Pull down resistor.
The pull resistance reduces the input voltage to ground (logic zero value), thus preventing an indefinite state at the input. the pull resistance must have a resistance greater than the impedance of the logic circuit, otherwise it may be able to lower the voltage too much and the input voltage at the spindle would remain at a constant low logic value – regardless of the position of the the switch.
You can use a pull-up resistor (or pull-down) as a poor man or a door. suppose you have three switches (for example three thermostats) and you want to allow one of three to activate an output. you wire the input of the three vcc switches and the outputs together, then use a 1k zipper to connect the output mass. When all switches are disabled, the pull-down menu will keep the output at 0v. if a switch is enabled, the output will change to vcc. since it will work with an unlimited number of switches, it is a simple and clever solution.
an additional reason, which has not been mentioned so far, is that a pull resistance can be used, especially in high speed digital circuits, to ensure the fast termination of standing waves along address or data bus. I already designed an accelerator for the Macintosh ic, which only had a connector for this purpose. the accelerator has worked in some ic, but not all. turned out that the data bus was charging too long after accessing the memory. adding dropdown lists on the data bus.
should never forget that, even when designing a digital system, it must be taken into account both from an alternating angle and even from a radio frequency aspect. these are used with logic circuits.
when an input is high impedance, this can come from unused input connected to a unipolar switch connected to the high logic to signal an event. driven by a three-state logic or wired logic logic output using open collectors or open drains or diodes. then there may be an indeterminate state when the input is left more or less high impedance.
The pull down gives it a lower impedance to ground to ensure that the input appears as a zero logic level when there is no undefined state. generally, pull-down is a resistance (sometimes called pull-down resistance) in the range of 1k to 100k or more, depending on the logical families. when the output that controls the input becomes active, it can overload the pull resistance and drive it to the logic level of the output. otherwise the input is grounded.
The same jobs for the booster resistors. except that they are linked to a positive power supply to force logic when the input is otherwise undefined.