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
