Project Overview
This project employs 11 servo motors, 5  in each leg, and one in its body to move the droids eyes. The microcontroller is an Arduino NANO EVERY, which has 50% more code space than a standard NANO and 3 times the RAM. I new I would need that extra space to store the servo movements. Between the eyes is a laser range finder, which give the droid an ability to react to object being placed in front of it. Leg movements and balance are quite complex, so I wrote a custom PC app which helped me to achieve this. The droid is controlled via a  Wii nunchuk, over a 2.4 GHz wireless link. This in turn provides a bidirectional data channel for telemetry, enabling the movement app to work wirelessly. If you are planning to use the PCA9685 16 channel servo driver board in another project, you might like to try the app I wrote for controlling that too. Click the image to the right to watch a video of my droid in action -->

All of the droids components were produced in PLA on my 3-D printer, and modelled using the free design package from RS Components, DesignSpark Mechanical. For this design I also constructed an assembly of the complete robot, to ensure that all of the parts aligned. This included simple models of all bought components like the servo motors and driver board. Given the size limitations of my 3-D printer, the chassis consists of a number of parts screwed together using 2 mm countersunk screws. The top plate has compartments for the two batteries and apertures for socket strips to take the micro and Wi-Fi transceiver. A finned ring surrounds the body to protect it, if it topples over.

The lower legs consist of plates, to provide mounting frames for the servo motors, and joints for hip, knee and ankle movement. The use of screws makes it easy to maintain the droid, should you wish to disassemble it to replace a part or change its appearance in some way.

The droid is powered from two 18650 Lithium batteries, delivering 7.4v to the system. An On/OFF switch is mounted at the base of its body, feeding power to the top plate electronics and the PCA9685 servo driver board, mounted on the rear. Two push button switches sit either side of the micro, with the 2.4 GHz wireless transceiver mounted centre of the top plate. Shortly after power-up the droid code drives all servos to their vertical rest positions, allowing it to stand freely with low power.

Programmable RGB LEDs are placed in each eye, and 5 in a strip on its front. The colours provide feedback on the modes of operation, and the front LEDs are animated when the droid is in motion. The servo cables are kept tidy with special clips on the rear of each leg. The foot servos need extension leads in order to reach the PCA9685 driver board, as slack is needed when the joints move at large angles..

The circuit diagram for the droid is shown here on the left, with the NANO EVERY in the centre, connected to the RGB LEDs below, the VL53L0X laser range finder left, and 2.4GHz transceiver above. The I2C bus wiring on the left is taken to the PCA9685 servo driver board in the body, along with an MPU6050 motion sensor. As the servo motors are only rated up to 6v I have used diodes to drop the battery voltage fed to the driver board. A simple resistor network is used to enable the Arduino to measure the combined battery voltage, which should not be allowed to fall below 3 volts per battery; this is monitored in the code.

The wireless transceiver command pin is connected to the NANO so that the code can set up the device using AT+ command codes. By default it is set to 9600 baud, but needs to run at 115200 for move telemetry programming.

A critical step in building a project like this is in attaching the servo motors leavers and calibrating their respective angles, by recording and storing PWM values in code. To simplify this task I developed a 16 channel controller app which provides adjustable sliders for each servo. Values are then simply cut-n-pasted into your code. Then, as leg movements can be complex, having 5 servos in each, I developed a Move Trainer app which enables you to create a series of movement points through which all 10 servos can be driven. This links with the code in the project, which also allows for linear and non-linear movements, like acceleration and deceleration. Complex moves up to 100 leg positions can be created and simply cut-n-pasted into your code, and copied back into the app for further refinement. This process also works over the wireless interface, so the droid is not tethered by the USB lead. The third app works as a simple plotter of range v angle values obtained from the VL53L0X laser range finder. I hope you find them useful.