- #36
petterg
- 162
- 7
jim hardy said:Edge triggered in my day meant it triggers on either a rising or a falling edge but not both.
Consider a negative going pulse.
Signal starts at zero, transitions negative to a non-zero value and holds a while, then transitions positive back to zero.
Just after its positive transition it has value zero. So if we look at it then and find zero, we know it was a negative pulse. We can ignore its negative transition
How do you make it ignore?
The differentiator show the rising side of the (positive) pulse as a negative value, the following falling side as a negative value. Hence the result has a falling edge, a rising edge, and another falling edge for each pulse. An edge triggered device after the differentiator should only trigger at one of those edges (doesn't matter which one). How can it be configured to only trigger on one edge?
I was thinking of two 555-based monostables, holding unstable state for 50us, one having inverted input. And connect output from each to the reset on the other. First edge would then trigger one of the 555's, which pulls the reset on the other 555. The second edge would trigger the second 555, but it will be ignored because the reset is held. Third edge would then bring the trigger signal back to 0 and not make any change as the first 555 had not timed out yet. Then the first 555 would time out and it's ready for next pulse in any direction... it all sounded so well, so I tried with the attached schematic. Output from U4 and U5 would trigger each of the 555's.
To make it work with small signals I had to include U3 to amplify the input.
So the circuit is:
U1: input high impedance buffer
U3: input amplifier
U2: differentiator
U4/U5: (inverting) amplifier (555 input will require at least 800mV from vGND to trigger.)
This seemed to give a reasonable signal to the 555's, when input signal was weak (the lower graphic). However, when the input was stronger, clipping occurred. The clipping itself doesn't matter, but the 3rd edge is so late that the reset is released before the signal gets below trigger level of the 555's.
If I increase the 555 timeout to more than 50% of the cycle, I see a risk that it will trigger on 2nd edge, and ignore 1st edge on the next cycle.