Double-clap to toggle the light. Costs between 6 and 30 EUR to build depending on how you count and what's already lying around.
Using ATTINY85 which I love for it's minuscule power consumption. For this build, I couldn't even measure it with a USB power meter since it's just 1mAh. This particular LED strip consumes 0.85Ah from power supply and 0.63Ah in my circuit. The difference is likely due to imperfect soldering and thin wires.
You could use a P-channel MOSFET and interrupt the positive LED line. See the parts list.
The code mostly works but triggers e.g. when someone talks in the room :-D Please post your improved version if you got one.
Since custom PCBs these days are dirt cheap, it would be awesome to design one for this project as well and fit this whole thing into AA battery sized footprint with USB and micro-USB ports on opposite sides.