Buying Ready-To-Use DIY Simple Electronic Project

I need a straightforward, fully-functional DIY project that I can assemble and demonstrate without extra troubleshooting. Any of the three domains—electronics, electrical, or instrumentation—are acceptable, and I am equally open to Arduino, Raspberry Pi, basic discrete circuits, motor-control rigs, power-supply modules, or lighting systems. Simplicity is key; the design should already be proven to work and require only routine tools to bring to life.

What I expect from you:
• A concise write-up that explains the underlying concept and its practical use.
• Complete schematic or wiring diagram in a common format (PDF, PNG, or Eagle/KiCad file).
• Full bill of materials with part numbers that can be sourced locally or online.
• Firmware or example code, if the build involves a microcontroller or single-board computer.
• Step-by-step assembly and testing instructions that let me reproduce your results on the first attempt.
• Photos or a short clip of the functioning prototype so I can see it works before purchasing.

If your design meets these points and can be handed off immediately, send a brief overview along with confirmation that all files and documentation are ready.