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.