Left side is what I usually find on Google. Lack of flyback diode scares me. Right side with diode + audio transformer to get better sine wave out of digital waves from Arduino PWM pin.
Perhaps the easiest way is to use a small class-D amp module to drive the speaker, such as a PAM8302.
This will be much more efficient than your circuit as power is drawn from the supply only in proportion
to the signal fed in.
The transformer is not an intrinsically inductive load, BTW, transformers reflect back the load impedance
to the primary, scaled by the square of the turns ratio. Speakers are largely resistive loads (except
at resonance). So your free-wheel diode is basically unnecessary - R4 is strongly snubbing the
Using a transformer is costly, transistor circuits can drive speakers directly, as they can handle low
impedances, you can simply reduce the value of R4 and drive the speaker direct (but through a DC
blocking cap of around 1000µF).
But as I said earlier this in inefficient as lots of power is wasted in R4.
Red on Black blackgroud in a small picture, just about impossible to read. Can you adjust it for other colors with white background?
I just did right-click > view image, to get the original sized image which is legible.
There are lots of tiny medium power class D IC PA modules around. Crazy cheap too.
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