Servos and Radios question

The_MasterThe_Master Posts: 61
edited May 24 in Robotics Vote Up0Vote Down
With each of my 4-channel R/C radios (72 Mhz) I was able to implement 5th and 6th channels to the transmitter (as ON-OFF functions for landing gear, etc) with the simple addition of a toggle switch and a few resistors. Under the cover of all my "four channel" transmitters was actually a six channel transmitter in hiding. At least for all the transmitters I owned.

So when one of the transmitter channels is used for an ON-OFF function (flaps, landing gear, bomb bay, etc) there is probably an industry standard PULSE WIDTH that should be used for the On position and for the Off position. Does anyone here have experience with this who could tell me what the pulsewidths are for ON and OFF? And would this pulsewidth be different for newer radios?

The reason I ask is because if I were designing an aircraft accessory that was activated by a fifth channel (e.g. camera shutter) I could just use the industry standard pulsewidth to detect activation. Otherwise I would need to add an adjustment pot on my accessory board, which I really don't want to do.

Also, does anyone happen to know if a newer cheap 2.4 Ghz transmitter will be able to be modified the same way? (or are all the 2.4 Ghz radios already 6 channel?)
I am the Master, and technology my slave.

Comments

  • 12 Comments sorted by Date Added Votes
  • ceptimusceptimus Posts: 58
    edited May 24 Vote Up0Vote Down
    Normal pulse width is nominally 1 to 2 milliseconds with centre at 1.5 milliseconds. Some go a bit beyond that range: +/- 512 us is common so that gives an 'off' pulse width of 988 us and an 'on' of 2012 us.

    The newer 2.4 GHz transmitters don't really use pulses internally (except for buddy box connections) - they process the inputs from the sticks, switches, and pots with their microcontrollers and then transmit a digitally encoded signal - but the receivers still mostly decode those signals and turn them into pulses to output to the servos with those same standard widths. There are 2.4GHz modules available from several manufacturers that allow you to adapt older transmitters. You just connect up two power wires plus a third wire with the PPM (pulse position modulation) signal that used to feed the 72MHz RF stage to the module and then the module turns the pulses into the digital format and transmits it. Different manufacturers use different digital encoding schemes so the 2.4 GHz receivers are hardly ever usable with transmitters or transmitter modules made by a different manufacturer. If you've not already got a bunch of 2.4 GHz receivers then I recommend the FrSky range which are common, well priced, and have exceptionally good performance. Beware that most manufacturers, FrSky included, have more than one form of 2.4GHz signal - so even within a single manufacturers range it's possible to purchase transmitters/modules/receivers that are incompatible with each other. Sometimes the transmitter modules can work with several different types of receiver, either by means of a switch or automatically when you bind a receiver to work with a particular transmitter. When you're buying the receivers and or transmitters just make sure they are compatible with 'X' or whatever name the manufacturer has chosen for that particular range.

    Almost all transmitters and modules will do at least six channels and eight is common. The modern transmitters (not modules though) are often capable of transmitting 16 channels though I don't know anyone who ever uses that many!
  • Good info ceptimus. So I should probably count on not being able to modify 2.4 Ghz transmitters.

    My guess is that for toggle functions (for new and old systems alike), it will be something like UP landing gear is 1.25 mS and DOWN landing gear is 1.75 mS. (equidistant from center servo position)

    I just want to verify there isn't some wacky standard I didn't know about for toggle switches. Like UP is 1.0 mS and DOWN is 1.2 mS. Having never owned a proper 6 channel system with toggles, I don't have the ability to verify it.

    I am the Master, and technology my slave.
  • ceptimusceptimus Posts: 58
    edited May 24 Vote Up0Vote Down
    The normal way two-position switches work - at least with the modern sets I've tried - is that they default to +100% or -100% throw unless you set them otherwise. So a switch in either end position gives the same signal as a joystick axis at maximum deflection. With FrSky +100% is 2.012 ms and -100% is 0.988 ms. It's always very easy to go into the transmitter's menus and alter the 'end positions' controlled by a switch, so often you might set them to, say, +75% and -63% or similar - to suit whatever your landing gear or flaps or whatever require. The transmitters also have multiple model memories so each model can be set up independently.

    You don't normally need to modify modern transmitters - the more advanced ones have a plethora of switches, knobs, and sliders - and using the built-in menus you can assign any switch or slider to operate any channel. You can also mix different inputs together before assigning them to a channel and implement delays, non-linear curves and so on if you wish.
  • xanaduxanadu Posts: 2,938
    edited May 24 Vote Up0Vote Down
    Chances are a non-computerized radio connected to a switch will move the entire range of travel. It should be easy to verify by plugging in a servo and seeing how far it moves.

    The Spektrum DX4 and DX5 are good examples of 2.4gHz controllers with hidden channels. They are both 6ch transmitters, and on the expensive side. I don't know of many others that are like that.

    Flysky makes a 6 channel transmitter that comes with a receiver for ~$45. That's going to be hard to beat.
  • xanadu wrote:
    Flysky makes a 6 channel transmitter that comes with a receiver for ~$45. That's going to be hard to beat.
    HobbyKing has a 2.4gHz 6-channel transmitter/receiver combo for $29.99:

    https://hobbyking.com/en_us/hobby-king-2-4ghz-6ch-tx-rx-v2-mode-2.html

    I've used them. They're more than adequate for the stuff I do and a bargain in a classroom setting. They have good range, but they do suffer from interference more than the Spektrums if a lot of them are being used at once, e.g. in a race.

    -Phil

    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • ceptimusceptimus Posts: 58
    edited May 24 Vote Up0Vote Down
    Or for the same amount of money you likely paid for your 72MHz sets (after adjusting for inflation) you can get something like this:

    FhkYgdul.jpg

    That's the FrSky Horus. Actually I think that one is overpriced and over engineered. The FrSky Taranis X9D+ is every bit as good (in my opinion) and less than half the cost - around $220.
  • Duane DegnDuane Degn Posts: 9,995
    edited May 24 Vote Up0Vote Down
    I think there are several 2.4GHz radios with protocols which have been hacked.

    I (Google) translated some directions I found on a German hobby site many years ago. I was able to protocol to send data to a transmitter module I pulled from a cheap Blade transmitter (one of those small transmitter which are included in many RTF helicopters). By combining the little transmitter module with a Propeller and a Wii Nunchuck, I was able to use the Nunchuck to control the helicopter.

    I think some control boards include ports for Spektrum satellite receivers.

    There are some transmitter modules which can be used as either transmitters or receivers. There's a lot of really interesting hack available nowadays.

    I'll try to find some links to these radio hacks and add them to this thread.
  • Duane DegnDuane Degn Posts: 9,995
    edited May 25 Vote Up0Vote Down
    Here's a video about using an Orange TX module as a receiver.



    IMO, this channel has lots of interesting RC related video.

    I'll add any other video/links I find on the topic of hacking RC gear to this post.

    Edit:

    I haven't watched the whole video but I think it has a lot of good information about some of the RC protocols.



    There appears to be a lot more information about the receiver side of RC gear than the transmitter side.

    BTW, the first video I posted used radio modules which allow much greater range than 2.4GHz radios.

    One of the properties of radio signals is lower frequency bands have greater range than high frequency bands at a given power level. You'll often see long range systems using 900MHz or lower frequencies. The high frequency systems can send more data in a given amount of time.

    Keep in mind if you want to add a video link, you'll want the video transmission on a different band than the control signal.
  • Wow. $29 for the radio is cheaper than the $45 Hitech replacement transmitter battery I was about to order.

    Phil, is that $29 radio a normal radio in the sense it can be used on an R/C airplane, not just a 'park flyer'? My understanding is that a lot of these cheap ready-to-fly helicopters and park flyers that have thrown-in radio systems are very weak close-range transmitters.

    I can't tell from the ad if it comes with batteries. And if not, does it take AAs, or a pack?
    I am the Master, and technology my slave.
  • The_Master wrote:
    Phil, is that $29 radio a normal radio in the sense it can be used on an R/C airplane, not just a 'park flyer'?
    It's a full-fledged radio set.
    I can't tell from the ad if it comes with batteries. And if not, does it take AAs, or a pack?
    It takes eight AA batteries, which are not included.

    You can also purchase one of these for $3.33, which I recommend, and which allows you to program the transmitter with various channel-mixing strategies:

    https://hobbyking.com/en_us/hobby-king-2-4ghz-6ch-tx-usb-cable.html

    -Phil

    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • Wow, that is a sweet deal. I had always thought it was non-programmable.
  • xanadu wrote:
    Wow, that is a sweet deal. I had always thought it was non-programmable.
    Their 4-channel radio is not programmable, but the 6-channel version is.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
Sign In or Register to comment.