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I have extracted a speaker frim a cheap radio. And I want to connect it to arduino uno. But, I dont know the rating of the speaker. And I am Afraid that If it is over rated, it can fry my arduino. The diameter of the speaker is 9 cm (approx). Whether this Speaker is more likely to be of 4 ohms, 8 ohms, or 16 ohms ??? I dont have a multimeter...What cam be the probable value of speaker ?

Kumar rajit
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2 Answers2

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I googled "measure speaker resistance" and found http://www.wikihow.com/Measure-Speaker-Impedance

Step by step instructions

You will need a multimeter - a great thing to have in any case.

AMADANON Inc.
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  • I dont have a speaker. What can be the probable value of The speaker. according to its Diameter (9 cm) ? – Kumar rajit Aug 27 '15 at 04:20
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    I have extracted a speaker from a cheap radio. ... I dont have a speaker. - Do you have a speaker or not? – Nick Gammon Aug 27 '15 at 05:57
  • Sorry that was mistyping. I have a Speaker but I dont have a Multimeter. What can be the probable value of The speaker. according to its Diameter (9 cm) ? – Kumar rajit Aug 27 '15 at 06:18
  • What difference will it be in your application if it is 4 or 8 Ohms? Do you really need to know? Just go ahead and test it with a series resistor. – Wirewrap Aug 27 '15 at 17:38
  • Get a multimeter. If you are doing anything electronics related, you will need one sooner or later. They're not expensive. Or find your local hackerspace/makerspace - they are usually happy to let people use their tools (take your speaker in). They can also teach you to use the tools. And they're great guys. I don't know where you live, so some of the above statements might not apply to your local hackerspace (but probably do). – AMADANON Inc. Aug 27 '15 at 21:00
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I'm not aware of any simple rule that relates speaker diameter to speaker impedance. Traditionally, 8 Ω is quite likely for typical cheap speakers, with 4 Ω not far behind in likelihood.

As pointed out in the Measure Speaker Impedance link that AMADANON gave, the DC resistance of the speaker's coil will be somewhat lower than the speaker's AC impedance. The link suggests 6 Ω DC resistance for a speaker with 8 Ω impedance.

As far as driving the speaker from an Arduino output pin, all of the likely values of speaker impedance are too low to allow driving it directly. Dividing the maximum allowed pin current of 40 mA into 5 V gives 125 Ω as the least resistance an Arduino output should ever see. When I attach a speaker to an Arduino output, I usually put a 200 to 400 Ω resistor in series with it. For my purposes (eg alert-buzzers) that gives enough volume without overstressing the Arduino.

If you still have the radio, look for a small transformer near the speaker connection. If the radio has an audio output transformer, it probably will have a few KΩ of resistance in the primary and a low-resistance secondary. If you attach the primary of that transformer to the Arduino pin you can deliver slightly more power to the speaker, but may need some clipping diodes or flyback diodes in the circuit to prevent damage from overvoltages.

James Waldby - jwpat7
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  • Indeed, and if you put 250 Ω in series, you are protected against a short circuit (as that only lets 20 mA through which is acceptable). Plus what jwpat7 said about a flyback diode. Alternatively drive the speaker from a small transistor which will probably make it louder and also protect the output pin. – Nick Gammon Aug 27 '15 at 05:59
  • What difference does it make anyway? If you are protecting the output pin with a resistor, whether the speaker is 4 Ω or 8 Ω isn't going to make much difference to your choice of current limiting resistor. FWIW I measured a speaker which claims to be 8 Ω and got 7.9 Ω on my meter. – Nick Gammon Aug 27 '15 at 06:03
  • @NickGammon, possibly I missed the context of your second comment. Anyhow, my phrase “all of the likely values of speaker impedance are too low to allow driving it directly” and the 200 Ω to 400 Ω range I mentioned are consistent with your “whether the speaker is 4 Ω or 8 Ω isn't going to make much difference to your choice of current limiting resistor”. – James Waldby - jwpat7 Aug 27 '15 at 06:18
  • Yes, I think we are in agreement. :) – Nick Gammon Aug 27 '15 at 09:01
  • To clarify, my comment was not aimed at you. It was aimed at the OP. Why does it matter what the impedance is? – Nick Gammon Aug 27 '15 at 09:16