Resistor suggestion?

-

i've got circuit 5v pin providing power rail. hanging rail several switches leading digital pins on mega board. each switch have 10k pull down resistor, want know should other resistor be? here's basic diagram of i'm having:

code: [select]
5v ----
      |
    switch
      |
      |--10k--gnd
      |
     ?r
      |
digital pin

i'm still noob electronics , first full project. gather, there should resistor @ ?r stop closed switch being short circuit, true? if so, apart being less 10k, should , why? in, why 5k rather 200 or 8000?

it wouldn't strictly necessary because reading pin - in other words won't draw current. worried being set output, , low (which short 5v gnd) work out this:

the pin can comfortably sink 20 ma. have 5v there. using ohm's law need protect pin with:

code: [select]
5 / 0.020 = 250 ohms

so next standard value 270 ohms. isn't critical.


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