Introduction

py-opc-ng is a small Python module to drive Alphasense Optical Particle Counters.

We currently support OPC-N2, OPC-N3, OPC-R1 and OPC-R2. The library doesn’t aim to be feature complete but provides support for most comon operations: turning on and off the device and the internal peripherals, querying device information, querying histograms and particle mass loadings readings.

Feedback, bug reports and pull requests to extend the module beyond this basic functionality are more than welcome. Or just drop me an email at filippo.argiolas@gmail.com if you need help or have any feedback.

Installation

$ pip install py-opc-ng

Requirements

The devices work with SPI interface. You can either connect them directly to a SPI bus using py-spidev (e.g. with the GPIO pins of a Raspberry Pi) or use a SPI to USB device, like the one Alphasense provides, using the pyusbiss library.

Examples

Opening a SPI channel

You first need a SPI interface. This can be either a direct one, e.g. from the MISO/MOSI headers in a RaspberryPi, or you can use a SPI to USB converters like the ones provided by Alphasense.

You can open a direct SPI connection with spidev:

import spidev

spi = spidev.SpiDev()
spi.open(0, 0)
spi.mode = 1
spi.max_speed_hz = 500000
spi.lsbfirst = False

Or you can use pyusbiss if you have a SPI to USB converter:

from usbiss.spi import SPI

spi = SPI('/dev/ttyACM0')
spi.mode = 1
spi.max_speed_hz = 500000
spi.lsbfirst = False

Reading PM data

Basic steps to operate the device and read some measurement:

from time import sleep
import opcng as opc

# autodetect device
dev = opc.detect(spi)

# power on fan and laser
dev.on()

sleep(1)

for i in range(10):
    # query particle mass readings
    pm = dev.pm()

    for k, v in pm.items():
        print(f'{k}: {v}')

    sleep(1)

# power off fan and laser
dev.off()

Querying device information

Here’s an example session where we read both metadata and histogram data from one of our OPC-N3:

>>> import opcng as opc
>>> dev = opc.OPCN3(spi)
>>> dev.info()        # query information string
'OPC-N3 Iss1.1 FirmwareVer=1.17a...........................BS'
>>> dev.fwversion()   # firmware version
(1, 17)
>>> dev.serial()      # serial number
'OPC-N3 177700019                                            '
>>> dev.power_state() # power state (only available for N2 and N3)
{ 'FanON': 0,
  'LaserON': 1,
  'FanDACVal': 255,
  'LaserDACVal': 140,
  'LaserSwitch': 0,
  'GainToggle': 3 }
>>> dev.on()
>>> dev.power_state() # power state after turning it on
{ 'FanON': 1,
  'LaserON': 1,
  'FanDACVal': 255,
  'LaserDACVal': 140,
  'LaserSwitch': 1,
  'GainToggle': 3 }
>>> dev.histogram()   # full histogram data
{ 'Bin 0': 1.883357398455647,
  'Bin 1': 0.4185238663234771,
  'Bin 2': 0.20926193316173855,
  'Bin 3': 0.0,
  'Bin 4': 0.0,
  'Bin 5': 0.20926193316173855,
  'Bin 6': 0.0,
  'Bin 7': 0.0,
  'Bin 8': 0.0,
  'Bin 9': 0.0,
  'Bin 10': 0.0,
  'Bin 11': 0.0,
  'Bin 12': 0.0,
  'Bin 13': 0.0,
  'Bin 14': 0.0,
  'Bin 15': 0.0,
  'Bin 16': 0.0,
  'Bin 17': 0.0,
  'Bin 18': 0.0,
  'Bin 19': 0.0,
  'Bin 20': 0.0,
  'Bin 21': 0.0,
  'Bin 22': 0.0,
  'Bin 23': 0.0,
  'Bin1 MToF': 5.0,
  'Bin3 MToF': 0.0,
  'Bin5 MToF': 10.666666666666666,
  'Bin7 MToF': 0.0,
  'Sampling Period': 0.51,
  'SFR': 9.37,
  'Temperature': 31.22453650720989,
  'Relative humidity': 40.822461280231934,
  'PM1': 0.29056867957115173,
  'PM2.5': 1.235719919204712,
  'PM10': 1.6337664127349854,
  '#RejectGlitch': 9,
  '#RejectLongTOF': 0,
  '#RejectRatio': 2335,
  '#RejectOutOfRange': 3,
  'Fan rev count': 0,
  'Laser status': 613,
  'Checksum': 35040 }