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I want to build a simple edge detector that triggers an interrupt once the power adapter is hot-plugged. Therefor IN will be around 18V and on the output I need 1V8 logic.

The generated pulse does not need to be accurate but should be around 50-100ms long.

I read that a simple RC-circuit can archive this and from a bit of experimenting in circitlab I found following values to work. However two questions:

  1. The BUF signal becomes negative on falling edges. Will this be a problem for the buffer IC? And how can I avoid this?
  2. What do I have to look for when selecting a part for the NOT gate. I read about schmitt-triggers, do I needs this? E.g. will something like SN74LVC2G14 work?

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  • \$\begingroup\$ There are ready power-on reset ICs, like the MIC803. There are different voltage ICs. Perhaps this is exactly what you are looking for? \$\endgroup\$
    – sx107
    Oct 8, 2019 at 9:24

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  1. The BUF signal becomes negative on falling edges. Will this be a problem for the buffer IC? And how can I avoid this?

Probably not. Most IC's have clamp diodes that clamp voltages to the lower rail or upper rail. You may not exceed the clamping current (defined in Absolute Maximum Ratings). But with 100kΩ that will not be the case.
Otherwise, you should implement clamp (Schottky) diodes yourself to protect the input.

  1. What do I have to look for when selecting a part for the NOT gate. I read about schmitt-triggers, do I needs this? E.g. will something like SN74LVC2G14 work?

You would need a trigger point with hysteris if the signal would be very noise. I think that is not the case (when R1 and R2 are located close to the gate), so you can use normal gates.

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  • \$\begingroup\$ Just to be sure I would always use schmitt-triggers. There is no thing like a noise-free signal. And it's not alone the noise, a simple unbuffered inverter will change its output quite slowly if its input voltage changes slowly, see its data sheets and application notes, in example for oscillators. \$\endgroup\$
    – the busybee
    Oct 8, 2019 at 11:05
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    \$\begingroup\$ An inverter with a slowly moving input will often give a burst of high frequency oscillations. Use a Schmitt trigger. \$\endgroup\$
    – EinarA
    Oct 8, 2019 at 22:25

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