Reading analog sensors through ADC in Raspberry Pi - Step-by-Step Execution

Concept Flow - Reading analog sensors through ADC

Start

↓

Initialize ADC

↓

Read analog value

↓

Convert to digital

↓

Use digital value

↓

Repeat or Stop

The Raspberry Pi reads an analog sensor by initializing an ADC, reading the sensor's analog voltage, converting it to a digital value, then using that value in the program.

Execution Sample

Raspberry Pi

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

This code reads an analog value from channel 0 of an MCP3008 ADC connected via SPI and prints the digital value.

Execution Table

Step	Action	SPI Data Sent	SPI Data Received	Digital Value	Output
1	Open SPI bus 0, device 0	-	-	-	-
2	Call read_adc(0)	[1, 128, 0]	[1, 2, 150]	662	-
3	Calculate digital value	-	-	((2 & 3) << 8) + 150 = 662	-
4	Return digital value	-	-	662	-
5	Print value	-	-	662	662

💡 Finished reading and printing the analog sensor value from ADC channel 0

Variable Tracker

Variable	Start	After read_adc call	After calculation	Final
spi	Not opened	Opened SPI bus 0,0	Opened SPI bus 0,0	Opened SPI bus 0,0
channel	N/A	0	0	0
adc	N/A	[1,2,150]	[1,2,150]	[1,2,150]
data	N/A	N/A	662	662
value	N/A	N/A	N/A	662

Key Moments - 3 Insights

Why do we send [1, (8+channel)<<4, 0] to the SPI device?

This command tells the MCP3008 which channel to read. The first byte '1' starts communication, the second byte selects the channel with a start bit, and the third byte is a placeholder. See execution_table row 2.

How is the digital value calculated from the SPI response?

The MCP3008 sends 3 bytes. We mask and shift bits from the second and third bytes to get a 10-bit digital value. See execution_table row 3 for the bit operations.

Why do we need an ADC with Raspberry Pi to read analog sensors?

Raspberry Pi GPIO pins read digital signals only. An ADC converts analog voltages from sensors into digital numbers the Pi can understand.

Visual Quiz - 3 Questions

Look at the execution_table, what is the digital value calculated at step 3?

A 662

B 150

C 2

D 128

💡 Hint

Check the 'Digital Value' column in execution_table row 3.

At which step is the SPI bus opened?

A Step 2

B Step 4

C Step 1

D Step 5

💡 Hint

Look at the 'Action' column in execution_table row 1.

If we change the channel to 1 in read_adc(1), how would the SPI data sent change at step 2?

A [1, 136, 0]

B [1, 144, 0]

C [1, 128, 0]

D [1, 8, 0]

💡 Hint

The second byte is (8 + channel) << 4. For channel 1, calculate (8+1)<<4.

Concept Snapshot

Reading analog sensors with Raspberry Pi requires an ADC like MCP3008.
Use SPI to communicate: send command bytes to select channel.
Receive 3 bytes, extract 10-bit digital value.
Convert analog voltage to digital number.
Use this value in your program for sensor data.

Full Transcript

This visual execution shows how Raspberry Pi reads analog sensors using an ADC chip like MCP3008. First, the SPI bus is opened to communicate with the ADC. Then, a command is sent to select the sensor channel. The ADC responds with 3 bytes containing the analog reading converted to a digital value. We extract the 10-bit number by masking and shifting bits from the response. Finally, the digital value is printed or used in the program. This process repeats to continuously read sensor data. The key is that Raspberry Pi cannot read analog signals directly, so it uses an ADC to convert analog voltages to digital numbers it can understand.

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