It drives on two wheels plus a coaster ball to support the batteries – although not very fast and veering slightly to the right. It will also turn, by rotating the servos in opposite directions. The batteries push down hard on the coaster ball, making the wheels slip when turning if the surface is not hard enough. The control mechanism is somewhat proportional, although it seems to work best by just pushing the joystick to its limits.

Here’s the current primitive control loop:

Some more close-ups:

From left to right: the Wii adapter, batteries with coaster ball underneath, on-off switch, JeeNode, and the edge of the servos.

Not much wiring needed to get it going:

The two resistors divide the battery voltage in half, so it can be measured by an ADC pin. Note the use of wire to “bundle up” the Nunchuk I2C “bus”.

Port layout so far: 1 = unused, 2 = left servo + battery level, 3 = right servo, 4 = Nunchuk via I2C.

I’ve also mounted the gyroscope (not connected yet) in the proper orientation to sense rotations around the wheel axis:

Will need to add an accelerometer as well, no doubt.

The center of gravity is just about on the power-switch, between the battery pack and the JeeNode. Which is quite low when this thing is placed upright, not sure the servos + control loop will respond fast / accurately enough for balancing.

Current consumption is around 30 mA when idle, 160 mA with wheels turning freely, and up to some 300 mA under load – these 4 NiMH batteries give about 5.2V under no load when full, but I’ve seen it drop to 3.7 V under load after a bit of use – not so good…

A slider drives the two servos in opposite directions, so you can make this thing turn on the spot.

Here’s the sketch:

Had to use SoftwareServo instead of the Servo library which comes with Arduino IDE 0015 – which didn’t work for me.

Turns out that only pulses from 1.4 to 1.6 msec actually have any effect on the speed of the servos, the rest of the 1.0 .. 2.0 msec range just makes them run at full speed. These ara Parallax (Futaba) Continuous Rotation Servos.

Note that the servos are driven from the PWR line, i.e. the full 5V – but the servo pulses are 3.3V, like the rest of the JeeNode.

It’d be nice to use a Wii Nunchuk controller as input. Even more fun would be an accelerometer / gyro combo to turn this thing into a self-balancing bot …

This is a setup with all the sensor interfaces documented in recent posts:

1. a SHT11 to measure relative humidity (and temperature)
2. a BMP085 sensor to measure barometric pressure (and temperature)
3. PIR + LDR sensors to demonstrate reading digital and analog signals
4. a Nunchuk with 2-axis joystick, 3-axis accelerometer, and 2 buttons

A new “combi_demo” has been added to the Ports library. The code is essentially the concatenation of the individual demo source files (you can browse the real thing here):

This example illustrates how the Ports library lets you mix and match drivers and ports at will. Note that two I2C interfaces are running in parallel at different speeds (sure, the BMP085 and the Nunchuk could also have been tied to a single port, running at 100 KHz).

Sample output;

This example compiles to 9114 bytes of code with the Arduino 13 IDE, so there is still plenty of room left to add, say, a wireless driver.

Although the “Wiichuck” adapter lets you connect an unmodified Nunchuk, I just ripped the whole thing apart, and cut off the proprietary connector. There are four wires, tied to pins 2..5 of port 4, respectively: green (SDA), white (GND), yellow (SCL), and red (+3.3V). The following “nunchuk_demo” has been added to the Ports library:

Sample output:

PS. I2C on port 4 = (on Arduino) SDA: D7, SCL: A3 (see JeeNode-v2.pdf page 3).

Update – swap the red and yellow wires for the JeeNode v3 and v4 (the above is an older unit).

It’s very easy to hook up to an Arduino and there’s a good example on the web to read out all the sensors. I’ve tweaked it a bit to use pins 8 and 9 as the I2C/TWI interface.

Connections above are as follows: red = +3.3..5V, white = ground, green = SDA, yellow = SCL.