Advice on PWM output from particle sensors

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kreid4
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Advice on PWM output from particle sensors

Post by kreid4 »

Hello

I am new here.
We are setting up a small network of air quality particle sensors. We are using SDS011 particle sensors connected to an ESP8266 node CPU/WLAN and a BME280 6 pin (temperature/humidity/pressure) sensor. Please see links below.

I am interested in observing/recording/analysing the PWM outputs from SDS011 devices. I am also interested in understanding how the electronics and serial comms are performing across all components.
Could you please advise me on the best hobbyist pico oscilloscope options to do this please.

SDS011
https://cdn.shopify.com/s/files/1/0943/8168/files/SDS011-DATASHEET.pdf?v=1626004123

ESP8266
https://randomnerdtutorials.com/esp8266-pinout-reference-gpios/

BME280
https://nettigo.eu/products/module-pressure-humidity-and-temperature-sensor-bosch-bme280

Thank you
Kind Regards
K Reid

bennog
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Re: Advice on PWM output from particle sensors

Post by bennog »

the SDS011 does not output PWM it outputs a serial data stream RS232 at 3.3V cpu levels.
So in theory you can monitor this with for example a arduino or a raspberry PI.

or with the ESP8266 as in your example

The BME280 has a I2C interface where you need to send a request and get the data back (there are buildin librarys to readout a I2C device so no problems expected).

Benno

kreid4
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Re: Advice on PWM output from particle sensors

Post by kreid4 »

No. Pin Name Function
1 NC No connection/Not used
2 1μm Range of PM2.5 value (0-999μg /m³) and provide output in PWM form on this pin
3 5V Connect 5V to this pin to power dust sensor
4 2.5μm Range of PM10 value (0-999μg /m³) and provide output in PWM form on this pin
5 GND Connect to ground terminal of power supply and common reference with Arduino or microcontroller
6 Rx Receiver terminal of UART module and used to receive commands from computer or microcontrollers
7 Tx Transmitter pin of UART and used to transmit output to the target device such as Arduino or other microcontrollers

Benno,
Thank you for your reply and advice.
The SDS011 data sheets indicates that pin2 and pin4 of the SDS011 are PWM outputs.
I want to be able to monitor the PWM waveforms so that I can make an assessment of the particle concentration, which is based on the pulse width. However I may be misunderstanding the table above.
Otherwise I would be grateful if you could advise me on an appropriate pico PC oscilloscope to monitor pin2 and pin4 of a SDS011.
Thank you
Kind regards
K

Gerry
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Re: Advice on PWM output from particle sensors

Post by Gerry »

Hi kreid4/Benno,

The SDS011 outputs BOTH a 9 bit UART signal on pins 6&7 AND two PWM output streams on pins 2&4 (which appear to be the concentration of particles for different particle sizes of 1um and 2.5um). The data sheet is a bit ambiguous on the low period of the PWM signal at the highest concentration (looks like its just 3ms, but why have they split it into 1ms and 2ms? Are they trying to show the precision of the pulse width, or something else?) My suggestion would be to go back to the source of where you got the data sheet from (manufacturer or Distributor) to get more clarity on the expected output from the sensor.

In order to determine the appropriate Data Acquisition Device for monitoring the PWM output you we would need to know what the minimum width of a high or low period of the PWM would be (which looks like it depends upon the minimum concentration detected for the particle sizes), the longest period that you need to be monitoring the PWM output or outputs for, and if you need to be monitoring anything else with the Device such as the UART output or outputs from the other sensors.

Regards,

Gerry
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kreid4
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Re: Advice on PWM output from particle sensors

Post by kreid4 »

Gerry, thank you for you reply.

There does not seem to be much more information available other than the Datasheet.

The air monitors sample the air 5 times at ~1 second intervals (ie ~ 5 seconds total ).
It then waits for 175 seconds and repeats this 24/7.

I wish to monitor both the PWM pins and UART pins but maybe not all at the same time.

I would only be monitoring the above pins with an oscilloscope during initial setup and/or calibration/maintenance periods.

I was thinking of purchasing a pico 2000 series for general use anyway. Would that type of device help me meet the requirement above? Also what overall general advantage would a series 2000 MSO provide me with.

Thank you
Regards
K

Gerry
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Re: Advice on PWM output from particle sensors

Post by Gerry »

Hi Kreid4,

Observations
OK, I can only base my comment on the information in front of me. Just for clarity, this Particle sensor is actually a Particle Transducer that contains a Particle Sensor. The Transducer takes the output from the sensor and transforms into Digital values of a Pulse Width Modulated signal, and a UART output, but the resolutions of these could be different.
The UART spec seems to indicate that the Resolution for the spread of the particle concentration is 12 to 13 bits (high byte * 256) + (low byte / 10). However, the Maximum sensitivity used from the sensor appears to be 0.3ug/m^3. For a maximum concentration of 999ug/m^3, this needs 4096 values (to be able to create up to 3,333 discrete values). 4096 values requires 12-bits, so I guess we can say that the UART has 12-bit resolution.

To answer your questions
The UART transmission rate is 9600 baud so the bit width is 104us, which means that you would need a Data Acquisition device that can sample at a rate of 4us to be able to recreate a good representation of individual data bytes. So the minimum sample rate that you can use would be 250kHz . If you're only making 5 captures every 180 seconds then it would be better to Trigger only when there's a waveform available. So, you could use a PicoScope 2204A and set it up to capture 1 seconds worth of data, once triggered by a start bit, and then save the 5 buffers of data every 180 seconds (using an alarm).

The Schematic for the output of the PWM seems to indicate that the resolution of the transducer for the converted output from the sensor, in the PWM is 1ms for every 1ug/m^3 of particle concentration. So, it appears that the resolution of the PWM is 10-bits, but even if was 12-bits the PWM has a much larger individual bit size than the UART, so you could also use the PicoScope to capture the PWM waveforms. However, if you want to capture both the 1um and 2.5um PWM output waveforms, along with both the RX and TX ports of the UART then, as the PS 2204A only has 2 Input Channels you'd need either 2 PicoScope 2204A's or a 2207B MSO (if the UART output and PWM output need to be synchronized and captured to the same data file), which would be the lowest cost devices that we have available for sale, with 4 Channels. The MSO allows you to capture Digital Signals as well as Analogue signals, so with it you can capture either the UART or PWM waveforms on 2 Digital Channels, and the other waveforms on 2 Analogue channel (my recommendation would be to use the Digital Channels for the PWM, as its switching at a level that can be easily accommodated).

Regards,

Gerry
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kreid4
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Re: Advice on PWM output from particle sensors

Post by kreid4 »

Gerry

Thank you very much indeed for your very useful and comprehensive information and help.
It looks like the 2207B MSO might be the best option.

I am also checking the availability of 2205a MSO from the link below.
https://www.sjelectronics.co.uk/picosco ... pe.html>

Would this be a suitable alternative to the to the 2207B, if it is in stock?

Regards
Ken Reid


Gerry
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Re: Advice on PWM output from particle sensors

Post by Gerry »

Hi Kreid4,

Yes, that would be a more cost-effective solution.

The only thing is that they don't have an image of it, and you would need the MSO connector: https://www.picotech.com/accessories/ms ... -mso-cable, along with a set of test clips: https://www.picotech.com/accessories/ms ... test-clips.

Regards,

Gerry
Gerry
Technical Specialist

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