555 timer output

[lawfreak]

I have a 555 (556 chip) circuit with variable output controlled by a pot.  5 volt power. 14 Hz to 11.9 kHz

When I probe at the output, I get a 0v-5v square wave.  When I "probe" with my headphones I get a soft pop (DC?) and the expected sound.  (Another pop when removing the headphones)

When I add a 0.1µF ceramic and probe post-capacitor I get -2.5v to +2.5v square wave.  When I "probe" with my headphones I get the expected sound and no pop.

I anticipated the cap would take away the pop (DC?), but didn't expect to see those voltages at the output.  Am I misusing the scope, or are negative voltages expected in this situation?

[obiwanjacobi]

Yeah, the pop is the DC. You typically remove that by using a cap - but the value you chose might be a little low so it has a higher resistance to lower frequencies, giving you a lower output voltage.

Your scope has DC- and AC-coupling. That means without (DC) or with (AC) cap - so that is already built into the scope.

Try a 100uF electrolyte (positive to the NEN555 output.

[SaintGimp]

If you're asking why you see a negative voltage after the capacitor, that's because it's doing exactly what it's supposed to be doing.

A capacitor doesn't pass DC current, as you know.  But you can get momentary movement of current on the downstream side of a capacitor as it charges or discharges.  As the capacitor charges up, it accumulates a significant negative charge on one of the plates.  That negative charge repels the electrons in the other plate so they leave the capacitor, creating a momentary flow of current.  But as soon as the capacitor finishes charging, everything settles down to steady state and you have no more current flowing downstream of the cap.

When the capacitor discharges, the negative charge on the upstream plate decreases, which allows electrons to return to the other plate, so you get a momentary flow of electrons *toward* the capacitor from downstream.  This is the negative voltage that you saw.

This is a desirable behavior to have for speakers because the neutral position for a speaker is right in the middle of its travel range.  You need a positive current to cause the membrane to move in one direction, and when you stop the current the membrane returns to the neutral position, but if you can provide a negative current then the membrane can also travel in the opposite direction so you get twice the travel distance and basically twice the work out of the speaker.

[ade]

Maybe another way to look at it... your original signal is an AC signal with a positive average value (2.5v, reflected as the DC component).

When you put the cap on, you are removing this DC component, so now you have the same AC signal but with a zero average value.  Hence the signal will be centered at 0, from -2.5v to +2.5v.

In this case, the signal is shifted to zero because the duty cycle is 50% so the average is exactly half the peak.  If you vary the square wave's duty cycle, the average value changes, so the amount of shift will also change.

[lawfreak]

Thanks for the explanations everyone.

Try a 100uF electrolyte (positive to the NEN555 output.

I tried a 10µF and 100µF, but didn't notice a difference on the output.  At least not a voltage difference.  I saw Dave's post on capacitor frequency response, so I understand what you're suggesting.  For these frequencies, I guess .1µF is enough...?

A capacitor doesn't pass DC current, as you know.  But you can get momentary movement of current on the downstream side of a capacitor as it charges or discharges.  As the capacitor charges up, it accumulates a significant negative charge on one of the plates.  That negative charge repels the electrons in the other plate so they leave the capacitor, creating a momentary flow of current.  But as soon as the capacitor finishes charging, everything settles down to steady state and you have no more current flowing downstream of the cap.

Thanks!

In this case, the signal is shifted to zero because the duty cycle is 50% so the average is exactly half the peak.  If you vary the square wave's duty cycle, the average value changes, so the amount of shift will also change.

Is this essentially how a switching power supply might produce a negative voltage?

[Zero999]

Yes, a negative charge pump is a type of SMPS which can create negative voltages.

What's the impedance of the headphones? There won't be any difference at higher frequencies but at lower frequencies it should.

[david-IT]

555 makes me pain when i am in colleague

[lawfreak]

What's the impedance of the headphones? There won't be any difference at higher frequencies but at lower frequencies it should.

I am testing with either an 8ohm (1/4 W rated) cheap speaker or 60ohm Sennheiser HD280s.  I actually didn't check for audible differences just probing with x10 probes.  I'll take another look at the situation.

Yes, a negative charge pump is a type of SMPS which can create negative voltages.

How could I take the AC out of this 555 circuit (with -5V to 0V) and end up with a usable DC current?

[Audioguru]

How could I take the AC out of this 555 circuit (with -5V to 0V) and end up with a usable DC current?

Look in Google for 555 charge pump circuit.

[Zero999]

Searching for 555 negative voltage generator yields better results:
https://www.google.co.uk/search?q=555+negative+voltage+generator&ie=utf-8&oe=utf-8&safe=active&gws_rd=cr&ei=lz3pVp_3A4H7aKmgjuAN