No firmware, no battery, no app. YouTuber ALTco built a palm-sized solar robot pet that wanders toward light using only analog BEAM circuits. Hackster covered the project on 5 Nov. The idea came after looking after frogs: he wanted a companion that mostly looks after itself, so the robot harvests energy from a solar cell and spends it in short motion bursts.
Miller solar engine and inverter drivers
BEAM (Biology, Electronics, Aesthetics, Mechanics) robotics favours simple analog behaviour over programmed state machines. Here two N20 gearmotors drive 3D-printed wheels with O-ring tyres. Stacked 74HC240 inverter chips act as low-quiescent-current motor drivers, which matters when every microamp comes from a small solar cell. A classic Miller solar engine charges a capacitor from the panel and dumps that energy into the motors once the voltage crosses a threshold, so the bot runs without a battery and only when there is enough light.
Two photodiodes act as eyes. Their signals feed a pair of coupled oscillators that set motor activity. In bright, even light the robot tends to crawl toward the source. In uneven light the oscillators drift in and out of phase, which produces the twitchy, pet-like wander that makes the build feel alive. The chassis splits into an electronics/solar top and a motor/sensor bottom with JST connectors and spring contacts so you can swap panels and tweak resistor and capacitor values.
Our take
This is a good teaching build for anyone who defaults to "put an MCU on it." Photodiode difference, oscillator phase and a capacitor dump already create seeking and chaos without a single line of code. If you want another modern take on the same tradition, see our earlier coverage of a solar-powered BEAM bot with minimal parts.
Image credit: ALTco / Hackster, cropped and resized.
About the author: Aswinth Raj is an electronics engineer and the editor of CircuitDigest, where he has published more than 150 hands-on projects and tutorials across Arduino, PIC, ESP32, Raspberry Pi and EV battery systems. He selects the stories on this page for what engineers and makers can actually learn or build from. Connect on LinkedIn
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