16 Jan
2014
16 Jan
'14
2:31 p.m.
I don't
know wh yDEC drew that 555 the way they did, but it is not easy to
figure out what is going on. It's not obvious at first glance which line
is the +12V power -- or at least it took me a few seconds to figure it out.
Yeah. It's annoying for someone new to this stuff.
I wonder... If a smoothing capacitor had dreied up, there migth be
excessiveripple on one of the power lines. This might cause the output of
the comparator stage -- the trigger input of that 555 -- to be toggling
at twice line frequency. At which point the 555 would be contiulaly
retrigggered, but maybe occasionally it drops resrt for long enough for
the CPU to do something.
That would explain why the behaviour is sporadic and unpredictable.
Have you put a 'scope on the power liens to check for ripple? And again,
I would look at that trigger input of the 555.
Don't have a 'scope yet. The one I was going to get needed repaired.