Project Overview
This robot uses 4 independent light sensitive resistors, mounted on a pan and tilt head, which is driven by 2 servo motors. The protruding shape of the head is such that parallel light coming towards it easily casts shadows on one or more sensors. The micro reads each sensor in turn at a high rate and can sense any imbalance in the light levels being received. This imbalance, when it occurs, is then used by the code to drive the pan and tilt servos, so as to bring them back into balance. Once calibrated the robot appears to have excellent characteristics for sensing illumination bright spots, and tracking things like moving torch light quite quickly.

Click on the image to the right to see the robot being put through its paces. The OLED display contents can be sent to a Windows application, over WiFi, to give a large screen view.

This design was created using a 3-D modeller and printer, to ensure that the assembly goes together with ease. The released design benefits from a range of improvements that were determined from building the first prototype. You will learn from the documentation how this design was put together and wired up.

The main body is essentially made in two halves, held together by small screws. This makes the overall assembly easier to build and offers a degree of maintainability. The pan mounting has graduations built into it to make the calibration process for the two servos simpler to achieve. This element of the design could be used as a stand-alone pan & tilt solution for another project.

In order to accurately calibrate the 4 light sensitive resistors, I designed a tube which fitted over the head. The inside of the tube was painted black, so as to reduce any external light hitting the sides of the tube, and I glued a piece of paper over the end of the tube to diffuse any light shone onto it. I could then use a external light source, like a torch, to illuminate the end of the tube and take measurements from all four sensors.

There is a calibration mode built into the code to assist with this process. It moves the head to the vertical position and presents all 4 readings on the display, averaged over 5 seconds, plus the average of all of them. With this method you can use these readings in map() functions to effectively offset the raw value received from the sensor. As the full scale on the ESP32’s ADC is 4095, I took eight readings, going from dark to bright light, at intervals of 500. The end result worked really well.

The circuit diagram is shown here on the left, with the ESP32 connected to the 4 light sensitive resistors, and to the OLED display using the I2C interface. Whilst the pan and tilt servo motors derive their supply voltage from a simple diode voltage dropper. A voltage divider is used to sample the input supply voltage, with code providing continuous battery health monitoring.

In construction I use wire wrap technology to make the process easier and more reliable. There is however an initial outlay in buying the wrapping tool and spools of wire. One benefit of this approach is that you can run and test your design before soldering the connections for improved reliability. Any errors can therefore be easily addressed during construction.

Give the variations found with light sensitive resistors I would advise that you purchase more than needed, then select four which provide very similar readings.

All of the 3-D models are provided as STL files, zipped together into one file. They can therefore be used with a slicing application of your choice, to create the g-code files for use with a 3-D printer. For my project I used PrusaSlicer, which is a free download from the internet. I’ve also included a simple stand, which clips onto the base of the PixyBot2 for testing or demonstration.

The project is built in two parts, one being the servo Pan & Tilt mechanism, with the light sensors, and the other being the main body. The mounting cup of the Pan swivel is graduated, as is the Tilt bearing plate, so as to make calibration of the two servos much easier. Information presented on the 64x128 OLED display can be sent wirelessly to a PC using the Wii Wireless Controller project, if you wish to build that and use it. The 3D Parts pdf file helps you identify each part by name, and also shows you what drills to use as pilot holes for the screws.