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. It holds the logic signal near zero volts when no other active device is connected.