Do I need to use a resistor for a small speaker?

I'm using a small speaker for a tone project. Right now, I'm not using any resistors for it. But, in the back of my head I have this tiny voice that is telling me I should be using a resistor. Any help on what size I should use, or if I even need to use one? The speaker is one I took out of an old speaker phone. Thanks for any help!

On the basis that the arduino can happily supply around 25ma at 5 volts, the output power is around 100mW. This can be heard on a small speaker OK but certainly won't blow your ears. The speaker circuit impedance should be around 200 ohms (5000/25). Most small speakers are about 8 ohms so you are going to need a series resistor of around 192 ohms (let's say 180) since the speaker will now only "see" 8/180 of the 200mW, ie around 8mW, you'll be lucky to hear anything.

Better to use an op-amp to act as an amplifier between the arduino and the speaker, as well as permitting the speaker to be connected directly to the amplifier (without a series resistor).

jack

Another possibility is using a small audio transformer to convert impedance. Always have a series capacitor to eliminate DC-bias in the speaker or transformer - this is important.

MarkT: Another possibility is using a small audio transformer to convert impedance. Always have a series capacitor to eliminate DC-bias in the speaker or transformer - this is important.

Hey that's a good idea, MarkT. Any pointers to a good online transformer?? I will look with my China contacts.

So. Hmmm.. 5.0 Volts at .025 amps R=E/I so 5/.025 = 200 ohms, yeah.. Maybe 150 to 250 ohms into 8 ohms or whatever speaker impedance is..

The capacitor should probably be 10 to 50 uF electrolytic, + to Arduino.... Hmmm.

A typical opamp usually will put out only about 20 ma as I recall?? Maybe a good choice out there; the 386 is widely used audio amp but not as simple..

Anyone got something simple working well??

The capacitor should probably be 10 to 50 uF electrolytic

While this is what you need remember that a discharged capacitor look like a short circuit so make sure that you don’t exceed the 40mA limit on the current peaks when charging the capacitor.

Thanks everyone! I'm not looking to make anything real loud... Just making a little chaos box that randomizes pitches and plays them based on a pot value. There's some cool stuff here to think about in case I do want to make some REAL NOISE in the future! I actually have an OP amp i grabbed for the heck of it one day, though I have no idea how to connect it. More and more reading to do every day! Again, thanks a ton. I greatly appreciate the help.

While this is what you need remember that a discharged capacitor look like a short circuit so make sure that you don't exceed the 40mA limit on the current peaks when charging the capacitor.

A capacitor in series with a 200ohm transformer looks like 200ohm at worst...