Electronic – arduino – Potentiometer volume control for a piezo buzzer in bridge (2-pin) configuration

Your NPN power transistor seems to be connected up wrongly.

I'd expect something like this ...

^{simulate this circuit – Schematic created using CircuitLab}

where

• R1 limits base current and protects P1. \$R_1 = 5V - V_{BE(sat)} / I_B(20mA?)\$
• R2 limits LED current. \$R_2 = (18V - V_{LED} )/ I_{LED}\$
• Q1's collector is connected to the LED's cathode.

Perhaps your LED is actually an 18V LED module that has current-limiting built in?

Note: I believe piezo buzzers don't draw much current, you probably don't need a TIP31 to drive it. Check the specs and choose a more suitable transistor. To be connected as shown, the piezo buzzer should be rated for more than 18V.

A stereo headphone audio circuit has two low-impedance paths: from signal left to ground, and from signal right to ground. However, it also has a third high-impedance path from signal left to signal right.

Normally the parasitic third path does not affect the overall system very much, since signal left and signal right prefer to return on the much lower-impedance path to ground. But when you increase the impedance on ground, say by adding a potentiometer and subsequently turning it up, you cause the third path to become much more prominent. The exact effect from doing so varies, but in general it could be said that it "turns the audio to sh*t".

Using a double-gang potentiometer instead will also increase the impedance on the normal return paths, but the added impedance on the third path will be double that added to the normal paths, reducing its effect overall and maintaining audio fidelity.