# What do Capacitors do in a System with a Speaker?

I made a circuit based on a diagram from my physics teacher. The circuit has a 555 chip connected to seven wires that make up a circuit. The circuit includes a photocell resistor that acts as a variable resistor. My question is why are capacitors needed to make the speaker work? I'm sorry if this question seems to be a duplicate of What is the job of the capacitor in a 555 circuit?, but the answers on that question don't explain how capacitors help the speaker work.

• @Dzarda: Is there anywhere I can draw a diagram of the circuit, or should I take a picture? – Jason Chen May 10 '14 at 22:24
• Feel free to edit your question - Ctrl+M then brings you to the editor. – Dzarda May 10 '14 at 22:33
• @Dzarda: here's your schematic. Hand-drawn. – Jason Chen May 10 '14 at 23:00

The capacitor in series with the speaker is to block DC.

The 555 output can only swing between the 6 V power and ground, so has a net DC bias of 3 V on average. This DC does nothing to make sound come out of the speaker, but limits how much the speaker can deflect in one direction and would draw significant current from the 555 timer if the speaker was connected directly. The DC resistance of a speaker is quite low. The capacitor allows only the AC component of the 555 output voltage to cause any current thru the speaker. You can only hear the AC component from a few 10s of Hz upwards anyway.

The short answer is that capacitors aren't needed for speakers to work. It's necessary for a 555 astable timer circuit though.

Capacitors have many functions in speaker circuits tho. For the 555 timer, they allow a timing to happen by charging and discharging it at a certain rate. For capacitors inline with the amplification or signal paths for the speaker, they can remove DC offset.

You could make a speaker work by using a digital ring oscillator and outputting the signal to it just as a counter-example to your assumption.

• So the 555 timer uses capacitors to discharge bursts of electricity to cause the speaker to vibrate at a certain rate? – Jason Chen May 10 '14 at 22:50
• No. The capacitor in series with the speaker removes the DC offset from the 555 output. The output of the 555 will switch between 0 and 5 volts. Without the capacitor, this will cause the speaker cone to move only one direction from its rest position - this is not good for the speaker. The series capacitor will charge to the average output voltage of the 555, so the speaker will see both positive and negative voltages, and will move equally in both directions from its rest position. – Peter Bennett May 10 '14 at 23:20
• @PeterBennett Yeah, he didn't have the circuit up at the time... I mentioned that in the second paragraph anyways: "remove DC offset". – horta May 10 '14 at 23:39
• @JasonChen Yes, that's the function of the capacitor on the left in your diagram. – horta May 10 '14 at 23:42