Reading Sensors on a Raspberry Pi 4 over I2C and SPI with Python

#!/usr/bin/env python3
# sensor_logger.py - read a BH1750 (I2C) and an MCP3008 (SPI) every 2 seconds
import csv
import time
from datetime import datetime

import spidev
from smbus2 import SMBus, i2c_msg

# ---------------- BH1750 light sensor (I2C) ----------------
BH1750_ADDR = 0x23          # ADDR pin low; 0x5C if ADDR is high
POWER_ON = 0x01
CONT_HRES_MODE = 0x10       # continuous, 1 lx resolution, ~120 ms per reading

class BH1750:
    def __init__(self, bus):
        self.bus = bus
        # The BH1750 takes single-byte commands with no register address,
        # so we send raw I2C messages instead of register writes.
        self.bus.i2c_rdwr(i2c_msg.write(BH1750_ADDR, [POWER_ON]))
        self.bus.i2c_rdwr(i2c_msg.write(BH1750_ADDR, [CONT_HRES_MODE]))
        time.sleep(0.18)    # wait for the first conversion

    def lux(self):
        msg = i2c_msg.read(BH1750_ADDR, 2)
        self.bus.i2c_rdwr(msg)
        high, low = list(msg)
        raw = (high << 8) | low
        return raw / 1.2    # datasheet conversion factor

# ---------------- MCP3008 ADC (SPI) ----------------
class MCP3008:
    def __init__(self, bus=0, device=0):
        self.spi = spidev.SpiDev()
        self.spi.open(bus, device)      # /dev/spidev0.0 -> CE0
        self.spi.max_speed_hz = 1_000_000
        self.spi.mode = 0

    def read(self, channel):
        """Return the 10-bit value (0-1023) of a single-ended channel 0-7."""
        if not 0 <= channel <= 7:
            raise ValueError('channel must be 0-7')
        # Byte 1: start bit. Byte 2: single-ended flag + channel in top nibble.
        reply = self.spi.xfer2([0x01, (0x08 | channel) << 4, 0x00])
        # Result: last 2 bits of byte 1 and all 8 bits of byte 2
        return ((reply[1] & 0x03) << 8) | reply[2]

    def close(self):
        self.spi.close()

# ---------------- main loop ----------------
def main():
    adc = MCP3008()
    with SMBus(1) as bus, open('readings.csv', 'a', newline='') as f:
        light = BH1750(bus)
        writer = csv.writer(f)
        if f.tell() == 0:                      # new file: write a header
            writer.writerow(['time', 'lux', 'adc_raw', 'volts'])
        try:
            while True:
                lux = light.lux()
                raw = adc.read(0)
                volts = raw * 3.3 / 1023       # VREF is 3.3 V
                now = datetime.now().isoformat(timespec='seconds')
                writer.writerow([now, f'{lux:.1f}', raw, f'{volts:.3f}'])
                f.flush()                      # survive a power cut
                print(f'{now}  {lux:8.1f} lx  ADC={raw:4d}  {volts:.3f} V')
                time.sleep(2)
        except KeyboardInterrupt:
            pass
        finally:
            adc.close()

if __name__ == '__main__':
    main()