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Author Topic: Looking for RF transmitters, 315 or 433 MHz the only choice?  (Read 463 times)
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I'm not sure what you mean.

If you put a mini arduino in each car, and want to use bluetooth you'd also need 1 bluetrooth adaptor per Arduino

e.g.

http://www.ebay.com/itm/1PCS-HC-06-Arduino-Wireless-Bluetooth-Transeiver-RF-Module-Serial-4Pin-Port-line-/271305987900?pt=LH_DefaultDomain_0&hash=item3f2b18d73c

That would still only cost less around $10 per car.

The Arduino PWM outputs can drive normal servo's as found in most mainstream RC cars.
The only thing you'd need is a 5V regulator (about $1) if the cars run on above 5V.
If the cars have less than 5V batteries, you'll need to use a 3.3V Arduno (around 10% more expensive as they are less common )
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What about taking the RC car's transmitter apart and using the arduino to control it?  The RC transmitter uses a pot for the throttle and a pot (if I am not mistaken) for the steering.  Might be bulky but you could use a multiplex circuit to control multiple cars.

Bob
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RC cars are also on 27 MHz?
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I really appreciate your help in all this, finally starting to understand how this is going to work. And extremely sorry for this huge wall of text and questions I'm about to bombard you with.

**For this hypothetical example I'm going to use the 315 MHz transceiver, just because the math is a little easier, however when I actually attempt to build this I will use 2.4 GHz transceiver.

So I have four 49 MHz RC cars that have four different channels. I'm trying to control all of them with the computer and preferably a single Arduino. My goal is to not modify the actual cars in any way and instead simply mimic the controllers RF output by using the transceivers on the Arduino.

If I understand this correctly, and was using the 315 MHz transceiver, for one of the 49MHz cars I would need it to modulate between (315 - 49) and (315 + 49), so between 266 and 364 MHz.

For the different channels this is then modified very slightly? So maybe the first channel is 49 MHz, the second channel is 49.2 MHz, third is 49.4 MHz and the fourth is 49.6 MHz, or something along those lines?

So if I had the transceiver, in order to find these slight differences in frequency I would use the receiver capabilities of the transceiver and select channel one on the RC car controller and select any random movement (forward, reverse, left, or right) and with the right coding I could get the frequency to be picked up by the transceiver and then transferred to the Arduino? And I assume I could get this to display on the serial monitor.

Then I could do this for every channel and be able to know their assigned frequencies.

One thing I'm confused about, however, is how I would figure out the pulse sequences of each direction. I read this article about how RC controls work and just as an example they state that:

  • Forward: 16 pulses
  • Reverse: 40 pulses
  • Forward/Left: 28 pulses
  • Forward/Right: 34 pulses
  • Reverse/Left: 52 pulses
  • Reverse/Right: 46 pulses

Is this information that can be picked up by the transceiver?

Assuming it is, at this point I now have the assigned frequencies and pulses for each channel, so now it's time to program the arduino to control the cars. So I need to set up variables that say

CAR_1 = 49 MHz

CAR_2 = 49.2 MHz

Etc.

But of course this is an extremely over simplified version of how this would go. In reality I have no idea how one would set the frequency.

I did, however, find this code online from a person setting up a circuit to remotely control their garage door opener and found this bit of code:

Code:
    // Set pulse length

    // NB Pulse length must be set AFTER Protocol,

    // because setProtocol(1) also sets pulse length = 350

    mySwitch.setPulseLength(232);


I assume "mySwitch.setPulseLength(232)" is referring to the pulses that in my case would indicate direction? So based on the example I found for the directional communication, if I wanted the car to go forward (16 pulses) then it might looks something like this:

   
Code:
mySwitch.setPulseLength(16)

Thanks again for all your help!
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You can't make 315mhz or 2.4ghz transmitters create signals on 49mhz.

If your cars operate on 49mhz you need a 49mhz transmitter, or possibly multiple 49mhz transmitters.

You need to research how radio data is actually sent to understand why this is true.
I.e you have to understand carrier frequencies and modulation methods.


IMHO the only way you could possibly get this to work without modifying the cars, is to take the transmitter units apart and put Arduinos and other electronics inside the transmitters, as its highly likely that these cars use some private undocumented data modulation method, which you'd need to replicate if you want to build your own transmitter.

I.e its totally possible to build a replacement transmitter, but you'd need to have access to an electronics lab with lots of test equipment like oscilloscopes, spectrum analyser, etc. and also up skill on how to design and build radion transmitters etc etc, which is a steep learning curve
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Ahh ok, I must have misunderstood one of the articles I read.

One thing that confuses me is despite the popularity of the 27 and 49 MHz transmitters for RC cars, I can't seem to find any transmitters for these frquencies. Is this due to, as you said, the proprietary technology the RC car manufacturers use?

Well this point it looks like I'm down to two choices.

1) Use relays soldered to the original RC controller to accomplish what I need. Similar to this method:
http://www.drivemeinsane.com/howtos/rccar/rccar.shtml?theme=0

or

2) Use the Arduino Micro's/Mini's and modify the RC cars' cricuit board with them.

Which would you recommend in terms of both cost effectiveness, simplicity, and fast response to inputs?
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The problem is mainly that these sorts of radio controlled cars etc are sold as toys rather than radio controlled "models", and are not designed to be modified in any way.

As such the designers often take a load of short cuts in the overall design and manufacture in order to make the cars as cheaply as possible, and you'll probably find just one control board in the car that is both the radio receiver and the demodulator and the power output control to the servo's and the motor.

You may find that the servo's don't use PWM, but that the control is direct from the single control board, so you may not even be able to put an Arduino in each car, as you won't be able to control either the steering servo or the motor control.


Your only practical option is to take the transmitter unit apart and design some electronics that makes the transmitter believe that a human is changing the inputs, but in actual fact the Arduino is controlling them.


All in all, I think this is a non-starter. You'd be better off buying some different cars that are controlled via bluetooth etc, or buy some real RC control cars that have separate internal modules e.g. separate receiver, real servo's and separate motor ESC s etc
But both options are going to cost $$$ which is probably why no one else has done this.
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Seriously can't thank you enough for the reply, you guys have been extremely helpful with this project.


Just a quick question though in terms of options available:

Would it be possible to remove all the circuitry inside the RC car, leaving only the motors and rechargeable battery, then use a micro Arduino to replace what was once the circuit board? Then buy transceivers for my main Arduino and each RC cars' micro Arduino, thus allowing my primary board (a mega 2560) to communicate with each micro Arduino on the RC cars.

I opened up one of the RC controllers and the circuitry was much more complex than I had imagined. My guess is that a lot of it is filters in order to eliminate all other signals other than the desired frequency?

I would build an RC car myself, but it would be nice to take advantage of the steering linkage, axles, wheels and body of a pre-built car that I could then modify.
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Can you take a photo of what inside the car, and post it to the forum ?

Its hard to tell what, if any, of the existing electronics in the car could be re purposed without even seeing a picture.

BTW. Do you have a volt meter (actually they are normally Digital Multi Meters), that you could use to probe voltages inside the car?


Quote
I opened up one of the RC controllers and the circuitry was much more complex than I had imagined. My guess is that a lot of it is filters in order to eliminate all other signals other than the desired frequency?

Can you take a picture of the TX as well.

When stuff looks like that, its normally because its analog electronics. (not digital)

You'll probably find the transmitter unit sends out various audio tones on several frequencies at the same time, where each tone controls a different part of the car. There will be an equivalent inside the car, lots of analogue filters to separate out the tones and turn them into voltages.
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