I just bought a vex transmitter/receiver “add-on” kit (It was cheap, os I figured it’d be useful 
). Is there a way to modify the Vex receiver so that it has outputs like a standard multi-channel receiver module? (several connections, one for each channel, instead of a single serial line for all channels that goes to a vex controller)
I don’t have a vex controller and I’d like to control normal servos and perhaps ESC’d motors off of the vex receiver.
I believe that the Vex reciever needs a microcontroller to decode the pulse stream. You might want to look at this link. They have done most of the work for you. http://kronosrobotics.com/Projects/vexradio.shtml I’ve used these chips for many things and like the whole system. There is more than enough sample code on the site to get going.
Did you buy it from allectronics.com? There is a spec sheet there that has a circuit and code:
http://www.allelectronics.com/cgi-bin/item/JS-6/search/6-CHANNEL_TRANSMITTER_AND_RECEIVER_.html
hey, just get a standard hobby reciver, and plug in the vex crystal and it works, the hobby reciver already has the chips in it to output pwm for servos
super simple
Devo
I’ve tried that with a JR R610M 6-ch FM receiver from an old RC airplane, popped the vex rx crystal into it and nothing works. The transmitter and receiver are connected though, as when I turn on the transmitter, servos attached to the receiver will twitch. Just none of the controls on the vex transmitter do anything.
Any ideas? Should I find myself another 6-ch receiver?
EDIT: The JR receiver has an “ABC&W” circuit in it, perhaps this has something to do with why it doesn’t talk to the vex transmitter?
JR Radio describes the ABC&W circuit as:
“JR has a unique signal processing system called ABC&W. ABC&W stands for Automatic Blocking Circuit with Window. ABC&W systems have distinct advantages over single or dual conversion receivers. ABC&W systems create a very small electronic window that looks for the specific frequency that the receivers crystal is on. Any signal that is off spec for this window is rejected, and then the allowed signal is cleaned up, amplified, and re-routed through the window many, many times till all that is left is the pure signal on the channel that is coming from your transmitter and then sent on to the servos. This all takes place without any noticeable delay of response from your stick movement to the servo. This technology is the reason that JR can produce single conversion receivers that have fewer components and weigh less than other single and dual conversion receivers without the worry about outside interference.”
Devo,
Does the ‘standard hobby receiver’ have to be PCM or just FM?
Does it have to be a 6 channel?
Have you tried any other brands besides Futaba?
Does the VEX transmitter have the same range as a Futaba?
Thanks,
BoyntonStu
Devo - Sorry to need to disappoint you, but I’m pretty sure that there is no such thing as a “standard” hobby receiver.
Like I mentioned in my other reply (in the other thread), part of what you are asking is easily found on the VexLabs Parts and Accessories web pages. Here are a few things that might not be there. The Vex transmitter emits pulses of two different frequencies. When one frequency is on, the other is off. In this way a train of pulses is modulated into/onto the frequency of the transmitted signal (i.e. it uses FM)
The width of these pulses (from which you can derive their position (in time) in the periodic emissions of the transmitter), is used to encode the state of the operator controls that are part of the transmitter. While some folks like to call this Pulse Position Modulation (PPM), I think that they are confused and that it is more properly called Pulse Width Modulation (PWM).
The current Vex Transmitter/Receiver (Tx/Rx) do not use PCM.
There are a few other Tx/Rx characteristics (like the polarity of the transmitted PWM signals and each channel’s location in the pulse train) that need to match before you can say that a non-Vex receiver will work with a Vex Transmitter. Sorry, but I forget if the idle state (what is output between pulse trains) of the Vex transmitter is its high freq (crystal freq plus ___ Hz) or its low (crystal freq minus ___ Hz) freq.
About the range question. You might have more luck asking “How long is a piece of string?” Without knowing anything about which of the many Futaba transmitters you have in mind, and without knowing (because I haven’t found anywhere where they tell us) the output power of the Vex transmitter, and without knowing if the receiver you have in mind is truly well suited for use with the Vex transmitter; there is no way to answer your question.
Even if “we” knew all these things, without extensive or sophisticated investigation the answer would still be a guess.
One thing that might interest you is that all RC transmitters operating in the band Vex uses are limited (by the FCC) to supplying a maximum of 0.75W to their antenna. This will tend to give similar Tx/Rx parts a similar maximum range, if the transmitter is designed to emit the max wattage. For this reason I think that many people will gues “Yes” if they answer your question about ranges without having actually tested a Vex and at Futaba transmitter side-by-side in the field, or having opened them up to see if their circuitry is identical/similar.
Blake
Blake,
I have read on several forums that Futaba receivers equipped with the VEX RX crystals worked well with the VEX TX.
The add-on VEX comes with 89, and I would imagine that any ‘hobby’ PWM receiver receiving on 89 would work.
Is my assumption accurate?
Maybe.
You say you are only considering PWM receivers. That is one correct criterion. Good.
But there are at least the other three criteria that I mentioned.
These are the thngs that come to mind as a result of studying this sort of Tx/Rx system for the last few days. There may be others.
Then you need to move to determining whether the receiver’s output wiring/signals matches the wiring/signals of the equipment you want to connect to it…
The subject is a bit tangled, but research and/or careful trial and error testing can get you the answer. Without specifics, you are stuck in the land of “Maybe”.
Blake
Blake,
Maybe is better than not possible.
Crystal: The channel 89 VEX RX crystal which comes with the add-on kit, is ‘standard’ sized and it apparantly fits Futabas, Bergs, and others. (Not so the TX crystal). (3 sets of VEX crystals (6) available for $10 on Ebay)
That leaves the polarity issue. TBD.
The TX takes 8 AA cells. Rechargeable NiCads in my fridge.
BoyntonStu
From: http://www.allelectronics.com/cgi-bin/category.cgi?item=JS-6&type=
Works fine with Futaba receivers 02/05/2007
Reviewer: Larry Gitchell from Northlake, IL USA I have tried this radio with 2 different types of Futaba receiver. Works with both just by plugging the Vex crystal into the receiver. Unfortunately, the transmitter crystal has a non-typical spacing on the pins, so you can’t change channels on the transmitter so easily.
and:
A very good 75 mhz transmitter 01/28/2007
Reviewer: A viewer from BROOKLYN, NY US
It’s very hard to find a transmitter in 75 mhz, with 6 channels with mixing and at this price. I tested with my all 3 types of 75 mhz synthesized receiver (no crystal needed) all work with the transmitter. But the channel 1 to 4 are in reverse order is a easy fix, 5 and 6 ok. the receiver come with the kit was a loss.
Interesting.
Would have been better had he described the specific receivers.
If you were to pick one to try, which one?
BoyntonStu
Boyn - The choice is yours to make. You are right that specifics are needed. The receiver that doesn’t need a crystal is intriguing. - Blake
It’s not really as exciting as you might hope. See the receiver here:
Live and learn. “Look Ma, no crystal!”
Please explain the 6 channels of the VEX.
The 4 and the 2 digital.
What do these 2 types of channels control, assuming they are successfully separated into 6 outputs of a ‘hobby’ receiver?
BoyntonStu
Hey guys, here’s some hard data for you. I bought the “GWS 4-channel R4PII 75Mhz Pico Receiver” from this page, and it works great with the Vex transmitter. All four of the analog channels come through, and in the expected order. I also had to buy the GWS crystals, because they are much smaller than the Vex ones. It seems that GWS makes generic radio parts that are meant to interact with a wide range of other manufacturers’ equipment (Futaba, JR, Airtronics, Hi-Tec), so I suspect that while there is no “standard” RC receiver, the Vex equipment is reasonably interoperable with other common parts. This thing was definitely not a bad deal ($21 for a 4.4 gram receiver, plus $7 each for crystals). BoyntonStu, be sure to let us know if your receiver also works.
Hey that is great news! Thanks.
The GWS is a ‘standard’ receiver. Is it + or 1 shift?
I would assume that any other receiver interchangeable with the GWS 4-channel R4PII 75Mhz Pico Receiver would also be OK.
The question remains whether or not any 72 Mhz Futaba, Hitec, etc with the same shift as the GWS would work with the VEX crystal?
What range did you test?
Many thanks for your most useful data.
BoyntonStu
This documentation is a little bit Engrish, but it sounds like this receiver is negative-shift. No guarantees there, though.
Yes! I checked and the Futabas are negative shift.
(JR’s are +)
What range did you try?
(The GWS is single conversion and they claim 1,000 feet.)
Have you considered building a decoder for the VEX receiver?
Stu
I built a circuit that converts the data stream to control 6 servos dirrectly without the use of a pic micro for those who don’t have the equipment to program pics. Board is simple as it uses only 10 components not counting connectors. A regulator on board supplies power for the Vex RX as well.
That is great news!
More info please.
BoyntonStu