Here's a quick log about a hack I built a couple days ago: Exploiting a SD card slot to power a LED light for my piano's sheet music tray!
There's nothing complicated about this hack and you can easily build it too with a few parts from your drawer. The key observation is that SD cards are powered by 3.3 V and can draw up to 100 mA. My piano doesn't have a host USB socket, so this is the only access to power on it.
There's one gotcha: The SD card slot is normally unpowered, presumably to save power (which doesn't really make sense for a keyboard without batteries). Luckily, after pressing the SD Card button, it shows "ERR: Format" on the display and leaves the slot powered until shutdown. So I not only have power for my light, but actually have a dedicated activation button as well!
For reference, here's the SD card pinout (source)
You can see that I cracked a micro SD card adapter, it's easily done with a knife. Then I removed the pins I didn't want to use as they'd only cause a short, and soldered my LED's wires. I glued the lid back on with some hot glue, careful to properly isolate the pins.
The used white LED has Vf=2.4 V and needs 20 mA. This yields R=(3.3-2.4)/0.02=45 Ω. I went with 47 Ω as the closest value I could find.
As a final note, no pianos were hurt in the process and the light works perfectly! I'd like the LED to have a bit wider angle, but it's good enough to see the notes.