Reading analog sensors through ADC in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the library needed to read from the ADC.

Raspberry Pi

import [1]

Drag options to blanks, or click blank then click option'

Aspidev

Btime

Cmath

Drandom

Attempts:

3 left

💡 Hint

Common Mistakes

Importing unrelated libraries like random or math.

Forgetting to import the SPI library.

The spidev library is used to communicate with SPI devices like the MCP3008 ADC chip.

2fill in blank

medium

Complete the code to open SPI bus 0, device 0 for communication.

Raspberry Pi

spi = spidev.SpiDev()
spi.[1](0, 0)

Drag options to blanks, or click blank then click option'

Aopen

Bconnect

Cstart

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like start or connect which do not exist.

Forgetting to open the SPI device before use.

The open method opens the SPI bus and device for communication.

3fill in blank

hard

Fix the error in the function that reads data from channel 0 of the MCP3008 ADC.

Raspberry Pi

def read_adc(channel):
    adc = spi.xfer2([1, (8 + [1]) << 4, 0])
    data = ((adc[1] & 3) << 8) + adc[2]
    return data

Drag options to blanks, or click blank then click option'

A1

B0

Cspi

Dchannel

Attempts:

3 left

💡 Hint

Common Mistakes

Hardcoding the channel number instead of using the parameter.

Using an undefined variable.

The function must use the channel parameter to select the ADC channel dynamically.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that reads ADC values for channels 0 to 3.

Raspberry Pi

adc_values = {ch: read_adc(ch) for ch in [1] if ch [2] 3}

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Arange(4)

Brange(3)

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using range(3) which excludes channel 3.

Using < which excludes channel 3.

range(4) generates 0,1,2,3 and <= includes channel 3 in the dictionary.

5fill in blank

hard

Fill all three blanks to convert the raw ADC value to voltage assuming a 3.3V reference and 10-bit ADC.

Raspberry Pi

voltage = ([1] / [2]) * [3]

Drag options to blanks, or click blank then click option'

Araw_value

B1023

C3.3

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1024 instead of 1023 as max ADC value.

Multiplying before dividing.

Using wrong reference voltage.

The formula converts the raw ADC reading to voltage: raw_value divided by max ADC value (1023) times reference voltage (3.3V).

Practice

1. Why do we need an ADC (Analog-to-Digital Converter) like MCP3008 when using a Raspberry Pi to read sensors?

easy

A. Because Raspberry Pi cannot read analog signals directly

B. Because Raspberry Pi has no USB ports

C. Because Raspberry Pi cannot connect to Wi-Fi

D. Because Raspberry Pi does not support Python

Solution

1. Step 1: Understand Raspberry Pi's input capabilities

   The Raspberry Pi's GPIO pins can only read digital signals (on/off), not analog voltages.

2. Step 2: Role of ADC

   An ADC like MCP3008 converts analog voltages from sensors into digital numbers the Pi can read.

3. Final Answer:

   Because Raspberry Pi cannot read analog signals directly -> Option A

4. Quick Check:

   ADC needed for analog input = Because Raspberry Pi cannot read analog signals directly [OK]

Hint: Remember: Pi reads digital only, ADC converts analog to digital [OK]

Common Mistakes:

• Thinking Pi can read analog signals directly
• Confusing ADC with Wi-Fi or USB functions
• Assuming Python support relates to ADC

2. Which of the following is the correct way to call a function named read_adc to read from channel 2?

easy

A. read_adc[channel=2]

B. read_adc{2}

C. read_adc(2)

D. read_adc 2

Solution

1. Step 1: Understand Python function call syntax

   Functions are called with parentheses and arguments inside, e.g., function_name(argument).

2. Step 2: Apply to read_adc function

   To read channel 2, call read_adc(2). Other options use invalid syntax.

3. Final Answer:

   read_adc(2) -> Option C

4. Quick Check:

   Function call uses parentheses = read_adc(2) [OK]

Hint: Use parentheses for function calls in Python [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Using curly braces instead of parentheses
• Omitting parentheses entirely

3. What will be the output of this code snippet reading from channel 0 if the sensor voltage corresponds to a digital value of 512?

value = read_adc(0)
print(value)

medium

A. Error

B. 0

C. 1023

D. 512

Solution

1. Step 1: Understand ADC output range

   MCP3008 outputs values from 0 to 1023 representing 0V to max voltage.

2. Step 2: Given sensor voltage corresponds to 512

   Calling read_adc(0) returns the digital value 512, which is printed.

3. Final Answer:

   512 -> Option D

4. Quick Check:

   ADC value matches sensor voltage = 512 [OK]

Hint: ADC returns 0-1023 values; print shows that number [OK]

Common Mistakes:

• Assuming output is voltage instead of digital number
• Expecting max value 1023 always
• Thinking code causes error

4. Identify the error in this code snippet that reads from channel 1:

def read_sensor():
    value = read_adc[1]
    return value

print(read_sensor())

medium

A. Incorrect print syntax

B. Using square brackets instead of parentheses to call read_adc

C. Missing return statement in function

D. Function name is invalid

Solution

1. Step 1: Check function call syntax inside read_sensor

   The code uses read_adc[1], which is invalid syntax for calling a function.

2. Step 2: Correct syntax for function call

   It should be read_adc(1) with parentheses to call the function.

3. Final Answer:

   Using square brackets instead of parentheses to call read_adc -> Option B

4. Quick Check:

   Function calls need parentheses = Using square brackets instead of parentheses to call read_adc [OK]

Hint: Use parentheses, not brackets, to call functions [OK]

Common Mistakes:

• Using brackets [] instead of parentheses ()
• Forgetting to return value
• Misusing print statement

5. You want to read analog values from 4 sensors connected to channels 0 to 3 using MCP3008 and store them in a dictionary with keys as channel numbers. Which code snippet correctly does this?

hard

A. values = {ch: read_adc(ch) for ch in range(4)}

B. values = [read_adc(ch) for ch in range(1,4)]

C. values = {read_adc(ch): ch for ch in range(4)}

D. values = dict(read_adc(ch) for ch in range(4))

Solution

1. Step 1: Understand the goal

   We want a dictionary with keys as channel numbers (0 to 3) and values as readings from read_adc.

2. Step 2: Analyze each option

   values = {ch: read_adc(ch) for ch in range(4)} uses dictionary comprehension correctly: keys are channels, values are readings. values = [read_adc(ch) for ch in range(1,4)] creates a list, not a dictionary. values = {read_adc(ch): ch for ch in range(4)} swaps keys and values incorrectly. values = dict(read_adc(ch) for ch in range(4)) tries to create dict from generator of values only, causing error.

3. Final Answer:

   values = {ch: read_adc(ch) for ch in range(4)} -> Option A

4. Quick Check:

   Dict comprehension with channel keys = values = {ch: read_adc(ch) for ch in range(4)} [OK]

Hint: Use dict comprehension: {key: value for key in range} [OK]

Common Mistakes:

• Using list instead of dict for key-value pairs
• Swapping keys and values in dict comprehension
• Passing wrong arguments to dict() constructor