Understanding the SDS011
Caveat Emptor
Much of this understanding comes from reverse-engineering the device. There are some cases where the SDS011 behaves in "unexpected" ways, but these might just be my misunderstanding of it's intended usage. Without more official documentation, it's hard to know if the explanations outlined here are actually correct.
The SDS011 is a small, low-cost sensor created by www.inovafitness.com, used to measure particulate matter in the air. It can measure both PM 2.5 and PM10 values simultaneously, and connects to devices over a serial port, or via USB with an included adaptor.
Because of its size and cost, it is often used on DIY projects with the Raspberry Pi, or Arduino boards, though it can be used on many other devices as well.
Features
Pollutants
The SDS011 measures both small PM2.5 particles, and larger PM10 particles, with a range of 0.0-999.9 μg / m3.
These pollutants can be used in part to calculate the EPA AQI.
Reporting Modes
The SDS011 can report in both ACTIVE and QUERYING mode, with ACTIVE being the factory default.
In ACTIVE mode, the device will always respond with pollutant data, regardless of what other commands are sent to it.
It constantly responds with this data, though there are caveats to this functionality.
In QUERYING mode, the device operates in a request/response model, where a request is written to the device, and then
it writes a response to it's output. In this mode, pollutant data must specifically be asked for.
Working Period
You can set the SDS011 to work continuously, reading all the time, or you can set it to turn on periodically and read for 30 seconds. The device allows you to set the period between work between 0 and 30 minutes. As an example, if you set the working period to 10, the device would wake up every ten minutes, read for 30 seconds, and then turn itself off.
Setting the working period to 0 makes the device work continuously.
Sleep/Wake
You can also manually turn the device on and off, by setting it to sleep or wake. This can be helpful if you want to manually control the device to get reads, or if you need to have a working period longer than 30 minutes. Note that in sleep mode, the device still draws some power; just the fan and diode are turned off.
Device IDs
The SDS011 has a 4 byte device ID, which seems to help support cases where you have multiple devices attached to a single
port, though I haven't actually tried to set this up. Every command that the SDS011 accepts can take a target device ID,
or you can send commands to target all devices by passing \xFF\xFF as the device ID.
Suggested Operation
The SDS011 ships with two modes of operation: ACTIVE and QUERYING. QUERYING mode, though not the factory default,
seems to be much more predictable in its behavior, is generally easier to use, has a more complete feature-set, and
likely can help extend the lifetime of the device. As such, I recommend using QUERYING mode.
It is also recommended to use either the working period functionality, or to manually put the device to sleep when not in use. Generally, continuously reading doesn't give meaningful differentiation of reads when compared to intermittent but frequent reading.
Lastly, it's usually good to give the device at least 15 seconds to warm up after waking before taking any read data. This gives the fan some time to flush out any air in the device, and pull in a more meaningful external air from the outside.
Device Lifetime
The SDS011 datasheet indicates that it has a lifetime of about 8000 hours, or a little less than a year of constant use. An easy way to extend the life of your device is to turn it on intermittently rather than constantly. There are two ways to do this, either via setting the working period, or by manually putting the device to sleep when it's not in use, and waking it when needed. By reading for even just a few minutes spread throughout an hour, you can extend the lifetime of the device to multiple years.
Quirks
Sending Commands in Active Mode
When in active mode, you can send commands to the device. However, because the device is essentially constantly
changing its response buffer to be pollutant data, you can't simply just read the latest data from the device, since the
latest data might be pollutant data instead of the response to your command. The implementation of the
SDS011ActiveReader handles this for you, but it's good to be aware of.
Serial Connections in Active Mode
Because active mode is constantly sending back data over the serial port, it's best to not leave the connection open,
since, unless you're reading from it constantly, the device will continually fill the response buffer with pollutant
data. Leaving this data there, might make you inadvertently read old data from the response buffer instead of the
latest. Instead, it's best to open and close the serial connection when executing queries and commands, so that you're
guaranteed to have recent data. Again, the implementation of SDS011ActiveReader manages this for you, but something
to keep in mind.
Delayed Responses to Sending Commands
When sending commands to the device, it typically takes about a second for the device to write a response back to the serial port. As such, all the readers are configured by default to have a 1 second sleep after a command is sent, so that the device has time to respond, and you don't receive errors when trying to read results back.
You can try and configure this to tune it if you'd like:
from sds011lib import SDS011ActiveReader
import serial
# Create a query mode reader, with a 2 second sleep instead
# of 1 after a command is sent.
reader = SDS011ActiveReader('/dev/ttyUSB0', send_command_sleep=2)