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Topic: Repurpose 4Ohm 5W Speaker (Read 1 time) previous topic - next topic

swerve8

Hey guys,

I was hoping to get some help with a small project I am working on.   I managed to salvage a speaker from a broken portable bluetooth speaker unit. (See picture below).

I have 2 questions:

1) Can I use a 4ohm 5W speaker with an arduino?

2) Why does this speaker have 4 wires? (Shouldn't it just be 2?)


dwightthinker

I'm not sure.
Some speakers have coaxial elements that you might want to
drive separately?
It is possible that one is used for feedback of distortion?
Maybe it is intended to be driven with 2 amplifier outputs
in parallel?
You still need a DC blocking capacitor and a current limiting
resistor if driving directly from I/O.
Dwight


swerve8

Dwight, thanks for the feedback. All 4 speaker wires were soldered onto the bluetooth micro controller.  It looks to the best of my knowledge that they are just 2 seperate + and - terminals.  I'd like to try to drive the speaker from the arduino unit utilizing one side just to see what happens...

What I need help with is figuring out how to properly drive a speaker with 4ohm 5W power reqs...  Unfortunately I have no good understanding of what is required when you say "You still need a DC blocking capacitor and a current limiting resistor if driving directly from I/O"  

My assumptions are naive at best but it would be that I can provide up to 5.5v from I/O (say pin # 9 for example) but I have no clue how that translates into ohms and watts.  Any additional help would be greatly appreciated.

CrossRoads

Absolute Max from an IO pin is 40mA, and the pin will not have a long life if driving it that hard.
Using Ohms Law, V=IR, if you have 5V and 40mA, then you must have 5/.04 = 125 ohm of resistance for a load.
So take your 4 ohm speaker and put  a big resistor in series with it.
Above 20mA, the pin will drop to 4.2V and lower as the current goes up.
Power P = I*V, so .04 x 5V = 200mW max volume, so not very loud.
If you want 5W, you're going to need an amplifier.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

dwightthinker

Although, with only 20 ma flowing through it, a DC current
in a speaker causes the cone to be displaced from center
position.
An electrolytic capacitor may be best to put in series
to avoid the DC offset.
100uf capacitor would remove the DC offset.
The output of the Arduino is a square wave. It will
be high in harmonics. These will be clear in the speaker sound.
Dwight

Paul__B

Hey, if you want information, you post a picture - with perfect focus, taken in outside daylight so we can see what it is.

jurs

#6
Feb 01, 2016, 01:37 pm Last Edit: Feb 01, 2016, 01:43 pm by jurs
2) Why does this speaker have 4 wires? (Shouldn't it just be 2?)
Do you have a multimeter and can measure the resistance of the two speaker connections?

Most likely it is a two-in-one speaker which is actually consisting of two speakers:
- a tweeter speaker for playing high pitch tones
- a woofer speaker for paying bass tones

And your speaker does not have a cross-over unit built in, so that it cannot seperate the frequencies and therefore needs different inputs for low pitch and high pitch signal input.

If you have a multimeter: Which is the resistance between the cables for one and the other speaker input?

In case that one of the inputs has a much higher resistance than the other, this is most likely for a piezo tweeter and you can use that at least directly with your Arduino to play high pitch tones fairly well (but bass playback for frequencies below 400 Hz will be very bad).

swerve8

Absolute Max from an IO pin is 40mA, and the pin will not have a long life if driving it that hard.
Using Ohms Law, V=IR, if you have 5V and 40mA, then you must have 5/.04 = 125 ohm of resistance for a load.
So take your 4 ohm speaker and put  a big resistor in series with it.
Above 20mA, the pin will drop to 4.2V and lower as the current goes up.
Power P = I*V, so .04 x 5V = 200mW max volume, so not very loud.
If you want 5W, you're going to need an amplifier.

CrossRoads, Thanks for that great answer with enough explanation for layman such as myself.

Do you have a multimeter and can measure the resistance of the two speaker connections?

Most likely it is a two-in-one speaker which is actually consisting of two speakers:
- a tweeter speaker for playing high pitch tones
- a woofer speaker for paying bass tones

Jurs, I didnt realize they made speaker units consisting of 2 speakers (tweeter/woofer) but if thats the case then this makes the most sense as to why it would have 2 inputs.  I have a multimeter and will try to measure the resistance.

For my project I was planning on using the speaker as an alarm so I the tweeter may be exactly what I would need to get a clear crisp alarm.  Thanks for the reply.

russellz

I have a multimeter and will try to measure the resistance.
Just bear in mind that just because a speaker is rated as 4 ohm doesn't mean that that is what you'lle measure with a meter.

The rated impedance is usually quoted as the lowest impedance measured at any frequency above the resonant frequency.  If you measure at dc as with a multimeter you will usually get a resistance of about half the rated impedance.

Russell.
Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.

swerve8

Just bear in mind that just because a speaker is rated as 4 ohm doesn't mean that that is what you'lle measure with a meter.

The rated impedance is usually quoted as the lowest impedance measured at any frequency above the resonant frequency.  If you measure at dc as with a multimeter you will usually get a resistance of about half the rated impedance.

Russell.
russellz, Thanks for this tip as well.  I didn't realize this, and would possibly have caused some confusion for me. 

Paul__B

We're waiting ...

on reply #5.

Zapro

I have come across that kind of speaker. The Bluetooth module has a stereo amplifier and the speaker has a dual voice coil. Both are full-range. You can connect them in series so it's easier to drive if you want.

// Per,

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