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How to detect edges from unknown circuits

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Hi. I've tried to teach myself how to built projects, but sometimes I run amuck. I built a sort of test instrument to measure the time between pulses on unrelated circuit boards each with independent voltage supplies. (I cascade a 10Mhz crystal though 8 CMOS 4017 dec counters, and demux the signal I want according to an appropriate scale - (manually, not automatically)

I use toggles to set whether I want to start with a rising or falling edge, and another toggle to select the edge for the end of the timed period.

Sometimes the timer works fine and sometimes the display runs really wild - not just fast. I do not have a scope, but suspect that my triggering circuits are the problem. What is a reasonable approach to sense voltage changes/edges? I am only looking for digital shifts, not incremental/analog ones. If I use an op-amp, how do I set the sensitivity, since a 'low' on the input may still be higher than neutral? Is a Schmit trigger enough? Any help would be appreciated. Thank you.


Well-Known Member
Flip flops can detect edges, but the FF clock must be running faster than edge being tested.

Another thought. You should google for a good logic probe circuit that you can build.
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