There are several controller boards out there (I'm most familiar with ones that control stepper motors.)
I'm assuming that most controller boards that have some kind of CPU have some filtering on the DC Power input side
so that when a heavy load like a laser pulsing or a heatbed cycles, the control board is somewhat protected from "noise".
Individual pololu driver carrier boards like the ones used on the Azteeg X1 or Barts "open-source-4-axis-stepper-driver"
have built in filtering for VMM (a 4.7 uF and a 0.1 uF capacitor) and for VDD (a 0.22 uF capacitor).
See
http://www.pololu.com/catalog/product/1182 for example, the filters are connected to JP1 & JP2.
Now you're talking about a schmitt trigger (used in a laser application?)
I would assume the chip/board you're using should have some filtering examples on the Schematic, data sheet or application note.
If you're rolling your own, then the filters connected to JP1 and JP2 would be a good place to start.
BTW, electrical noise can come from AC powered stuff as well,
air conditioners, drill press, and even the CFL bulbs can output electrical noise.
That kind of noise should be handled at the AC input end of your AC-DC power supply,
but you might want to check that as well.