Edge-detector

  
The 555 is a monostable that wants a negativegoing trigger. If the pulse you`re feeding it with is positive-going, you can run it through an inverter made up of either an inverting gate or, if you`re tight on space, a single transistor. Both ways are shown. The circuits shown in Fig. 25-9b are edge detectors as well, and are usually referred to as half monostables, since they can`t be used in every application.
Edge-detector - schematic

The width of the output pulse is determined by the RC value, but there are a few rules governing their use: The input pulse has to be wider than the output pulse. The input pulse can"t be glitchy. The circuit can"t be retriggered faster than the RC time.




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