555 or FPGA and speakers

[mbird]

I was experimenting with getting sound from my FPGA board or from a 555 Timer (so I create a digital circuit that can send out square wave at various frequencies).

The thing I don't understand is this:
My square waves are all zero to 3.3 volts (they are all positive). I run them through a simple NPN transistor to get some amplification (my 555 is 10mA source max) then to the speaker. So is it correct to say then that I am only using half the speaker's ability (so should a speaker be driven from -x to +x rather than 0 to x -- or does it not matter) -- and if it does go -x to +x then how do I go from a 555 that is 0-x to use the full range of the speaker.

Can I drive the speaker directly from the 555 or FPGA or do I need a capacitor in between.

As you can see I am just trying to learn about how to get digital sound to a simple speaker so any links that would help would be greatly appreciated!

Thank you!

 

[Hero999]

It depends on what you want to do.

An FPGA is overkill for just a squarewave.

A 555 is perfect.

A TTL 555 can drive loads up to 200mA.

A CMOS 555 can drive loads up to 100mA.

A NE555 can easily power an 8Ω speaker, depending on the supply voltage you might even need a resistor to limit the current.

You can use a capacitor to couple the speaker to the 555 which can also be used to limit the current to the speaker.

 

[duffy]

So is it correct to say then that I am only using half the speaker's ability

Yes, if it's all positive like that the speaker isn't going the other way. If you put a cap in there it will travel in both directions and might sound louder. If you use a 4Ω speaker it will be louder.