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A pulldown resistor seems to always be used for configurations where when the switch is open the normal state is low. For example enter image description here

The reasons always given are to avoid a short between voltage and ground when the circuit is closed, and also to avoid the input floating when the switch is open.

I understand the first point about the short but why would the input still be floating if we did in fact short it to ground and didn't use a pulldown resistor? Can someone explain? Wouldn't the noise / random charges have a path of least resistance to go into just the same as with a pulldown resistor?

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  • \$\begingroup\$ " but why would the input still be floating if we did in fact short it to ground and didn't use a pulldown resistor..." If you short the input to ground then it is, by definition, no longer floating. \$\endgroup\$
    – SteveSh
    Nov 20, 2022 at 13:46
  • \$\begingroup\$ That's what I thought, intuitively, which is why all the stuff I've read online have confused me. Therefore am I correct in surmising that the only reason for the pulldown is to prevent a short between ground and source? \$\endgroup\$
    – Daniel
    Nov 20, 2022 at 13:49
  • \$\begingroup\$ Do you really mean TTL, as you mention MCUs and surely they are CMOS these days, not TTL? \$\endgroup\$
    – Justme
    Nov 20, 2022 at 14:55
  • \$\begingroup\$ Either, I suppose. I'm just trying to make sense of pull-up abd pull-down resistors for starters \$\endgroup\$
    – Daniel
    Nov 20, 2022 at 15:05

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If you had a 1000 Ω pull-down resistor (for example), when the switch closes, the current from 5 volts to ground would be 5 mA. If you used a 1 Ω resistor, the current would be 5 amps. Either will work for the MCU but it would be ridiculous to pull 5 amps just to send a signal to an MCU. A short circuit can also be regarded as a pull-down resistor.

Wouldn't the noise / random charges have a path of least resistance

I'm picking you up on this sentence because it's a common mistake to assume that all current or charge flows through the path of least resistance. Current distributes across a range of parallel paths with the highest current density being the path of least resistance.

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