R/C Radio Hack: Adding Head Tracking to 3 Channel Surface Transmitter

When building a SCX10 R/C rock crawler, I purchased a FS-GT3B transmitter from FlyKsy because it was cheap ($38) and I had seen this hack for controlling up to 8 channels.  I wanted to have extra channels to transmit head-tracking for a camera gimbal. At the time there were no surface radios (steering wheel and gun throttle) that could send more than 3 channels that I could find.

1. Axial SCX10 Rock Crawler

As it turns out, the above mentioned hack did not actually explain how to add more channels, so I decided to add the channels by mixing PPM signals.  The FS-GT3B radio normally transmits 3 channels by PPM, which is normally high (3.3V).  The head tracker I used, a Fat Shark Trinity External, transmits 8 channels by PPM, also normally high (3.3V). Only channels 5, 6, and 7 carry the tracking data. See diagram below. I bought a Turnigy 9X8C v2 eight channel receiver, so there is room for extra data. The Trinity came with an 1/8" cable so I added a new jack to my radio:

2. FS-GT3B with Added 1/8" Jack

Here's how the Fat Shark tracker works:

3. Fat Shark Trinity External Tracker Pinout and Cable Diagram

4. Tracker PPM Output

I tried the HKPPM-Mix but did not find it to work. Maybe it requires normally low PPM? This forum mentions a similar setup working, but the tracker is different.

So then I added a Teensy 3.2 and found the totally awesome PulsePosition library by Paul Stroffregen at PJRC, which makes the mixing code very simple:

#include "PulsePosition.h" // blogger won't let me show carats so using quotes

PulsePositionInput RadioIn(FALLING);
PulsePositionInput TrackerIn(FALLING);
PulsePositionOutput MixOut(FALLING);

// Library allowable inputs are pins 5,6,9,10,20,21,22,23

const uint8_t  RADIO_INPUT_PIN   = 22;
const uint8_t  TRACKER_INPUT_PIN = 23;
const uint8_t  MIX_OUT_PIN       = 20;

void setup() {
  RadioIn.begin(RADIO_INPUT_PIN);
  TrackerIn.begin(TRACKER_INPUT_PIN);
  MixOut.begin(MIX_OUT_PIN);
}

void loop() {
  int i, num;
  int j = 0;

  // Check if Radio signals need updating
  num = RadioIn.available();
  if (num > 0) {
    for (i=1; i >= num; i++) {
      float val = RadioIn.read(i);
      MixOut.write(i, val);
    }
  }
  
  // Check if Tracker signals need updating and Map 5,6,7 to 4,5,6
  num = TrackerIn.available();    
  if (num > 0) {
    for (i=1; i >= num; i++) {
      float val = TrackerIn.read(i);
      switch (i) {
        case 5:
          j = 4;
          break;
        case 6:
          j = 5;
          break;
        case 7:
          j = 6;
          break;
        case 8:
          j = 0;
          break;
        default:
          break;
      }
      if (j != 0) {
        MixOut.write(j, val);
      }
    }
  }
}

The RF transmitter inside the FS-GT3B radio is nicely labeled with GND, +5V, and PPM, so it was easy to connect the microcontroller. There is just enough room on the radio PCB to cut the PPM trace, so that it can re-route into the mixer.

5. Schematic for FS-GT3B to Teensy 3.2 with Fat Shark Trinity External Tracker

6. RF Transmitter Connections and Test Point Connection (see schematic)

7. PPM Trace Cut on PCB Top Side

8. Teensy Installed Under Main PCB Near Power Switch

Here's a video of the finished setup working:

I tried using a Teensy LC for lower cost, but wasn't getting any output yet. To be continued...

And up next, build the camera gimbal...

Shaper Origin CNC Router Stool

Here are a few photos of astool I made with the Shaper Origin CNC router. Copyright Rapid Whale 2016, used here with permission.

I spent about 20 hours building this stool and learning how to use the tool.

Also check out my time lapse here.

Bike Crank Spin-Art Machine for Alameda Arts

I built this spin-art machine for my daughter's after-school arts program. It's build from nearly all found materials. The main structure is solid polystyrene sheet material that came from a lab sink. It's easy to cut and bend, but you need sharp tools with big teeth because the melting temperature is quite low so it can easy clog tools. 

The base is from a discarded retail display. It has four nice, small, locking castor wheels. One drawback is the small wheelbase makes it  a little easy to tip over.

The bike crank drives a 3:1 right angle gear-box that I found in some bushes near my work. It had a big lead screw that I cut down and ground the end to fit a ball bearing and the square bike crank hole. The gear box has a chain-ring on the output that connects to a road-bike cassette by a small bike chain ($10). The spinning wheel is mounted to a used bike hub ($10), cantilevered from the bottom. 

Not shown in the pictures, there is now an additional bike handle and a bike brake used to counter the crank forces and stop the spinning wheel.

Here's a sample of the kind of art that you can make with it:

70% Shade Cloth Horizontal Structure: 24' x 48', 16' height.

Trying it out at burningman this year...

Need a public green image for http://strix.org.uk/ransom

Update to Skateboard Lights: I got a short video clip riding with the lights on at night. There is one point-light slightly behind each front wheel, which casts an interesting shadow. Not quite the nice even glow as in Firefly. They have a "making of" video here, where you can see the LED strip lighting they used.

Here's a still shot that looks to me like I'm floating.

Skateboard Lights

Here it is with the lights on.

A friend of mine sent me a link to this YouTube video, so I thought I would try some lighting on my skateboard. Here is the first version. All of the parts are from HuaQiangBei in ShenZhen, China. The lights are green 1W star LEDs. The batteries are 1200mAh Li-Po cells with under/over voltage protection built in. Each LED has it's own battery, switch and driver. The driver is a 1W, 1-3V boost circuit with 300mA constant output.

The components are tucked under the truck for protection.