Pull-up and pull-down resistors in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pull-up resistor?

A pull-up resistor is connected between a signal line and a positive voltage supply to ensure the line reads a high voltage (logic 1) when no other device is driving it.

beginner

What is a pull-down resistor?

A pull-down resistor is connected between a signal line and ground to ensure the line reads a low voltage (logic 0) when no other device is driving it.

beginner

Why do we need pull-up or pull-down resistors in Raspberry Pi GPIO inputs?

They prevent the input pin from 'floating' which can cause unpredictable readings by ensuring the pin has a default voltage level when no button or sensor is connected.

intermediate

How can you enable an internal pull-up resistor on a Raspberry Pi GPIO pin using Python?

Using the RPi.GPIO library, you can set up a pin with pull-up like this: <br> GPIO.setup(pin_number, GPIO.IN, pull_up_down=GPIO.PUD_UP)

beginner

What happens if you don't use pull-up or pull-down resistors on an input pin?

The input pin may 'float', causing it to randomly read high or low, leading to unreliable or noisy input signals.

What does a pull-up resistor connect to?

A Ground

B Positive voltage supply

C Output pin

D Battery negative terminal

A pull-up resistor connects the signal line to a positive voltage supply to keep the input high when not driven.

Which resistor prevents a GPIO pin from floating low?

A Pull-down resistor

B Pull-up resistor

C Series resistor

D No resistor

A pull-down resistor keeps the pin at a low level, preventing it from floating high.

How do you enable an internal pull-down resistor in RPi.GPIO?

A GPIO.setup(pin, GPIO.IN)

B GPIO.setup(pin, GPIO.OUT)

C GPIO.setup(pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

D GPIO.setup(pin, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

Use GPIO.PUD_DOWN to enable the internal pull-down resistor on a GPIO pin.

What problem occurs if no pull-up or pull-down resistor is used on an input pin?

A Pin will short circuit

B Pin will always read high

C Pin may float causing unreliable readings

D Pin will always read low

Without pull resistors, the pin voltage can float and cause random input values.

Which of these is a correct way to set a GPIO pin as input with pull-up resistor in Python?

A GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP)

B GPIO.setup(17, GPIO.OUT)

C GPIO.setup(17, GPIO.IN)

D GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

This command sets GPIO pin 17 as input with an internal pull-up resistor enabled.

Explain what pull-up and pull-down resistors do and why they are important for Raspberry Pi GPIO inputs.

Think about how to keep a wire from 'floating' when nothing is connected.

Describe how to enable internal pull-up and pull-down resistors on Raspberry Pi GPIO pins using Python code.

Look at the parameters inside GPIO.setup for input pins.