randomhero421

Me and a friend are currently working on a bettlebot-like project.
It involves many vehicles operating at once.
I plan to put a Futaba receiver in each, which controls 6 servos.
The problem is, all will be 75mhz, but we need to have 32 of them working at once.
Is there a way we can do this at minimum cost?

Thank you,
-Random Hero-

ZAGNUT

My Feedback: (1)

you probably already know that you only have 30 channels on 75mhz, right? if any of the contestants have an amateur radio license they could use 50 and 53mhz which would add another 18 channels.

operating 32 radios at a time would require that attention is payed to proper rx antenna size and placement. this might be a bit difficult considering that most of these robots are probably metal and you also need to protect the antenna from your opponents.

just my opinions and i am NOT an expert on radios

good luck,
dave

randomhero421

When you say '30 channels' are you referring to the total number of servos that can be controlled on the frequency or the number of contestants that can run their bots? And how would I go along with setting it up so the transmitter controls the right robot?

Thanks,
Random Hero

amcross

My Feedback: (2)

Random,
each robot needs a transmitter on one of the 30 legal frequencies for 75mhz, and then you can also use one of the 4 legal 27mhz frequencies, to get your 34 robots functiona.

each needs a matching receiver on the same frequency as the transmitter setup for each robot. the transmitters and receivers are sold in matched sets, normally, so when you buy one you get both.

lastly, most 75mhz systems have just 2 or 3 channels, so they can only operate, for example, a 2-wheeled robot to work them together for forward/reverse and opposite to turn, but have 1 or no channels left for any other controls.

did i understand your question correctly, is this what you're asking?

randomhero421

I am new to this, seeing as I never ran 10+ bots together.. But, are you saying there is no way to have 10+ bots (on 75mhz) without using other frequencies (27, 72, etc)? And, I am still having difficulty understanding how the whole thing works. If someone could explain how this all goes together that'd be great.

Edit: I have seen 20+ robots at a time on the tv show "BattleBots" and they each have about 6 servos/motors being controlled. How was this achieved? Multi-freq radios?

Edit 2: We need 6 functions fior each bot: 4 servos reverse and forward, and 2 to activate relays. 2 of the servo outputs will go to a Servo-pulse to PWM driver and that will go to an H-Bridge control for 2 drive motors.

Correct me if I am wrong with the design

Thanks for your time
-Random Hero-

ZAGNUT

My Feedback: (1)

let's clarify things a bit

on 75mhz you have 30 channels from 75.410 to 75.990 with .020 mhz spacing, each robot can use one of these channels for all of its functions

radios with 6 or more functions are no problem to find but are usually on 72 mhz and are for aircraft use only, you cannot use these legally for robots. search out the manufacturers and see if any of them offer 75 mhz rf modules and receivers for use in these radios.

i've seen plenty of battlebot shows on tv where the guys are using high end 8+ function radios. either they are using a frequency that's legal for surface use or they are illegally using 72 mhz. worth looking into...

dave

amcross

My Feedback: (2)

Zag,
They are using radios like the 9C and 9Z with 75mhz modules. Futaba even sells the 9C with a 75mhz module already in it.

Random -- there are 30 75mhz FREQUENCIES, so you can run 30 robots at once, all using 75mhz. more robots? need to go to 27mhz and then get ham licenses and go to 50mhz. of course, most events dont ahve all the robots running at once and oyu take turns with what channels you use.

how do they work?

transmitters and receivers, matched to use the same frequencies....receivers in the robot with an antenna picking up the signal.

here's a high end 9 channel radio on 75mhz as an exapmle for you. they are NOT cheap...
INFO
http://www.futabarc.com/radios/futj89.html
PURCHASE
http://www2.towerhobbies.com/cgi-bin...**&FVPROFIL=++

Mac_Man_UK

Over here in the UK we have what I imagine is a similar program to your battlebots, we call it Robot wars. I am led to believe that they have special dispensation to use the Aircraft frequencies.

I would imagine that it would be just as easy for the TV producers of your Battlebots to get similar dispensation.

randomhero421

I understand all that, but one more question: How does the transmitter control the right bot? Is there a switch or dial? Or is it autodetect?

Sorry for all the questions, we wanted to know all about this before we buy.

Thanks,

-Random Hero-

amcross

My Feedback: (2)

Random,
the transmitter has a crystal and is tuned to a specific frequency. the receiver in the correct bot is on the same exact frequency.

there ARE systems you can buy that are "select a channel" (called SYNTHESIZED) so they could operate receivers on differen channels but they are not available for 75mhz.

randomhero421

Alright, I get that but here is where I am confused!! You said there are many channels (75.410 ie.). THe receivers just say if they are "75Mhz", that doesn't tell me if it is at 75.410 or 75.430 etc! Is there a site that can explain it to me very simply?

Thanks for your time,

Random Hero

ZAGNUT

My Feedback: (1)

i think i understand your confusion....if a receiver says 75mhz then it is designed to operate somewhere on that band. it should also have a socket for a plug in crystal, this crystal will have a sticker printed with the exact frequency (75.xxx mhz) that it provides. the transmitter will have a removable rf module that needs to be the same frequency within the 75mhz band in order for it to talk to the receiver.

go to an rc car race in your area and everything will become clear. you'll see that if two guys that have the same frequency on their radios want to compete against each other one of them will need to change out his rf module and receiver crystal. all the racers can run on the 75 mhz band but each needs to be on his own frequency within the band.

dave

randomhero421

Yes, that is what I needed. I understand now, but one more thing, where is a good place to buy crystals?


THanks

Random Hero

Ladyflyer

My Feedback: (2)

Not again !!

amcross

My Feedback: (2)

Random,
You purchase receiver crystals with the receivers when you purchase them, or through the same sources you are purchasing your receiver.

NOTE: you ARE NOT allowed to change the TRANSMITTER CRYSTAL to change its frequency. You CAN change the frequency by having a modular radio and changing the module.

randomhero421

Got it.

Thanks for all your help. I appreciate it.

End of thread

-RandomHero-

David Cutler

I think you might be getting channels for the frequency and channels for each servo connected to each receiver mixed up.

As has been said, there are 30 channels on the 75 meg band. That means 30 different receivers can be actuated at any given time, as each could have its own channel.

Each receiver, in turn, can actuate as many channels as it has been designed to do. An 8 channel receiver can run 8 servos, and it's only using one frequency channel to do it.

The 'splitting' of each servo channel is done internally in the receiver, while the receiver itself is only using one frequency channel.

Don't worry about what happens internally, just remember that one frequency channel is being used by each receiver, and the internals in the receiver is handling each servo.

Does that help?



-David C.