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Many electronic input terminals are sensitive to stray voltages when they are floating (not connected to a voltage). Voltage signal based inputs don’t let current flow through them. Which means that an undesired voltage can easily build up on them from stray electromagnetic activity in the air.
A relatively high value pull up resistor can be used to connect the input to the positive supply in order to keep the output high instead of floating.
Or, a relatively high value (pull down) resistor can be connected between the output and negative supply voltage to keep the output low instead of floating.
A switch of some kind is generally also connected to the output, but heading to the opposite supply voltage. When you want that other voltage, you simply close the switch so that it is shorted to that supply voltage. The pull up or down resistor will pass some current, but that won’t affect the voltage that is output.
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