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How to detect edges from unknown circuits

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hlstahl

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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.
 
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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