detectordigital-logicedge
I have a signal that changes from high state to low every few minutes, after changing state it will remain constant, all level changes are clean.
I'm looking for the smallest possible circuit that can detect a rising edge and falling edge (could be two different circuits) and will output a pulse when the the edge is detected.
Ideally, I'm looking for a single circuit that will output a positive pulse when one type of edge is detected and a negative pulse when the other type is detected.
You have unfortuantely run into a relatively subtle problem with this particular flip-flop. (BTW, this is not metastability; that's a different problem.) It's designed to operate at high speed over a wide range of supply voltages, and one of the compromises made in its design is that it has a rather strict requirement on the clock input transition speed.
If you look at section 9 of the datasheet, the input transition rate is given as 10 ns/V maximum. This means that you need to make the clock rise or fall by 5 volts in no more than 50 ns in order for the chip to operate correctly. With an RC time constant of 10 ms, you are about 6 orders of magnitude too slow.
You may implement this digital design for detecting rising edge.
simulate this circuit – Schematic created using CircuitLab
The output will go high as soon as a rising edge is detected on the D input. The output is cleared on the next rising clock edge.
Best Answer
A CD4098 dual one-shot will meet all your stated requirements and is very cheap and available. It can operate from 9V, trigger on either the rising or falling edge, and provides positive or negative going pulses. An R-C pair sets the pulse duration for each half of the chip.