What are pull-up/pull-down resistors?

・In Japanese

■What are pull-up/pull-down resistors?

A pull-up resistor/pull-down resistor is a resistor that is set to obtain a stable voltage between terminals and circuits. It is used to determine the ON/OFF state of switches and accurately detect sensor values.

■Pull-up resistor

A pull-up resistor is attached to the power supply side, and pulls up the voltage of A when the switch is turned off. By measuring the voltage of A, we can know the ON/OFF state, so we actually calculated.



From the above results, there is a clear difference in voltage between ON and OFF, so it is possible to judge whether it is ON or OFF. So what happens if there is no pull-up resistor?



In this way, the voltage when the switch is ON is approaching the voltage when the switch is OFF. This is a very unstable condition, as even variations in the resistance of the copper wire can make a big difference.

Pull-up resistors can detect not only the ON/OFF status of switches, but also the variable resistance values of sensors such as thermistors, as shown below.

■Pull-down resistor

The pull-down resistor is attached to the ground side as shown below, and the voltage of A is set to 0 when the switch is turned off. Like the pull-up resistor, the pull-down resistor also has the role of stabilizing the voltage.

■How to properly use pull-up and pull-down resistors

Both pull-up and pull-down resistors can be used to detect the voltage of the connected device. As for how to use it properly, it is decided whether it is better to keep it LOW (0V) or HIGH (5V in this case) in the event of a disconnection failure. (Pull-down resistor if you want to keep it LOW)

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