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.
You can measure the impedance. Speakers generally have lower impedance than microphones. Otherwise they are really the same thing, a transformer between sound and electrical energy.