This is my 3rd or 4th device for PM2.5 display.
I developed this portable value as I want to know everywhere I go what are the condition of pollution for running inside or outside.
The condition of this device was to be essential, meaning just measure the value, calculate the hourly average and the AQI. To be portable, it must be power on battery and keep charged for days and weeks.
For simplicity, I add a LED display color for easy reading.
Technical description: Hardware :
- Sensor is the PMS7000. this choice because it is small and fan embedded
- The uC is a PIC18F2520
- The sensor is powerd by a real shutdown DC/DC boost: Vbat=>5v (100mA)
- When a measurement need to be done, need power 30 sec before the first measurement is valid
- The LED are low consumption (2ma max)
- Display with 8*2 cars. This display is B&W, very low consumption and easy to buy on internet
- I add a sensor SHT21 for temp and humidity (not necessary just nice to have)
- LDR for night detection
- Battery charging with USB. Option for solar power recharge
- Overall the device consumption is 2mA, except during measurement phase which is 100mA
Technical description: Software :
- Develop with MikroC for PIC18F2520
- Several display mode:
* Last measure and AQI
* Hourly average and AQI
* Battery voltage and Last measure, Hourly average and AQI
* Instant measurement: Sensor always ON and real time display value
- Choice to display the LED based on last measyre, hourly measure or AQI
- 2 choice for the AQI calculation: USA standard one (24h average rolling basis) or the new calculation method, which is 12h basis rolling with some weight on the latest measurement
- Sampling particule measurement choice, from every 10min to every hour. this is the major impact on the autonomy. Every hour, an average of all the measurement made during this hour is recorded and used for the AQI calculation
- Choice to keep or remove the LED display during the night (LDR light detection)
After mock up develop and validated, create a small PCB with SMT components, 5cm*4cm.
All the components can feet into a standard small casing 9.5cm*6cm.2.5cm. I'm designing a dedicated small casing for 3D print to optimize more the space.
Some consumption optimisation need to be done but overall, this device is working fine, maybe too fine as it can show some bad environment. At least, we can see that an air purifier in a kids's room is working very well.
PCB need to be improve as connecters are too big and some layout error.