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<\/a><\/p>\n\n\n\nIn an earlier post <\/a>I described the development of an aerosol sensor that measures how much fine dust is present in the air and sends these values to an MQTT broker. The measured values can then, for example, be used to control the fan of a central ventilation system.<\/p>\n\n\n\n Hier<\/a> kun je een Nederlandse vertaling<\/a> van dit project vinden.<\/p>\n\n\n\n It is surprising to discover how much variation there can be in the amount of fine dust, depending on factors such as traffic, wood stoves, ventilation, or simply where you are located. That gave me the idea to build a small, portable device that allows me to take local measurements.<\/p>\n\n\n\n Instead of the PMS5003 sensor that I used in my previous design, I now want to use the SPS30 sensor (both sensors are made by Sensirion).<\/p>\n\n\n\n To truly be portable the design must meet a number of criteria:<\/p>\n\n\n\n The solution is a so-called soft latch circuit<\/strong>. With this circuit, using a p-channel MOSFET, the power between the battery, the ESP32, the sensor and the e-Paper display is completely disconnected. Power consumption in the \u201coff\u201d state is only a few microamps.<\/p>\n\n\n\n The result is a handheld fine dust meter based on an ESP32 and a Sensirion SPS30 sensor with an e-Paper display. The device is intended for quick measurements at a location: outside in the street, inside the house, near an open window or for example in a workshop. Because the device runs on a battery it is truly portable and handheld.<\/p>\n\n\n\n The SPS30 sensor measures the mass concentration of fine dust in several fractions: PM1.0, PM2.5 and PM10. These are the concentrations of particles smaller than 1 \u00b5m, 2.5 \u00b5m and 10 \u00b5m respectively.<\/p>\n\n\n\n The sensor uses a laser and a fan to draw air through a measurement chamber and count the particles. This produces fairly reliable readings.<\/p>\n\n\n\n As mentioned earlier, I want to use standard modules as much as possible. In this case:<\/p>\n\n\n\n I also bought this module from Amazon for a few euros.<\/p>\n\n\n Note that the module can produce several output voltages (5 V, 8 V, 9 V and 12 V).<\/p>\n\n\n\n To obtain an output voltage of 5 volts the two 0-ohm resistors (next to A and B) must be removed. The SPS30 particulate matter sensor is made by Sensirion.<\/p>\n\n\n There are literally dozens of different ESP32 boards available. For this project and the PCB I based the design on a 30-pin ESP32-DevKit-V1 board.<\/p>\n\n\n\n When the button (SW1) is briefly pressed, Q101 starts conducting and the ESP32, the SPS30 and the e-Paper display receive power.<\/p>\n\n\n\n The ESP32 will first set the latch pin (GPIO_PIN_LATCH) HIGH. This keeps the circuit latched so Q101 continues to conduct.<\/p>\n\n\n\n The SPS30 sensor and the e-Paper display now also receive power.<\/p>\n\n\n\n For reliable measurements the SPS30 must first warm up. This warm-up time is configurable (via the platformio.ini file). According to Sensirion the warm-up time must be at least 30 seconds.<\/p>\n\n\n\n After this warm-up time the device performs five measurements (also configurable), which are shown directly on the display. Finally the average of all measurements is displayed and the ESP32 sets the latch pin LOW. Q101 switches off and the power is disconnected.<\/p>\n\n\n\n For correct operation the soft latch circuit is a crucial component. Many variations can be found on the internet, some better than others. After some research I ended up with the following circuit:<\/p>\n\n\n It consists of two MOSFETs:<\/p>\n\n\n\n a p-channel MOSFET (AO3401) used as the power switch and an n-channel MOSFET (BSS138) for control.<\/p>\n\n\n\n Two signal diodes (1N4148) are included so that the push button, after power has been applied, can also be used as a normal switch on GPIO33.<\/p>\n\n\n\n Dave from EEVblog <\/a>and PKAE Electronics<\/a> have made videos explaining how the circuit works.<\/p>\n\n\n\n To test this I connected a timer output to the \u201cExt.Sw\u201d jumper (J5) and repeated the following cycle 700 times: Using KiCad I designed a PCB with space for most of the components. Only the SPS30 and the e-Paper display are not mounted on the board but can be connected with standard 2.54\u2033 pin headers or JST-XH connectors.<\/p>\n\n\n\n On the top side of the PCB:<\/p>\n\n\n\n On the bottom side of the PCB:<\/p>\n\n\n\n\n
Design choices<\/strong><\/h2>\n\n\n\n
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![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/ePaper_Data-1024x768.jpg\")
<\/a><\/figure>\n<\/div>\n\n\n![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/HH_Construction-1024x768.jpg\")
<\/a><\/figure>\n\n\n\n![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/HHSensor-540x1024.jpg\")
<\/figure>\n<\/figure>\n\n\n\nThe SPS30 sensor<\/strong><\/h2>\n\n\n\n
The basic design<\/strong><\/h2>\n\n\n\n
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![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/HHSensor.drawio.png\")
<\/a><\/figure>\n\n\n\nCharging module<\/strong><\/h2>\n\n\n
![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/laadModule-TP4056.png\")
<\/a><\/figure>\n<\/div>\n\n\n
The charging module is a very common one (I bought several for a few euros each from Amazon).<\/p>\n\n\n\nDC-DC step-up module<\/strong><\/h2>\n\n\n
![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/DC-DC-BoostConvertor-1024x714.png\")
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<\/a><\/figure>\n<\/div>\n\n\n
Do not forget!<\/strong><\/p>\n\n\n\nSPS30 (or PMS5003) fine dust sensor<\/strong><\/h2>\n\n\n\n
![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/SPS30-and-PMS5003-1024x387.png\")
<\/a>Waveshare 1.54\u201d e-Paper<\/strong><\/h2>\n\n\n\n
![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/1.5422-e-Paper-1024x708.png\")
<\/a><\/figure>\n\n\n\nESP32 DevKit 30-pin<\/strong><\/h2>\n\n\n\n
![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/ESP32-DEVKIT-V1-30pins-1024x628.png\")
<\/a><\/figure>\n\n\n\nGlobal operation of the device<\/strong><\/h2>\n\n\n\n
Simplified code example<\/strong><\/h2>\n\n\n\n
void setup()\n{\n \/\/ -- Switch off WiFi and Bluetooth as early as possible\n \/\/ -- This saves power and avoids starting hardware that is not needed\n disableRadios();\n\n \/\/ -- Keep the power latch active so the device stays on\n digitalWrite(pinLatch, HIGH);\n\n \/\/ -- Initialize the display and prepare the first screen contents\n epdInit();\n\n \/\/ -- Read battery voltage early\n \/\/ -- If the battery is too low, show a message and shut down\n batteryVoltageLast = readBatteryVoltage();\n\n if (batteryVoltageLast < 3.4f)\n {\n switchOff(\"BATTERY TOO LOW\");\n return;\n }\n\n \/\/ -- Initialize shared communication buses\n \/\/ -- I2C is used for the SPS30 and optionally for the BMP280\n i2cInit();\n\n \/\/ -- Detect which sensor hardware is available\n \/\/ -- The code can work with more than one particle sensor type\n detectAvailableSensors();\n\n \/\/ -- Try to start the particle sensor measurement\n \/\/ -- When successful, the software enters the warm-up phase\n if (!particleSensorInitAndStart())\n {\n messageText = \"Particle sensor error\";\n }\n else\n {\n messageText = \"Warming up...\";\n }\n\n \/\/ -- Reset all software timers used by the state machine\n restartTimers();\n\n} \/\/ setup()\n\n\nvoid loop()\n{\n \/\/ -- Run small background jobs that must be serviced regularly\n \/\/ -- Example: button handling, display housekeeping, sensor polling\n serviceBackgroundTasks();\n\n \/\/ -- If a shutdown was requested somewhere else in the code,\n \/\/ -- execute it here as soon as possible\n processSwitchOffRequest();\n\n \/\/ -- Run the main application logic\n \/\/ -- This is the state machine: warm-up, sample, validate, average, show result\n runMainStateMachine();\n\n \/\/ -- Check again after the main logic,\n \/\/ -- because the state machine may have requested a shutdown\n processSwitchOffRequest();\n\n} \/\/ loop()<\/pre>\n\n\n\nThe soft latch circuit<\/strong><\/h2>\n\n\n\n
![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/Latch-Circuit-1024x784.png\")
<\/a><\/figure>\n<\/div>\n\n\nHow good is this soft latch circuit?<\/strong><\/h2>\n\n\n\n
\u2022 close for one second (which starts the fine dust measurement cycle)
\u2022 open for two minutes (within this time the sensor switches itself off again)
Starting with a fully charged 18650 cell, after those 700 cycles the cell voltage was still above 3.8 volts (about 50%).
Quite efficient!<\/strong><\/p>\n<\/div>\n<\/div><\/div>\n<\/div><\/div>\n\n\n![\"\"](\"https:\/\/willem.aandewiel.nl\/wp-content\/uploads\/2026\/03\/handhelAerosolSensorSchematic-1024x705.png\")
<\/a><\/figure>\n<\/div>\n\n\n
Here is the complete schematic of the handheld fine dust meter:<\/p>\n\n\n\nPCB<\/strong><\/h2>\n\n\n\n
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