Best way to interface to a 27 Mhz Remote

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profmason

Member
Help!

I want to use a picaxe 08M to create a serial interface from the PC to a 27Mhz remote control. There are 6 inputs on the remote, 4 analog and two digital. There is a microprocessor on the remote that takes the 6 inputs and creates the appropriate 27Mhz pulse encoding (I haven't been able to figure out the pulse encoding yet.)

I have thought of three techniques so far:
1. Use a DAC (Like the TLC5620 or Max518) and have the picaxe take a serial stream and feed the appropriate analog levels to the microprocessor on the remote.
2. Use digital pots and do the same thing.
3. Figure out the pulse encoding on the transmitter and just tie the picaxe to the transmitter section of the remote. (So far I have probed each of the pins on the microprocessor. I found the PWMout but it is not obvious how this is being modulated)

Suggestions ideas help! Before I start buying parts......

I am trying to make the new VEXplorer robot capable of computer control. (In the US Vex is the premier student robotics platform, unfortunately their new product is a highly glorified remote control car.)
Here are links to the project so far:
http://profmason.com/?p=499
 
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moxhamj

New Member
If you have a CRO and you don't mind hacking a bit then go looking around for the pulse signals. I've done this before and it does work. You want the point on the circuit which interfaces between the digital and the RF at the 5V level. Usually it is an output from a chip. Look for similar pulses coming out of the Rx and if you are lucky they will be 0-5V as well (but if not then a transistor can be used to translate the levels). Cut the track on the Tx PCB and insert your own pulses from a picaxe.

But this may still not be the best solution. What exactly are you trying to do? If for instance you want more channels (eg you have channels for flying a plane but want one to lower the wheels, turn on landing lights etc) then you can take just one channel and send data via that. At a very simple level you could move a servo one way or the other to sent 1 and 0 and decode this at the other end using slow binary or morse code.

There are other cunning solutions as well but it depends on the problem. Hacking a 27Mhz Rx/Tx might end up being expensive if you zap the unit!

Then again, 433Mhz units at $4 each might end up being cheaper - I've build a radio controlled car out of those units as well.
 
P

profmason

Member
Thanks for the info folks!

Those were great links to general 27Mhz encoding!

I got it now!!!!! The website
http://webpages.charter.net/rcfu/HelpsHints/RadioOps.html

gave a great explanation. I can see the pulse train coming off of pin 20 from the microprocessor. One of the things that threw me was that the remote only has six channels, but it is transmitting eight pulse spaces in a frame. (Nine pulses total.) Still I can clearly see the 20ms cycle and can see the spacing change from 1 ms to 1.5 ms when one of the digital channels is brought high.

You guys are awesome!

Here is my writeup so far:
http://profmason.com/?p=508

It should be pretty straight forward to get the picaxe to do this. One question to you guys, do I need to make the carrier and AND it with the pulse stream, or can I just send out the pulse stream with 300us pulses separated by the appropriate spacing? (I hope that makes sense, do I need to generate a 27Mhz carrier which is modulated by the PPM signal?)

Thanks again!
 
P

profmason

Member
The project is done.
I can use the picaxe to generate the appropriate stream of 300 us pulses to control the VEXplorer robot. (This would work with any remote control device that uses the PPM standard)
Details are at:
http://profmason.com/?p=513

Picaxe triumps again!
 
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