• Electronics

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    A complete radio system for your R/C model boat includes the transmitter, which sends the command signals that open and close the carb butterfly and move the rudder to drive the boat through the desired direction, the receiver, that inside the boat receives and decodes those signals and the servos, small electric motors that, under the receiver command,move the shafts and cables that control the throttle and the rudder. Just like ordinary transmitters, that you could have at home or in your car, RC radios are allowed to transmit only in specific frequencies ranges:

    27 MHZ for airplanes, boats and cars - not much used, because of the risk of putting together planes and other vehicles.

    72 MHZ exclusively used for planes

    75 MHZ for boats and cars.

    Always use the frequencies range authorized for the vehicle you are driving, to avoid accidents. For each of those groups there are channels with different frequencies, so several pilots can use their vehicles at the same time, without danger of interfering each other. The AM transmitter is more susceptible to external interference and, whenever used on a gas boat we must filter this interference, mainly that originating from the engine, using resistor spark plugs and accelerator cables made from non metallic materials. The FM transmitter is less prone to those interferences and, according to Futaba, has a 20% longer range. The PCM type radio encodes the TX signal. At the RX this signal is decoded and so only the "true" signals are accepted by the receiver. This way, interference is drastically reduced. Additionally, the PCM radio has a inboard fail-safe that may be adjusted to kill the engine if the interference supersedes the system filtering.


    The battery/receiver connectors are fragile and offer no resistance against salt water. On all our packs we use Dean connectors, much more strong and recoverable when corroded. Look at the pics bellow:

    Fail safe

    A dangerous situation occurs when you lose contact between the transmitter and the receiver. The immediate consequence is a total loss of control of the boat. It's hardly fun. Futaba solved the problem designing what they call a Fail Safe. It's a little electronic device linked between the receiver and the accelerator servo that closes the carb and kills the engine as soon as the receiver looses the transmitter signal. Add on fail safes are now manufactured by quite a few different companies and are available from your local hobby supplier. Many of the newer radios (PCM and 2.4 GHZ) will have a built in fail safe function. But is this enough?. The fail safe depends on battery power to be effective. And if the battery is discharged or disconnected? Additional protection can be added that takes care of this: used together with the fail safe, it cuts the engine if the battery disconnects from the receiver or discharges. Total protection. The picture below is a simple wiring diagram from Jim Nissen that uses a relay to achieve this. Note that this system will only work on spark ignition engines and not the glow type engines.

    Turning the engine OFF with a micro switch

    There is no worse feeling, when operating an R/C boat at speed than discovering something went bad. The rudder doesn't turn, the accelerator is stick or debris clogged the cooling system and the engine, in the middle of the lake, is getting hot. To solve this problem, we developed a device on our radio boxes: a micro switch installed in such a way that, from the transmitter, we can turn off the engine and stop the boat. Recovering your stalled but otherwise perfect boat is much less trouble than fixing a hull that crashed into a rock or rebuilding an overheated engine

    The micro-switch is easily available at electronic parts shop (Radio Shack is a good source); anyone rated to at least 5A and 125V will do. The device has 3 poles: neutral (N), one normally open (NO) and the last one normally close (NC). Let's see how this works.

    The neutral pole is permanently connected to one of the other two (NC or NO), according to the activating lever you see at the above pic. Without pressing the lever, N pole is connected to NC pole; press the lever and it will disconnect from NC and will connect to NO. That's all we need to kill the engine, has it 2 or 1 coil, kill switch or not. The drawings bellow show ho

    a. Two coils and a kill switch: on these type of engines, there are 2 coils, a low voltage one that is the one witch controls the engine timing and commands the spark from the high voltage and the high voltage that produces the spark at the low voltage command and has the spark plug wire. A low voltage wire connects the 2 coils. Whenever grounded, the kill switch cuts the current to the high voltage coil and, so, kills the engine. The kill switch wire is connected to the MS normally open (NO) pole, meaning that, when not pressed, the N pole (ground) is not connected to it. Pressing the MS connects the NO pole to the N pole and from there to the ground, so the engine is shut off.

    b. Two coils without a kill switch: the coils operation is exactly the same as item (a), but the kill switch. For killing the engine, the wire that connects the 2 coils is interrupted; one tip is connected to N pole, the other to NC pole. Without pressing the MS, current flows from N to NC pole and the engine runs. Pressing the MS disconnects the 2 wires and the engine stop running

    c. One coil and kill switch: the system is found on Homelite engines, just one coil has the low and the high voltage internals. In this case, the kill switch is wired to the micro switch NO pole, the N pole is wired to ground, much alike the (a) described.

    Radio range

    Note: I copied the table bellow from the Futaba web site. There are a lot of recommendations on how to check your radio equipment. Although specifically made for our flying cousins, considering that they have to be much more concerned than us on this subject (this doesn't mean that you should not be aware of this, also) the recommendations are useful for us.
    Range Testing Your Futaba R/C Aircraft System

    Please note that different systems demonstrate different range checks, and the same system will range check differently in different conditions. Also, the receiver antenna's installation effects the range test -- exiting the top of the model is ideal.

    Before you range test: equipment preparation and mounting

    1. Pre-installation inspection:
      • Be sure the receiver antenna is intact, with no scratches, tears, hard creases or other damage, and that it is securely attached to the receiver.
      • Be sure the receiver crystal is fully seated in the receiver.
      • Always pad the receiver, and cover the crystal location to ensure the crystal does not fall out should it become loose.
      • Never install an aftermarket "whip" or shortened transmitter or antenna. These are not recommended by Futaba and may affect your overall range.

    2. Receiver antenna routing recommendations:
      • Keep the antenna as far as possible from servo wires, metal pushrods, ignition wires, anything carbon fiber, or anything else that may conduct electricity. Be aware that truly metallic (metal-colored MonoKote is not) and carbon fiber finishes can affect radio range as well.
      • Exit the antenna above the aircraft, not close to the ground, if possible. This dramatically increases the accuracy of a ground range test.
      • Remember: if this is a gasoline-powered model, EVERYTHING radio system related must be isolated by at least 12 inches. This includes the antenna. Don't be tempted to run the antenna forward up the fuse if your receiver is rear-mounted, or that isolation protection is lost.
      • Ideally, have the antenna exit the fuse within inches of the receiver, and mount to the top of the vertical fin (run the antenna through 2 holes of a cut off servo arm, then rubber band that servo arm to a t-pin out the top of the fin).
      • NEVER cut, fold, twist, coil, or knot your receiver antenna wire.
      • If you wish to run the antenna within the model, consider running it inside a plastic guide tube to minimize the chance of metal contact. In some installations, straight out the wing is a great choice!
      • Strain relief is critical! Be sure to secure your receiver antenna with a cut off clevis arm or other method to protect it in case someone steps on your antenna!

    3. Gasoline powered model recommendations:
      • All ignition equipment, including an electronic kill switch, must be mounted at least 12", and preferably 14", away from all radio equipment, including throttle servos, radio system power switch, receiver antenna, etc.
      • Ignition kill switch should always be on opposite side of fuse from your radio/receiver's on/off switch.
      • All pushrods going to anything related to the engine must be non-conductive (just non-metal clevises is not sufficient. Use a nylon, plastic, or similar pushrod to the throttle servo, kill switches, etc).Transmitter antenna: Leave the transmitter's antenna retracted, and be sure both batteries are fully charged. (For the 9Z family of radios, you must pull the antenna fully out, then collapse the upper sections into the bottom segment. If the bottom segment is not locked in place, you will not have any range at all, as the case shields the antenna from having sufficient output.)

    4. Position the aircraft away from wires, other transmitters, radio towers, etc.

    Test one - engine/motor off, minimum of 100 ft. range
    • Have a friend view the model but not hold it, engine off.
    • Walk away from the model, holding the radio as if you were flying, and working all controls constantly. Stop when the servos jitter significantly (a jitter here and there is normal), control movement stops (PCM), or you lose control altogether.
    • Measure the distance. If greater than 100 feet, great! Proceed to test 2.
    • Less than 100 feet of range check means you need more information to determine if your system is safe to fly.

    • Are both batteries fully charged? Peak batteries, check voltage, and repeat test to be sure.
    • What does your specific radio call for in its manual? Different models' manuals will provide you the accurate range for its particular performance.
    • Is the radio a 9Z? If so, did the thickest segment lock in place? If not, you will not get an accurate range reading.
    • Are you using a genuine Futaba FM/PCM dual conversion receiver? Small, park flyer receivers and some other non-Futaba receivers demonstrate different ranges of performance than Futaba's FM dual conversion receivers. Please consult your receiver's documentation
    • If you have previously range checked and flown this system in a similar location, similar weather, etc, is the range you are now seeing less than 90% of what you have previously seen? If so, DO NOT FLY unless you are aware of specific differences in this set of circumstances that explain the change.
    • Ask a fellow modeler with a similar system to duplicate your range check to see if his range is similar to yours.
    • Repeat the range check with a different transmitter/same receiver and different receiver/same transmitter to try to isolate if either is not performing properly
    • Repeat the test with your friend holding the model exactly as he will in test Note that a person may increase or decrease the range demonstrated.

    Test two - engine/motor on

    • Check for significant vibration in the model, particularly loose engine or engine mount bolts.
    • If gasoline-powered, double-check all items listed above. Consider having the ignition, if any, tested.
    • Check that the receiver crystal is securely seated in the receiver
    • Has the receiver been in even a minor crash? If so, fragile internal parts may be damaged and will not operate properly under the normal vibration/load of flight.
    • Be sure the receiver antenna has a proper strain relief and is not being pulled out of the receiver when the engine is running.
    • If you are unable to find the cause of the problem, DO NOT FLY. Send the receiver for service immediately.

    What your fully operational system demonstrates is the normal range for your system in those conditions. Before every flying session, it is critical that you perform a range check. It is also required by the AMA Safety Code. If you notice a significant decrease in range with fully charged batteries, do not attempt to fly.

    Radios Systems

    Your boat's radio system is of utmost importance, for the single reason that it is responsible for the boat going to where you want it to. Enough said.

    From the Futaba site I collected and adapted a few tips that I think can be useful for us. Why Futaba? Because it is the best know and most used in radio system.

    Battery: all Futaba systems are designed to operate on either 4.8 volts or 6 volts, except certain servos which are specifically labeled for use at one voltage or the other only. The increased power input provides increased performance, speed and torque. While 6 volts provides you more torque and speed from your servos, it also provides you a significantly shorter run time for the same milliamp hours of capacity and may shorten the life of your servos proportionally

    Battery cycling: the transmitter batteries need to be cycled only rarely, so you may be able to simply unplug the Tx pack from the board inside the radio to do so. But if your particular radio's battery jack is difficult to access or you are cycling frequently, you may want to consider cutting the battery lead inside the battery compartment, soldering a male J connector onto the battery portion of the lead and the female J connector onto the transmitter portion of the lead.

    "Black Wire Corrosion": over time, the negative lead from the battery pack, through the switch harness, to the receiver will corrode until the copper wire becomes dark, almost black, and brittle and no longer flexible. The cause is storage of the system in a damp environment with the battery installed. The corrosion usually starts at the battery pack and works its way towards the switch harness. There is no cure once black wire corrosion starts. You can only replace the wires. Prevention requires that your equipment be stored in a clean, dry environment, and maintain your batteries. Store them fully-charged and cycle them regularly. If you can't do that, then at least remove the batteries from your models, and store them, along with your transmitters, inside, where the temperature and humidity are fairly stable, compared to a garage or shed.

    Extensions: standard servo leads and extensions are 26 gauge wire. If you need extensions (much more used by our planes cousins), heavier gauge extensions are recommended, as much for the distance as for the high draw of high torque high speed, ball bearing servos.

    FailSafe: If your PCM receives interference, it maintains the last task it was doing until a preset time you program in your radio. If it continuously receives interference past the preset time it then obeys the commands you set in the transmitter.

    Interference: in general there is no need for noise suppressors in current generation Futaba equipment Twisting your servo extensions lightly and gently is a good idea, as it helps minimize the combination of the electrical fields from the 3 leads. If you see radio interference operation, look first to the gasoline engine installation.

    Noise reduction: going to PCM will make a marked improvement in noise reduction and in safe performance during the time noise is received. Basically, PCM takes your radio's FM signal and 'scrambles' it. Then the PCM receiver descrambles it and utilizes it. Random noise from other items are not going to be mistakenly read as proper servo instructions as can happen with an FM.

    Range: the ground PCM systems will typically give you a slight increase in range over the ground FM systems. An FM system will give you approximately 20% more range than an AM system. An air system converted to ground will give you the farthest possible range, typically approximately 10% farther than a system built for ground use.

    Reversing the TX/RX crystals: they are not interchangeable.

    About this issue, a discussion at Jim's Board went far more deep in the subject:

    Is there a difference between the crystal in transmitters versus receivers?
    I thought they were the same until I saw some with tags on them.
    Can anyone elaborate?

    Lots of differences...
    Transmitter crystals typically operate at the fundamental frequency your trying to transmit on (in this case 27 or 75Mhz for surface use). Both are labeled TX and can have FM and AM designations.
    The receivers is where it can get real tricky. They have AM and FM type crystals as well but there is also single conversion type crystals and dual conversion type crystals. Conversion refers to the number of intermediate frequency (IF stages in the receiver. A single will normally have a crystal frequency around 455Khz under the target frequency that is being transmitted. The way the receiver works is it takes in the received signal and combines it with this locally generated source. The result is a 455Khz signal that gets demodulated. The demodulated signal is what is used to drive the servos.
    In a dual conversion the receiver crystal frequency typically is 10.7Mhz below the transmitted frequency. Two stages of IF are employed. The first being a 10.7Mhz stage and the second a 455-470Khz stage. A second crystal is used (almost always 10.7Mhz) and is inside the receiver (you don't change that one). The result is that dual conversion receivers typically have very good rejection of spurious (noisy) signals. The signals had to go through two levels of filtration instead of just one. The resultant signal is very likely the intended signals for the servos.
    The airplane guys typically use dual conversion receivers if they value their equipment. Some surface radios use dual conversion but not many.
    There is a fairly easy to understand article here. It goes into a bit more detail but not a lot of theory. If you really are interested I suggest you get in contact with a local HAM fanatic or two.

    We didn't even bring FM/PCM and PPM into this discussion! That is basically a method of modulation for the signals.

    Hi Jim, No argument, just trying to stay current. I thought the crystals were cut to a certain harmonic that is specific to the receiver circuitry i.e. single or double etc. conversion circuits. Am I off base here?
    Marty Manross

    Yes your right Marty. The fundamental frequency that the crystals are 'cut' to is intended to work with certain frequencies. Transmitters require one type (27 or 75Mhz) while the receiver's frequency is a set number of KHz offset from the transmitters. The amount of offset is dependent on single or dual conversion receivers.
    Jim Nissen

    I thought the crystals were cut to a certain harmonic' crystals are cut to work on harmonics and the receiver takes advantage of this, it's easier to cut crystals at lower frequencies. So your 75mhz crystal may in fact be a 25mhz crystal running in your RX/TX oscillator on its 3rd harmonic.
    Marty Manross

    Your right. Some mfg's use overtones to achieve the desired frequency. I really don't have enough in depth knowledge of how most major radio mfg's spec their crystals. I can ask around if anyone is really interested. I work with a guy at work that is Doc Crystal. No kidding this guy has collections of oddball crystals and probably forgot more than I know on the subject! I'm sure he would have the info to tell me if they really operate on fundamental modes or overtones.

    The more I think about it I bet it is an overtone. Would make the crystal blank less prone to vibration and mechanical damage. Golflake you know if this is the case in most radios?
    Jim Nissen

    Single Conversion Receiver

    When a radio has one IF amplifier it is called a Single Conversion Receiver. Although adequate for most applications it has one serious weakness. The intermediate frequency is a product of the difference between the wanted signal and the crystal frequency. In this case the wanted frequency is 455Khz above the crystal frequency. Unfortunately a signal 455Khz below the crystal frequency will also produce 455khz interfering signal at the input of the IF amplifier. If you accidentally swap the Tx and Rx crystals in a single conversion system your RC system will probably still work but the transmit range is likely to be much reduced. Do not try this at home, you will be operating illegally outside the designated RC bands.

    Servo mounting steps:

    1. Insert a rubber grommet into each of the four servo holes.
    2. Insert a metal eyelet from the bottom side of the rubber grommet. This way the wide portion of the eyelet will be in contact with the servo tray when mounted.
    3. Test fit the servo in the tray, and enlarge the openings so the servo will not touch the tray. The rubber grommets will isolate the servo from the hard vibration
    4. Position the servo, then mark the location of the mounting holes. Drill pilot holes with a 1/16" bit at each mark.
    5. Use the servo screws supplied with your radio to mount the servo(s) in the servo tray. Tighten the screws until they just touch the top of the metal eyelet

    Servos shake or 'jitter': if this occurs whenever you get the transmitter really close to the boat, especially if the antenna is up, this is a behavior commonly known as 'swamping'. Basically, your transmitter is overpowering your receiver because it is so close. Try collapsing your antenna, and keep your transmitter at least 2 feet away from your model. If this behavior occurs when you are more than 2 feet away or especially during range checking, DO NOT RUN.

    Turning off the radio system: in AM and FM radio systems, if the receiver is not getting clean data from a transmitter then the servos will respond relatively randomly. Only a PCM system (or a system with a failsafe unit installed) will hold the last known position. For this reason, ALWAYS turn your transmitter on first, then receiver, And when turning off, always turn off receiver first then transmitter.

    The signal travels the same speed through the air if AM or FM. FM is less susceptible to interference than AM (listen to your car radio and you know this!).

    Good FM's are available from HiTec, Futaba, Airtronics. Get a new set. A lot of crashed car racers sell their stuff online and all you need is a flaky radio to drive you nuts! It's worth it to get a known good set. Make sure you purchase a surface frequency radio only. This is a radio that operates on 75Mhz or possibly 27Mhz.

    The 72Mhz radio's are strictly for aircraft. DO NOT USE AIRCRAFT frequencies. The flyboys adhere to this as well and avoid the surface frequencies. PCM radios are Pulse Coded Modulation. This is a digital encoding scheme that the transmitter sends out to the PCM receiver. The actual transmission takes place via FM modulation. The receiver decodes the digital data and determines if it's valid or not. If valid the servos are adjusted. Standard FM does not encode the signal. What ever the receiver gets - it's demodulated and sent to the servos.

    Install the receiver and servos in a good radio box that does not leak. Make sure zero water gets inside. You can wrap the receiver in a balloon or condom for added protection from water. If you use the four dry cell battery pack make sure you wrap tape around the pack after installing the batteries. Vibration can easily dislodge a battery and your up the creek after that.

    Wrap the receiver in foam rubber as well to minimize vibration to the receiver. They are sensitive to vibration and drops. The crystals inside the receiver (as well as the transmitter) are very thin and can crack. The frequency will change slightly if the crystal is damaged. Best to have a pro test the radio if you ever drop it hard. Radio sets are tuned to a particular frequency or channel. Popular ones are the 75Mhz bands. They go by numbers 61 through 90. The 27Mhz channels are A1 to A6. The main thing to know is that the transmitter and receiver both have to be tuned to the same channel. The receiver on channel 64, the transmitter on 64. You can control multiple models with one transmitter as long as each model has a receiver tuned to the transmitter's channel. Only one user of a particular channel can operate his/her model at one time. NEVER turn on your radio until you have a frequency clip (if managed boating site) or you check to see what frequencies the others may be using.

    Use a 1/4 scale servo for steering and a standard for throttle. Always use plastic servo horns to minimize electrical noise from migrating into your radio system. 99% of the servo's sold today have nylon servo horns for this reason.

    Servo Jumper

    Don Betz discovered that one on a sail-boat site. For servos that demand a large amount of current, a jumper from the battery to the servo may help the speed of the servo:

    Protecting the receiver

    We are using this on all our receivers and it is working very well. Cut the terminals from the servos and battery wires that are connected to the receiver, leaving just the plain tips. Open the receiver plastic box, solder the wire tips on the places where originally the terminals go, heat bee wax till it melts and fill the box with the melted wax, with the crystal in place. Close the box, wait till the wax cools and you have a completely water-proof receiver. It works.