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:
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.

Practice

1. What is the main purpose of using pull-up or pull-down resistors with Raspberry Pi GPIO pins?

easy

A. To set a default voltage level on input pins to avoid floating values

B. To increase the speed of the GPIO pin

C. To power external devices connected to the GPIO

D. To change the output voltage of the pin

Solution

1. Step 1: Understand the problem of floating pins

   GPIO input pins can read random values if not connected to a stable voltage, causing unreliable readings.

2. Step 2: Role of pull-up and pull-down resistors

   Pull-up sets the pin to high by default, pull-down sets it to low, preventing floating and ensuring stable input.

3. Final Answer:

   To set a default voltage level on input pins to avoid floating values -> Option A

4. Quick Check:

   Pull-up/down resistors prevent floating = A [OK]

Hint: Pull resistors fix input pin voltage to high or low [OK]

Common Mistakes:

• Thinking pull resistors power devices
• Confusing pull-up with output voltage control
• Believing pull resistors speed up GPIO

2. Which of the following is the correct way to enable an internal pull-up resistor on GPIO pin 17 using the RPi.GPIO library in Python?

easy

A. GPIO.setup(17, GPIO.OUT)

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

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

D. GPIO.setup(17, GPIO.OUT, pull_up_down=GPIO.PUD_UP)

Solution

1. Step 1: Identify correct pin mode for input

   Pull-up resistors are used with input pins, so mode must be GPIO.IN.

2. Step 2: Use correct pull-up argument

   GPIO.PUD_UP enables the internal pull-up resistor; GPIO.PUD_DOWN is for pull-down.

3. Final Answer:

   GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP) -> Option B

4. Quick Check:

   Input pin with pull-up = D [OK]

Hint: Pull-up means GPIO.IN with GPIO.PUD_UP [OK]

Common Mistakes:

• Setting pin as output when enabling pull-up
• Using PUD_DOWN instead of PUD_UP for pull-up
• Omitting pull_up_down parameter

3. Consider this Python code snippet using RPi.GPIO:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(22, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
input_state = GPIO.input(22)
print(input_state)

If no button or wire is connected to pin 22, what will be printed?

medium

A. 1

B. Error

C. None

D. 0

Solution

1. Step 1: Understand pull-down resistor effect

   Pull-down resistor sets the pin to low (0) by default when nothing is connected.

2. Step 2: Check input reading

   Since no button is connected, the input reads the default low level, so GPIO.input(22) returns 0.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   Pull-down resistor means input reads 0 when floating = C [OK]

Hint: Pull-down means default input is 0 [OK]

Common Mistakes:

• Assuming input reads 1 without connection
• Expecting an error without a device connected
• Confusing pull-up and pull-down effects

4. You wrote this code to read a button press on GPIO pin 5:

GPIO.setup(5, GPIO.IN)
if GPIO.input(5) == 1:
    print("Button pressed")
else:
    print("Button not pressed")

But the output is random even when the button is not pressed. What is the likely problem?

medium

A. You forgot to enable a pull-up or pull-down resistor on pin 5

B. You set the pin mode to output instead of input

C. You used the wrong pin number

D. You need to call GPIO.cleanup() before reading input

Solution

1. Step 1: Identify cause of random input readings

   Without pull-up or pull-down resistors, input pins can float and read random values.

2. Step 2: Fix by enabling internal resistor

   Use GPIO.setup(5, GPIO.IN, pull_up_down=GPIO.PUD_UP) or PUD_DOWN to stabilize input.

3. Final Answer:

   You forgot to enable a pull-up or pull-down resistor on pin 5 -> Option A

4. Quick Check:

   Floating input needs pull resistor = B [OK]

Hint: Always enable pull resistor for input pins reading buttons [OK]

Common Mistakes:

• Not setting pin as input
• Assuming cleanup affects input reading
• Changing pin number without hardware change

5. You want to connect two buttons to Raspberry Pi GPIO pins 6 and 13. Button on pin 6 should read HIGH when pressed, and button on pin 13 should read LOW when pressed. Which setup correctly uses pull-up and pull-down resistors to achieve this behavior?

hard

A. GPIO.setup(6, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
GPIO.setup(13, GPIO.IN, pull_up_down=GPIO.PUD_UP)

B. GPIO.setup(6, GPIO.OUT)
GPIO.setup(13, GPIO.OUT)

C. GPIO.setup(6, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.setup(13, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

D. GPIO.setup(6, GPIO.IN)
GPIO.setup(13, GPIO.IN)

Solution

1. Step 1: Understand button logic for pin 6

   Button on pin 6 should read HIGH when pressed, so use pull-up resistor to keep default HIGH and go LOW on press.

2. Step 2: Understand button logic for pin 13

   Button on pin 13 should read LOW when pressed, so use pull-down resistor to keep default LOW and go HIGH on press.

3. Step 3: Match setup calls to logic

   Pin 6 uses GPIO.PUD_UP, pin 13 uses GPIO.PUD_DOWN to achieve desired readings.

4. Final Answer:

   GPIO.setup(6, GPIO.IN, pull_up_down=GPIO.PUD_UP) GPIO.setup(13, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) -> Option C

5. Quick Check:

   Pin 6 pull-up, pin 13 pull-down = B [OK]

Hint: Pull-up means default HIGH, pull-down means default LOW [OK]

Common Mistakes:

• Reversing pull-up and pull-down resistor roles
• Setting pins as output instead of input
• Not using pull resistors causing floating inputs