Hello. I'm a college student majoring in EE, and unfortunately, so far I've never worked with sensors of any kind, really, aside from the mic as an acoustic sensor. I'm wishing to take a toy piano and modify it for velocity-sensing (possibly via some kind of light sensors) and for MIDI functionality. However, first and foremost I would like to know just what kind of sensors are there? I hear names like "light sensors", "IR sensors" but I've never really seen them or used them yet. I'd like to look them up, see their pictures for sizes and read their datasheets for information. Could someone please give me a decent list? Are there IC-packaged sensors? Could I get some names of them too?

 

Interesting idea.

Here's some info on CdS cells:
http://www.ladyada.net/learn/sensors/cds.html
CdS cells are rather slow; you wouldn't want to use them for sensing velocity - however, they are usually readily available and inexpensive.
Photodiodes are light-sensitive and fast, but their output is quite small.
Phototransistors are also light sensitive and fast, and provide a much higher output than photodiodes.
You can actually generate a small voltage output from an LED by exposing it to light.
There are IR detectors available which are primarily intended for the reception of remote control commands. These detectors usually require a signal to be on a carrier in the frequency range of 32kHz-42kHz; this is to nullify the effect of ambient light.
There are combination IR emitter/detectors that sense when an object is blocking the path between them. These are very frequently used in computer mice that have the ball underneath.

There are also sensors that don't involve light, such as Hall-effect sensors; they detect the proximity of a magnetic field, or ferrous metals.