Pull-down resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages. When the switch is open the voltage of the gate input is pulled down to the level of ground. When the switch is closed, the input voltage at the gate goes to Vin.
A pull-down resistor weakly "pulls" the voltage of the wire it is connected to towards its voltage source level when the other components on the line are inactive. When all other connections on the line are inactive, they are high-impedance and act like they are disconnected. Since the other components act as though they are disconnected, the circuit acts as though it is disconnected, and the pull-down resistor brings the wire up to the low logic level. When another component on the line goes active, it will override the low logic level set by the pull-down resistor. The pull-down resistor assures that the wire is at a defined low logic level even if no active devices are connected to it.
It holds the logic signal near zero volts when no other active device is connected.
- ↑ Arduino. "Pull up and pull down resistor." Accessdate: 1/2/16. http://playground.arduino.cc/CommonTopics/PullUpDownResistor