ECE118 Robot
An autonomous robot that finds opposing robots by their IR beacons and shoots ping-pong balls at them, built for ECE118, a mechatronics course.
Mechanical and Electrical
In ECE118 (Intro to Mechatronics, a capstone style robotics course at UCSC), we spent five weeks designing, and building a robot in order to compete in a tournament-style competition at the end of finals week. Our final bot passed the minimum specification for the competition, and made it to semi-finals in the competition.
The assembled robot: differential drive base, laser-cut plywood layers, stepper-driven turret with flywheel cannon and the IR detector array on top.
The robot navigates by following the field’s middle tape line to the shooting zone — harder logic than the side-tape route most teams took — using four IR reflectance modules for tape detection and two microswitch bumpers behind a laser-cut whisker for collision recovery. The interesting engineering, though, is in how it finds the other robot.
Beacon detection
Each robot carries a 2 kHz IR beacon, and the course adds obstacle beacons at 1.5 kHz and 2.5 kHz that have to be ignored. Our answer was an eight-channel phototransistor array on the turret, multiplexed through a single analog filter chain so the whole thing reads out over three digital pins.
Getting the optics right took three iterations. A 3D-printed PLA housing leaked IR straight through its walls and lit up every channel at close range. Bare aluminum tubes fixed the crosstalk but had a field of view that changed drastically with distance. The final design press-fits a convex lens into each machined aluminum tube, focusing light onto the phototransistor with enough margin to absorb assembly tolerances.
Exploded view of the final detector array: machined aluminum tubes, press-fit lenses, and 3D-printed shrouds in a common housing.
We characterized every sensor on a purpose-built test stand, sweeping angle and distance and reading amplitude on an oscilloscope. The data caught a badly seated lens in one sensor — reseating it roughly tripled long-range performance.
Test-stand data for one sensor before and after reseating its lens.
The analog chain — 30× pre-amplification, a fourth-order Chebyshev bandpass centered at 2 kHz with a 200 Hz passband, post-gain, peak detector, and a comparator with hysteresis — was designed in KiCad and built on perfboard.
The full per-channel signal path: pre-amp, bandpass filter, post-gain, peak detection, and comparator.
The boards as built. A counter IC drives the mux select lines, so eight channels sample through one filter using three digital pins.
With seven active sensors spaced 7.33° apart (44° total field of view), the assembled array detected the beacon from under 1 ft out to 16 ft with no dead zones between channels.
Turret and shooter
The cannon and detector ride on a stepper-driven turret — a TMC2209 driver salvaged from a 3D printer, a laser-cut plywood gear reduction for holding torque, and a lazy Susan bearing gluing the whole rotating mass to the chassis with low friction. Decoupling aiming from driving means the robot shoots accurately regardless of chassis orientation.
CAD of the turret rotating subassembly: stepper pinion, plywood ring gear, and lazy Susan bearing.
The shooter is two flywheel assemblies clamped onto a PVC barrel whose inner diameter happens to fit a ping-pong ball snugly. A servo-driven rack-and-pinion trigger feeds balls in, with ball bearings salvaged from a drill gearbox pressed into the rack to kill slop and friction.
Software, briefly
A hierarchical state machine on the Uno32 ties everything together: a scan phase finds the opponent and fixes the robot’s orientation, then it follows the middle tape to the shooting zone — handling bump recovery and tape corrections as nested sub-states — before entering the shooting routine. Because aiming runs independently of the drivetrain, a bumped chassis never throws off a shot.
The one place the firmware got fussy was the stepper. Step pulses run off a dedicated timer interrupt at tenth-of-a-degree resolution, since the framework’s 1 kHz tick would have capped step rate and added visible jitter. Firing is event-driven for the same reason it matters mechanically: the flywheels spin up the instant any detector first sees the beacon, so the shot is ready by the time the turret finishes centering.
Budget
The whole robot came in at $122.36 against a $150 cap — laser-cut plywood structure, salvaged stepper and driver, and a free IKEA drill that donated its gearbox bearings.
The full report is still being finished; competition results and the complete writeup will land here when it’s done.