Special "dual" pulse generator circuit (follow up)

(A note to Chris Foley regarding the circuit he proposed on 10/10/04.)

I now have the circuit breadboarded and working pretty well. Since
we last talked I added another pair of 555 ICs between the 4017 and
the optoisolators so that I can control the output pulse width. This makes
my tester a little more versatile. I got that working with a little help
from
other people in the group (thanks, guys!).

As you might have noticed in related threads I started, I also had to
switch CLK and CLKEN from the way you had shown it, and ground RESET.
Now it works, and I can adjust both speed and pulse width.

The remaining question is regarding output pulse shape. If I connect a 10k
resistor between +12 and the optoisolator collector, and ground the emitter,
I can see the optoisolator switch from open to conducting. When it's open
the emitter is at +12 and when it conducts this voltage drops to 0 almost
instantaneously. However, when it opens again the rise back up to +12
is an exponential, taking about 2-3 milliseconds. Is this to be expected, or
am I overlooking something.

Ed

 

Subject: Special "dual" pulse generator circuit (follow up)

You might want to improve the risetime on the optotransistor by placing a 270K
resistor between the base and GND. That should improve things a bit. Also,
you might want to play around with the pullup resistor --a lower resistance
will improve the risetime. Try 4.7K and see if the collector output when on is
still saturated. If so, you will have cut your risetime in half right there.

Phototransistors are inherently slow, though.

Good luck
Chris

 

Thanks, Chris. I'll try a resistor connected to the base. From the data
sheet
I see that it's available at pin 6.

By "pullup resistor do you mean the one I have connected between +12
and the collector of the 3083?

Ed