pulldown

Pulldown refers to a resistor or network used in electrical circuits to define a default low logic level on a signal line when no active device is driving it. The resistor connects the signal to ground, ensuring the input does not float and is interpreted as a logical low.

In digital electronics, pull-downs are commonly used on input pins, including open-collector or open-drain outputs that cannot drive a line high by themselves. A high-impedance or unconnected input can pick up noise, so a pull-down keeps it near zero volts. Typical resistor values range from about 1 kΩ to 100 kΩ, with common choices around 4.7 kΩ to 10 kΩ, balancing noise immunity and static current. Using a pull-down differs from a pull-up, which ties the input to a high level; the choice depends on the surrounding circuitry and active-high versus active-low signaling.

Other uses of the term include 3:2 pulldown in video and film production, a technique for converting 24 frames per second film to approximately 29.97 frames per second for NTSC video. The process inserts extra video fields across frame pairs so the display rate matches the target video standard.

In user interface design, “pull-down” (often written pull-down menu) describes a menu that remains hidden until activated and then reveals options in a dropdown. This UI element helps conserve screen space while providing access to related commands.

Overall, pulldown concepts share the common goal of establishing a known state or accessible options when an explicit signal is not present.