How do you tell the difference between speakers and microphones?
I pulled the following two components out of an old cell phone and cannot tell them apart.
Each have two metal contacts on the back.
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The speaker is the yellow device on the right-hand side, the microphone is the device on the left, with blue plastic/metal visible.
The speaker will almost inevitably be larger then the microphone. Also, you can actually see the voice-coil in the speaker - it is the copper colored oval.
@markrages - is somewhat correct about measuring impedance, but not correct about the internals. The speaker should read as a fairly low impedance - less than 100 ohms.
However, in all likelyhood, the microphone is a Electret Microphone, which uses a capacitor which varies in capacitance in response to air pressure waves, e.g. sound.
Getting a meaningful signal out of an electret microphone is pretty simple, but they do not produce an output without being biased with a power supply. Basically, they need a little bit of power to run, and you have to supply them with that.
Fortunately, there are lots of schematics freely available, and they're generally pretty simple.