Working of pull-down resistor is almost similar to that of pull up resistor. Pull down resistor is always connected to ground as shown in the following image. It helps to keep logic level near zero, when no other active devices are connected.
In the following image, switch is kept open and hence active devices are disconnected from the system. So the logic gate input get connected to ground through a 10 K pull down resistor and the input of the logic gate will remain at LOW level.
If the switch is closed, voltage Vin get dropped across the 10 K pull down resistor. So voltage across the gate input will be the voltage drop across the pull down resistor ( Vin ). This voltage will keep the gate input at HIGH level.
In the following image, switch is kept open and hence active devices are disconnected from the system. So the logic gate input get connected to ground through a 10 K pull down resistor and the input of the logic gate will remain at LOW level.
If the switch is closed, voltage Vin get dropped across the 10 K pull down resistor. So voltage across the gate input will be the voltage drop across the pull down resistor ( Vin ). This voltage will keep the gate input at HIGH level.
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