealing, jittery movements or jumping during travel, or intermittent lack of response. While none of these symptoms by themselves necessarily indicate a servo about to quit, they do warrant a little extra attention to determine the exact nature of the problems and their repair. The following are some things to check if any of the above symptoms are occurring in your equipment:

Slowing movement or increased power drain	Motor wearing out
Black residue inside of case	Motor brushes worn
Squealing	Worn bushings or gears
Jittering	Pot worn at that point (could also be improper dead band built into servo electronics or a servo incompatible with your transmitter)
Jumping during travel	Bad pot or worn gears
Intermittent movement	Broken wire, short or bad solder joint in wiring of components of amplifier. Bad connection in receiver block or in connectors.

Good maintenance of your servo, care in mounting and of exposed wiring and connectors will all help keep your servo from suddenly quitting. If you see any of the above symptoms, pull your servo out and determine the cause. Get it fixed! They won't repair themselves.

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Getting Started in R/C

Don't go out and by the most expensive and best car there is, you'll only break it and it will cost a fortune to buy spares. For your first car get something you can gradually upgrade as your driving gets better and stick with it.

Join an R/C club in your area!

Try not to go racing and do every thing it takes to win. Go there and practice new cornering techniques and learn the way your car handles. Then when you're good at this you should start to win anyway.

One of the most important things you can do is check you car over after every race or run. Not only do you get know your R/C Car from head to toe, but your also teaching yourself to find problems before they happen.

Don't get frustrated, just have fun and you will have a better time and a better lap time if you relax.

Watch the more experienced drivers race see if you can work out what there doing with the controls, if you know, try it yourself.

Also don't go out and get the hottest motor about. If you're not experienced you won't be able to control the car and you'll end up crashing. Start slow and get good!

Your transmitter and how it works <<<click **

The Right Radio ?

Choosing the right radio system can be a bit of a nightmare for new modelers. Whatever you buy, you are likely to use for many more years so it is very important that you choose your system carefully. The brand that you buy should be able to offer you a broad selection of receivers, servos and accessories with the right specifications and quality to meet your aspirations in the future.

You need to decide what you will be using it for right now, what you may want to use it for in the future and how much you are willing to spend. If all you are going to do is drive a basic car and have no aspirations to anything hotter, then stick with the minimum requirements. If you know you want to drive different cars in the future which will require more functions then buy something a little bit better. Of course, all will be dependent upon how much you want to spend, however, often paying a little bit more initially can save you money in the long run!

Most model car enthusiasts will generally start off with a basic 2 channel system sold with most deals. As skills are improved and the interest grows perhaps into competition racing, drivers tend to look towards more advanced stick and steer wheel sets which will allow them to use many more functions which may enhance their performance.

ONLY THESE FREQUENCIES MAY BE USED

YOUR RADIO AND HOW IT WORKS

When considering any electronic system, such as your radio transmitter and receiver, it is simpler to explain when looked at as blocks with inputs and outputs, which do not require an explanation of the electronics within them.
Let us consider a 2 channel 27meghz transmitter and receiver set.

THE TRANSMITTER

1. The Power Supply.

Power for the transmitter is obtained from batteries, usually eight connected in series, of which there are four common types (AA size) in use.

1.1 ZINC CHLORIDE. 

These are probably the cheapest, but are NOT rechargeable, each cell gives 1.5 Volts, hence 8 times 1.5 equals 12 volt total. An indicator, either a meter or a set of Red, Amber and Green L.E.Ds will show how much power is left 

1.2 ZINC ALKALINE.

Similar to the Zinc Chloride, but with a higher capacity, hence slightly more expensive, but should last that little bit longer. Each cell is again rated at 1.5 Volts , giving a 12Volt supply from 8 cells. There are some rechargers for these on the market but I would not recommend them unless you are prepared to watch them continually when in use (they can be dangerous).

1.3 NICKEL CADMIUM

These are rechargeable using the correct rated CONSTANT CURRENT CHARGER. Each cell is rated at 1.2 Volts, with 8 cells giving a total voltage of 9.6 Volts, lower than the Zinc Chloride, but they do hold the voltage for a longer time, so don,t worry if the Amber light stays on that little bit longer. DO NOT USE A LEAD ACID TYPE CHARGER FOR THESE BATTERIES.

1.4 NICKEL METAL HYDRIDE.

Like the Nickel Cadmium Cells these are rated at 1.2 Volts, with 8 cells giving 9.6 Volts. They are the highest capacity of the AA cells, and hence more expensive. They must be used with a SPECIAL CHARGER a NICAD UNIT WILL NOT DO ESPECIALLY IF FAST CHARGING.

2. THE CRYSTAL.

This part of the transmitter is the reason we USE THE PEG BOARD.  The Crystal is a piece of Quartz cut in such a way, that when a voltage is applied across the faces of the crystal it vibrates or oscillates at a very precise frequency, such as the GREEN at 27,195,000 times per second or 27.195MEGAHERTZ. The output of this crystal is applied to the MIXER (MODULATOR) unit.
These type of crystals find use in all manner of electronic units especially computers, indeed you probably have one on your wrist at the moment in the form of a Quartz watch, in which a crystal is oscillating at precisely 4,194,304 times per second. If we divide this number by 2 a total of 22 times we end up with 1 hertz.

The oscillator frequency in the transmitter is used as a CARRIER for the information we wish to transmit to the model Receiver, I.E. Throttle and Steering Position. 

If we have two transmitters using the same crystal frequency (colour) there will be conflict between the transmitted information sent to each model.

HENCE: USE THE PEG BOARD

3 & 4 - STEERING AND THROTTLE JOYSTICKS.

Channel1 is the Steering Joystick with a left and right movement, whilst Channel 2 is the Throttle Joystick with a forward  movement. With each joystick in it's central position the associated electronic unit produces a pulse of voltage which is 1.5 milliseconds long (1.5 thousandth of a second in duration), by moving the joystick (throttle) forward to it's maximum position this pulse is increased to 2.0 milliseconds in length, and by moving the stick to full reverse it is decreased to 1.0 milliseconds. The same effect is achieved on another pulse by moving the Steering stick left and right from centre. Moving the joystick to the half-way position forward will produce a pulse of 1.75 milliseconds duration, with other positions producing it's associated pulse length . This alteration or change in the width of the pulses is called PULSE WIDTH MODULATION, Modulation merely means a change to the signal. further START pulse is also produced of only 0.5 milliseconds duration and is used to synchronize the timing of each pulse so that the receiver will know which is which. These three pulses are applied to the MIXER( MODULATOR) along with the 27megHz signal from the crystal oscillator.
The combined output of units 3 and 4 is shown below.

5. MIXER ( MODULATOR)

If the crystal oscillator equates to the heartbeat of the transmitter the Modulator equates to the heart. This electronic unit takes the Throttle and steering pulses along with a Start Pulse of 0.5 milliseconds and combines them with the specific 27 megHz Carrier wave, the Start pulses and the Throttle and Steering pulses are repeated 100 times per second and when mixed with the 27megHz carrier alter the height of the carrier wave as shown below. You may ask why do we need the carrier wave, and the answer is that to transmit just the pulses, the range of reception would be about 1 metre at the most, however the 27megHz signal has a much higher range.

The alteration in height of the 27megHz carrier waveform is called AMPLITUDE MODULATION, (the amplitude is the height of the waveform above and below a midpoint). Hence 27megHz A.M.

The combined waveforms are shown in the diagram below.

6.POWER AMPLIFIER.

The combined signals are then increased in voltage and current by the power amplifier to give sufficient signal strength to be applied to the aerial.

7. AERIAL.

This extendable rod radiates the signal to your car. It is designed to a specific length and will not have the range if not extended, if it is allowed to touch the ground the signal will be lost, it is much more efficient if the aerial is held vertical.

All the above is applicable to the new 40megHz A.M. set now on the market, but in this case the carrier wave operates at 40megHz instead of 27megHz. The pulse width modulation of the steering and throttle signals is exactly the same, but by using the higher carrier frequency is it possible to increase their repetition rate to 200 times per second.

40megHz F.M. is an entirely different carrier wave system,but again the same pulse width modulation is used for the two channels. F.M. means FREQUENCY MODULATION and in this case it is the frequency of the carrier which is changed instead of the amplitude. The major advantage being better interference suppression. The AM. and F.M. systems should not clash with one another even when using the same frequency, but you may have problems if two transmitters one on A.M. and the other on F.M. at the same frequency are within about 2metres of each other in that one will tend to swamp the other.

THE RECEIVER.

1. AERIAL

This is a piece of wire of a precise length (DO NOT CUT IT SHORT) which picks up the transmitted signal. Remember that metal will shield the aerial from doing it's job, plastic and wood tend not to.

2. PREAMPLIFIER.

Because the signal at the aerial is reduced in strength due to the range over which it is transmitted it is first amplified by this section in order to be applied to the DEMODULATOR unit4.

3.POWER SUPPLY.

Power for the receiver is usually from a set of AA batteries giving 6.0 Volts, or from the Motor battery if B.E.C. (BATTERY ELIMINATOR CIRCUIT) is used . This power is also used to drive the steering servo and the throttle servo in the case of a BOBS BOARD,as well as the control circuit of the Electronic Speed Controller, if used.

4. THE DEMODULATOR AND RECEIVER CRYSTAL.

This unit is very similar to a portable radio set, but it does not have a tuning dial, instead it has a fixed frequency crystal matched to your transmitter crystal. This precise matching of transmitter and receiver crystal means that only your transmitted signal is processed ( and someone else's if they switch on their transmitter with the same colour crystal) FLY THE FLAG AND USE THE PEG BOARD 
The signal is first stripped of the 27megHz carrier waveform, we now don't need it, leaving just the Start , Steering and Throttle pulse width modulated waveform. Th Start pulse effectively tells the next units 5 and 6 which pulse is applied to the steering unit and which is for the throttle unit, every 1/100th second. These pulses are therefore separated and applied to the next units 

5 AND 6 STEERING AND THROTTLE UNITS

These units within the receiver convert the steering and throttle pulses into a positive or negative voltage which is variable plus or minus 200millivolts depending on the length of the pulse which you started with by moving the steering stick left or right , or the throttle forwards or reverse on the transmitter. This variable voltage from each channel is applied to the relevant servo which moves accordingly as shown below.

Information and Write-up provided by Phil Bowker.