Mike C said:
Ken: I'm not sure if "the pulses have a period less than or equal to
0.1 seconds" can be considered the same thing in this case... because
you could still have less than 10 pulses in a second even if they have
a .1 sec period.. but maybe i'm not understanding how it would work..
Imagine this: the AC line frequency is 60.0Hz or so. It varies a bit. Yet,
there are many household clocks which keep time precisely by this standard.
Why aren't the power companies screwing these all up? Because, although the
frequency itself may not be precisely regulated, the number of cycles per
day, let's say, *is* kept in check. Although the clocks may cycle faster
sometimes, they also cycle lower sometimes, keeping it accurate over long
average periods.
The same works here. You can tap out nine pulses really quick, but if no
tenth pulse comes before this counting second finishes, your counter is
going to say fail. The average pulses per second is still ten, but they
didn't come evenly. THIS is the question we are asking you.
If the pulse length is roughly equal for all ten pulses (i.e., the rate
doesn't change much during that second), then a missing pulse timer type
circuit will work very simply. It will not work if the pulses are
significantly displaced (as elaborated above), because that long empty space
between the first 9 pulses and the 10th will trigger the timer to send a
fail.
What is your signal source?
1. If and only if the pulse train coming in >=10Hz (10 "up" cycles
per second with no particular duty-cycle pattern)
Alright. Then please, don't call it hertz -- if you must, call it _average_
hertz. The reciprocal duration (i.e., Hz == 1/s) of each cycle varies, and
could be 2Hz or it could be 50Hz, so long as the average during that second
is 10Hz (10 pulses per 1 second, explicitly so).
Again, if it changes slowly (perhaps 9.7 to 10.3Hz), a missing-pulse
detector (set for perhaps 0.103 or 0.104 second trip time) will suffice for
reasonable accuracy. Which is another concern- how accurate must this be to
exactly 10 cycles? Are the cycles what count, or the time or frequency?
2. Otherwise, if the pulse train count goes below 10 cycles at any
point, turn off the transistor at once.
....And it'll take one second to reach this decision, correct? Obviously it
can't know if ten cycles have passed in that second unless they have.
(Arguably, you could have a circuit count the average *continuously*, with a
time constant of one second. Even a simple RC integrator would accomplish
this, roughly speaking. But this also reduces to a missing pulse detector.)
So the output should be HIGH only if 10 cycles per second (or higher)
are counted, otherwise output is LOW. Here is an example of the
input and its required output :
http://tiberian5.googlepages.com/
io.JPG
That looks like it is varying pretty widely, though the Hz input count makes
me feel like the input is some constant frequency during that count period.
Tim