user11772: 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 ?

user6740: 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.

user3917: 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.

11772: <p>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 ?</p>
 6740: <p>I googled "measure speaker resistance" and found <a href="http://www.wikihow.com/Measure-Speaker-Impedance" rel="nofollow">http://www.wikihow.com/Measure-Speaker-Impedance</a></p>

<p>Step by step instructions</p>

<p>You will need a multimeter - a great thing to have in any case.</p>
 3917: <p>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. </p>

<p>As pointed out in the <a href="http://www.wikihow.com/Measure-Speaker-Impedance" rel="nofollow">Measure Speaker Impedance</a> 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.</p>

<p>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.</p>

<p>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  <a href="https://www.google.com/search?q=clipping+diodes&amp;biw=1112&amp;bih=593&amp;tbm=isch&amp;tbo=u&amp;source=univ&amp;sa=X&amp;ved=0CCwQsARqFQoTCNXa8qC6yMcCFckziAodpccJmw" rel="nofollow">clipping diodes</a> or <a href="https://www.google.com/search?q=clipping+diodes&amp;biw=1112&amp;bih=593&amp;tbm=isch&amp;tbo=u&amp;source=univ&amp;sa=X&amp;ved=0CCwQsARqFQoTCNXa8qC6yMcCFckziAodpccJmw#tbm=isch&amp;q=flyback+diodes" rel="nofollow">flyback diodes</a> in the circuit to prevent damage from overvoltages.</p>
