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555 output not what I expected

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ampedtech

New Member
I was expected a square wave of just under 12v all above zero volts.


Why is output not square?

Why does output go below zero? (zero is at aprox the "1" on the left side of the scope pic)

I will indicate that I did not follow high frequency tactics (wires not shortened on bread board)

Your wisdom is welcome.
 

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Les Jones

Well-Known Member
Most Helpful Member
I think you have the scope set to AC coupled mode. The square wave is being differentiated.

Les.
 

JimB

Super Moderator
Most Helpful Member
I agree with Les, ensure that the scope is set for DC coupling.

Also, you are looking at the voltage on the LED according to your diagram, connect the scope probe probe to pin 3 of the 555.
Are you using a x10 divider probe on the scope? Is the probe correctly compensated?

JimB
 

cowboybob

Well-Known Member
Most Helpful Member
I think you have the scope set to AC coupled mode. The square wave is being differentiated.

Les.
As Les notes.

Switch the scope vertical input to DC coupled.

<EDIT> oops. Ditto JimB. Took me too long to sim the (interesting) little ckt...
 

AnalogKid

Well-Known Member
Most Helpful Member
Disagree. The low frequency corner of a scope in AC mode usually is Hz, not cHz. Looks like 1 time constant is approx. 200 us, for a corner frequency of 800 Hz. I suspect that the wired circuit does not match the schematic.

ak
 

ampedtech

New Member
I think you have the scope set to AC coupled mode. The square wave is being differentiated.

Les.
Thanks for your response Les, but that ain't it, got my probe on DC couple.

I agree with Les, ensure that the scope is set for DC coupling.

Also, you are looking at the voltage on the LED according to your diagram, connect the scope probe probe to pin 3 of the 555.
Are you using a x10 divider probe on the scope? Is the probe correctly compensated?

JimB
I appreciate you jumpin' in too JimB, but no cigar just yet, probe is 1x, my level of knowledge tells me that if the square wave (5v 1khz) coming out of the front scope looks crisp then the probe is properly compensated. I tossed in my knowledge level cause it's been a LONG time since I played with this stuff.

I did move the probe to the other side if the resister to pin 3. (LONG time) thanks


Disagree. The low frequency corner of a scope in AC mode usually is Hz, not cHz. Looks like 1 time constant is approx. 200 us, for a corner frequency of 800 Hz. I suspect that the wired circuit does not match the schematic.

ak
AK your suspicions will be investigated.
 

ronsimpson

Well-Known Member
Most Helpful Member
Many scopes have a "test point" on the front. Most modern scopes have a output of 1 or 2 volts p-p at 1khz. Please measure your scope's "calibration point" and see if it looks good. It might have the same problem.
 

ChrisP58

Well-Known Member
I agree with ak. Though a different model, my Tek scope shows a little sine wave symbol in the channel indicator area if that channel is set to AC coupling.

But the signal definitely appears to be AC coupled. So I would look for some connection that isn't as DC coupled as you think it is. And that could even be the ground connection.

As for trouble shooting, the first thing that I would do is look at the power supply rail with the scope. When you probe pin 8, does the trace jump up to +12 and stay there?
 

ampedtech

New Member
I agree with ak. Though a different model, my Tek scope shows a little sine wave symbol in the channel indicator area if that channel is set to AC coupling.

But the signal definitely appears to be AC coupled. So I would look for some connection that isn't as DC coupled as you think it is. And that could even be the ground connection.

As for trouble shooting, the first thing that I would do is look at the power supply rail with the scope. When you probe pin 8, does the trace jump up to +12 and stay there?

Winner Winner Winner

"...isn't as DC coupled...could even be the ground connection...power supply rail with the scope"


thanks ChrisP58 for pointing me toward the path of the righteous square wave. thanks 2 all!


I'm sure I will need to plead another case soon!
 
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