30 year old Futaba Silver Wheel radio converted to 2.4GHz

[TheObstacle]

The only radio I've ever used to drive R/C cars is my first transmitter -- a Futaba FP-T3FG "silver wheel" that I got around 1982, with my first real R/C car (a Tamiya Sand Scorcher). I still use it today to drive my (now heavily modified) Edinger RC10.

I've been thinking for a while that I'd like to update it to newer technology. I'm not even sure if the original 75MHz band it uses is technically still legal... although I haven't looked into that too closely.

My initial thought was just to try and find a broken transmitter of the same model, so I could gut it and install the internals from some other wheel radio, maybe a cheaper Specktrum, or possibly build something out of an Arduino and a standalone RF module using something like http://www.reseau.org/arduinorc/ .

At some point while doing research on the web, I ran across this "FlySky 2.4 GHz hack idiots guide-3.doc":

http://www.rcgroups.com/forums/showthread.php?t=967207&page=63#post12420518

I've recently gotten into flying quadrotors, and I use a FlySky 9x for those. I've picked up a few spare receivers that are compatible with that system, and being able to use the same system in my ground vehicles has a great appeal...

I recalled from looking inside my transmitter before that it had a main board and a separate RF board. A closer look showed that the RF board is connected to the mainboard with 3 wires, which turned out to be power, ground, and (as I determined with an oscilloscope), PPM.

The power wire delivers full battery voltage, which in my case is around 10v (since I'm using a 3S LiFE transmitter pack). The PPM signal from the main board turns out to be an "open collector" output, which means that the voltage of the output signal is set by an external pull-up resistor to a reference voltage.

One of my goals was to keep the original setup intact, so I could put it back to the stock 75.5MHz FM setup if I ever feel the need. To this end, the first actual change I made to the Tx was to install a 3-pin servo plug and socket inline between the main board and the Rx board. The new module and attendant circuitry will plug into this in place of the old RF, so it should be completely reversible. I verified that everything still works with the old RF module and receiver after this change:

Unplugging this and breadboarding a simple voltage regulator and pull-up resistor showed that the main board could generate a clean ppm signal with the RF section completely disconnected. This looks _very_ promising.

The wheel radio is a 3-channel, but you may notice that's a 4-channel ppm signal. It turns out that channel 3 is encoded into both channels 3 and 4 in the ppm signal, which is kinda interesting but not exceptionally useful.

The donor radio I picked to harvest an RF module from was the Hobby King HK-T4A 4-channel unit, which includes a 6-channel receiver. Under $30 shipped from Hobby King's U.S. warehouse.

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=15142

3-channel surface receivers compatible with this system (model number HK-GT2R) are also available for about $6 each, plus shipping:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=36509

These are identical to the FlySky FS-GR3E, and probably other re-brands as well.

Before removing the RF module from the donor radio, I powered it up and put it on the oscilloscope. The wires going to the RF module are ground, 5v power, bind (which is shorted to ground when you push the "bind" button) and a 2.5v PPM signal which is inverted relative to the one the Futaba generates (i.e. the signal is at 2.5v with pulses to 0v, instead of 0v with pulses to higher voltage).

To provide 5v to the module, I'm using an OKI-78SR-g 5v switching regulator (I've used these in some other projects before, so I had one lying around):

http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/2259781?cur=USD

After teaching myself a bit about open collector outputs through the magic of google, I determined that turning the open collector PPM signal into an inverted one at 2.5v requires a relatively simple circuit consisting of a pull-up resistor, an NPN transistor, and a voltage divider to pull the inverted signal up to 2.5v. I initially put this together on a breadboard, then transferred it to a piece of perf-board cut to the same size as the original RF module so it would fit into the original board holder.

The perfboard is big enough that I was able to attach the new RF module to it with velcro, and still had room to mount a "bind" button to it that's accessible via hole in the case where the original module's crystal stuck out.

The antenna on the donor TX is made entirely of plastic -- I had to break part of the case's plastics to get it free, and I couldn't come up with a very good way to mount it to the Futaba's case. To make the setup more flexible, I replaced it with an RP-SMA pigtail and screw-on antenna:

https://www.sparkfun.com/products/662

https://www.sparkfun.com/products/145

I just cut the small U.FL connector off the end of the cable and soldered the coax to the RF module.

I was able to take a piece of aluminum from an old heat sink and hand-craft it into a shape that slides into the square slot in the top of the case, with a hole in it for the SMA connector to mount into.

Here's the new RF module in its final form:

It works great, and the new receivers are TINY.

[jwscab]

excellent work and very thorough thread!!!!!

[GreenHell]

That was my first radio as well. Good read!

[sj701]

Nice conversion. I may have to do that to my Magnum Racer transmitter.

[filzemail]

Nice work! The older we get, the more nostalgic we get. Should be an article out of an RC magazine. Nicely done..
regards
Philip

[kink]

Very clever and what a cool looking retro radio!