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In my class today, I hooked a 555 in astable mode to an o'scope.
Before powering up the scope, I centered the trace. After powering
up the 555 I noticed the output from pin 3 was going from +v to -V (I
didn't check the voltage scale, so I don't know how large V is). I
don't understand why that happened. I should have gone from 0V
to +V.

Am I missing something?

Paul

Just Catching up on my old email reading so someone
may have answered already ....


Yeah ... what happening is the the oscilloscope is
probably set to "AC" coupled instead of "DC" coupled
input. It's a switch... usually near the vertical
volts per division switch.

In "AC" mode, the scope tries to remove the DC
component out of the waveform. And because the 555
output is from 0 to Vcc, the average (DC component) is
1/2 of Vcc.

Hope this helps!!

Curtis




--- verhap@... wrote:

In my class today, I hooked a 555 in astable mode to
an o'scope.
Before powering up the scope, I centered the trace.
After powering
up the 555 I noticed the output from pin 3 was going
from +v to -V (I
didn't check the voltage scale, so I don't know how
large V is). I
don't understand why that happened. I should have
gone from 0V
to +V.

Am I missing something?

Paul

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verhap,

In my class today, I hooked a 555 in astable mode to an o'scope.
Before powering up the scope, I centered the trace. After powering
up the 555 I noticed the output from pin 3 was going from +v to -V (I
didn't check the voltage scale, so I don't know how large V is). I
don't understand why that happened. I should have gone from 0V
to +V.

Did you have the scope in DC mode. If it was in AC mode the input capacitor
might DC shift the voltage. That's. what the capacitor does, it removes the
DC component and 'averages' the output. It's also a good idea to pay close
attention to the voltage settings, you can learn a lot from them. Before
connecting the signal position the trace on a convenient gratical (grid
line). In the DC mode the output should only rise in the positive direction
from that zero base line. In the AC mode a square wave should swing
half above and half below that line.

Jim