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Hello!

I have a simple question concerning this circuit I have built: 555 VCO Circuit

It is a 555 Voltage Controlled Oscillator I have built as a foundation for my signal generator circuit. The circuit works great, changing the frequency as I turn the pot – but I am confused about the speaker connection.

I have one cheap speaker I use to test my circuits (3W, 8Ohm) that (obviously) has two input wires, + and -. I tried connecting the + wire to the pin no.3 (OUT on the schematic) and – wire to ground, but there was no sound.

By trial and error, I figured out the speaker is only working when I connect one wire (I know they’re interchangeable) to the OUT (no.3) pin and the other to VCC, which is +9V to ground in my case.

I am really confused by this, since I thought DC will damage the speaker, but in my case it’s working just fine. Can anyone please explain why is the speaker working with the circuit like this? The only thing that came to my mind is that the output is inverted somehow. I am sure it’s trivial, but I just can’t get my head around it.

Thanks for enlightening me!