When the switch is open, instead of the input to be floating to an uncertain value, it becomes grounded, therefore LOW, through the resistor. We fix this by using a large resistor (say, 10kΩ or larger) to connect the input of the logic circuit to ground. So how do we fix it? Have a look at the next schematic: Let’s repeat: the problem here is that the input of the logic circuit is not connected to a source of defined value, and logic circuits really don’t like this! Another term people often use for this kind of situation is “floating”. The input of the logic circuit is not connected to anything with a specific value, it’s just hanging there. This looks fine, the switch is closed and the input of the logic circuit is directly connected to Vin, so it is HIGH. Schematically, you would have something like this: An example could be an ATMega micro-controller, like the one you find in an Arduino board. A logic circuit is a component that expects certain values only at its pins, usually HIGH or LOW, and it doesn’t really work well with anything else. One pin is connected to a 5V source, and the other to a logic circuit. When I think about this, it really is simple. I’ll try to answer this question here as simply as I can. What is the purpose of a pull-up or pull down resistor, and how does it work? A question I get asked frequently by people new to circuit design is about pull-up and pull down resistors.
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