: What is the purpose of pullup and pull down resistors?
Pullup resistors are used to pull logic signals up
(to logic 1). So when some inut signal needs to be
set to logic 1, but might need to be changed for
some reason to 0 at some other time, a pullup resistor
can keep the signal at logic 1 until the signal is
pulled down by something. Typical application for
pullup resistors is to connect 4.7 kohm (or some
other suitable resitor value usually between 1 kohm
and 10 kohm) from the circuit operating voltage (+5V usually)
to the input pin. This resistor keep the signal at logic 1.
When the signal needs to be set to 0 it is pulled
down by conencting that input pin to ground
(usually throuhgh a button, DIP switch or open
collector output of some other part of circuit).
Pulldown reistors work in the opposite way.
They keep signal a logic 0 until somethign
connect te signal input to +5V (or whatever
the operating voltae of the logic circuit is).