Project Design Breakdown

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Project
Environmental Exposure Risk Meter – Detailed Design Review 1. Project Purpose
A hand-held, solar-assisted environmental exposure meter that:
  • Measures indoor / outdoor air quality (CO₂) and ultraviolet (UV) index.
  • Connects to Wi-Fi / Bluetooth via the on-board ESP32 to push data to mobile or cloud dashboards.
  • Runs from a small Li-ion battery that can be charged by a photovoltaic (PV) panel and boosted to power-hungry peripherals when needed.
Typical use-cases: personal health monitoring, classroom air-quality indication, building-HVAC feedback, wearable sun-exposure tracking.
2. Core Hardware Blocks & Interaction 
Diagram



  
    
  
  
    
  














VBAT

3.0-4.2 V

5 V BOOST

3.3 V

UART

I²C










"Solar Panel"
"CN3791 MPPT Charger(SL2)"
"Li-ion Cell (VBAT)"
"MT3608 BoostConverter (SL1)"
"AP2112 LDO 3.3 V(U1)"
"ESP32-WROOM-32E(U2)"
"MH-Z19 CO₂ Sensor(UART)"
"VEML6075 UV Sensor(I²C)"
"LED + Two Push-Buttons(LED1, SW1, SW2)"
"Headers & JST Qwiic(J1-J6)"

Block-by-Block

BlockKey Parts (linked)FunctionNotable Design Choices / Trade-offs
PV ChargingSL2, +V_SOLAR, J4MPPT charger maximises solar harvest while safely charging Li-ion.CN3791 offers >90 % PV utilisation but needs good layout for thermal dissipation.
Battery RailVBAT, J51-cell Li-ion removable.Directly feeds LDO and boost; no ideal-diode ORing, so reverse-polarity protection is absent.
Boost ConverterSL1, VBOOST, BOOST_ENSteps 3-4 V up to ~5 V for sensors needing higher V or USB-UART flashing.MT3608 is easy to source, but >1 MHz switching may radiate – careful PCB plane design needed.
3.3 V LDOU1, C2Quiet 600 mA rail for ESP32 and logic.LDO wastes power at high Vin (boost active). Could be bypassed by feeding ESP32 directly from battery for better efficiency.
MCU / CommsU2Wi-Fi, BLE, I²C, UART, ADC; central data processing.Module avoids RF layout complexities, eases certification, but fixes antenna position constraints.
Level-Shift & AutoprogramQ2, Q3, R3, R4MOSFETs form the classic RTS/DTR auto-reset circuit for USB-UART programming & IO0 boot mode.Resistor values 10 kΩ are standard; ensure gate actually reaches 3.3 V for reliable switching.
User InterfaceLED1, R1, SW1, SW2Basic status LED and two tactile buttons.LED current set to ~3 mA (1 kΩ) – power-friendly.
Sensor InterfacesNotes reference: MH-Z19 CO₂ (UART) and VEML6075 UV (I²C) modules. Connect through headers/JST.Modular sensors allow swaps / upgrades.UART vs I²C split reduces bus loading, simplifies software.
ConnectorsJ1–J6, QwiicExternal access for sensors, programming, power.PH series provides locking, but thru-hole raises board height; could switch to SMT to save assembly time.
3. Step-by-Step Operation
  1. Energy Harvesting & Storage
    • PV panel feeds CN3791 → tracks max power point, charges Li-ion at up to 2 A (config-dependent).
    • Battery voltage (VBAT) available even without sunlight.
  2. Power-Up Sequence
    • VBAT enters U1 → stable 3.3 V rail for logic.
    • If firmware asserts BOOST_EN, MT3608 turns on, creating 5 V (VBOOST) for high-current sensors or external peripherals.
  3. Boot & Programming
    • RTS/DTR from USB-UART header (J6) toggle Q2/Q3 to pull ESP32 EN and IO0, enabling auto-flash.
  4. Sensor Polling
    • ESP32 I²C pins (IO22/IO21) communicate with VEML6075 via Qwiic connector (J3).
    • UART0 secondary (IO16/IO17 via J1 & J2) exchanges data with MH-Z19 CO₂ module.
  5. Data Processing & Transmission
    • ESP32 calculates UV index & CO₂ ppm, assesses risk, lights LED1 or pushes notifications over Wi-Fi/BLE.
  6. Low-Power Mode
    • Boost disabled, peripherals off, ESP32 in deep-sleep (~10 µA). AP2112 quiescent is ~55 µA, still the dominant drain.
4. Key Technologies & Dependencies
  • 2.4 GHz Wi-Fi/BLE stack (ESP-IDF or Arduino-ESP32).
  • I²C & UART drivers for sensor modules.
  • MPPT charging firmware optional (CN3791 is HW-only).
  • Power-budget analytics to balance sample interval vs battery.

5. Challenges & Performance Considerations

AreaRisk / ChallengeMitigation
Power EfficiencyDual regulation (boost → LDO) wastes ~30-40 % under certain loads.Feed ESP32 directly from battery (3.0-3.6 V safe) or swap boost for buck-boost.
RF InterferenceMT3608 1.2 MHz switch node near ESP antenna degrades RSSI.Place boost far from module, shield in ground pour, keep FB loop short.
ThermalCN3791 in TO-252 or QFN must dissipate up to 3 W.Large copper plane on bottom, thermal vias.
I²C Pull-upsNone present on SCL/SDA nets; bus may float.Add 4.7 kΩ to 3.3 V near connector.
DecouplingOnly two 1 µF caps shown; ESP32 spec calls for multiple 0.1 µF close to VDD & VDD3P3.Add at least 3 × 0.1 µF + 10 µF bulk near module.
DRC ErrorsCurrent design shows “Overlapping Copper”, “Floating Copper”, “Airwires”. Must be fixed before fab.Re-run layout, clear errors.
6. Opportunities for Improvement & Future Enhancements
  1. Electrical
    • Add reverse-polarity & input surge protection (Schottky + TVS).
    • Replace discrete level-shift with dedicated USB-to-UART (e.g., CP2102N) for simplicity.
    • Integrate charging fuel-gauge IC for accurate battery state-of-charge.
  2. Power Architecture
    • Adopt single buck-boost PMIC (e.g., TPS63070) to generate 3.3 V directly from 2.5-5.5 V, eliminating LDO + boost.
    • Use deep-sleep wake via RTC timer or UV/CO₂ threshold interrupts to extend battery life >1 week.
  3. Sensing & UI
    • Add temperature/humidity (SHTC3) and particulate matter (PMS5003) for full AQI.
    • Provide small e-Paper display for always-on exposure index.
  4. Firmware & Cloud
    • MQTT/HTTPS push to open-source air-quality networks (e.g., Ambient Weather, ThingSpeak).
    • BLE beacon mode for smartphone dashboards without network.
  5. Mechanical / Production
    • Conformal-coat for outdoor use.
    • Custom enclosure with integrated solar cell window and light guide for status LED.
  6. Scalability
    • Design as stackable module; additional sensor “shields” on same I²C bus.
    • Upgrade to ESP32-C3 (RISC-V, BLE5 LE) for lower power and Matter/Thread compatibility.
7. Conclusion
The current schematic provides a solid foundation for an IoT-connected environmental exposure meter with solar charging and dual-sensor support. Key strengths are the use of a proven ESP32 module and MPPT charger. Addressing power-path efficiency, decoupling, and DRC issues will greatly improve reliability. With iterative hardware tweaks and richer firmware, the design can scale into a versatile platform for portable air-quality and UV monitoring in consumer, educational, or occupational-safety settings.
Optimize power design
Improve decoupling strategy
Address RF interference
Reassess sensor interface
305 days

Ready.

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SL1
UART0_RX
U1
Manufacturer Part Number
AP2112K-3.3TRG1
SW2
ESP32_EN
UART0_TX
UART0_RX
+3.3V
BOOST_EN
+3.3V
ESP32_EN
ESP32_RX2
RTS
ESP32_EN
UART0_TX
IO0
ESP32/I2C_SCL
+3.3V
ESP32/I2C_SDA
SW1
ESP32_TX2
VBAT
+3.3V
Note 1
Line 1
Sensor MH-19
Line 2
Intelligent Infrared CO2 Module
Line 3
------------------------------------------------------------
Line 4
https://www.winsen-sensor.com/d/files/PDF/Infrared%20Gas%20Sensor/NDIR%20CO2%20SENSOR/MH-Z19%20CO2%20Ver1.0.pdf
J1
DTR
ESP32/I2C_SCL
+3.3V
Note 2
Line 2
Sensor VEML6075
Line 3
UV Light Sensor
Line 4
-----------------------------------------------------------
Line 5
https://www.sparkfun.com/products/15089
+3.3V
LED1
U2
J3
DTR
ESP32/I2C_SDA
BOOST_EN
C1
Capacitance
1u F
VBOOST
ESP32_RX2
J2
R4
Resistance
10K Ω
+3.3V
IO0
VBAT
J5
ESP32_TX2
ESP32/I2C_SCL
R1
Resistance
1K Ω
IO0
RTS
Q3
Manufacturer Part Number
BSS138
ESP32/I2C_SDA
R3
Resistance
10K Ω
Q2
Manufacturer Part Number
BSS138
C2
Capacitance
1u F
VBOOST
J6
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ENVIRONMENTAL EXPOSURE RISK METER thumbnail
Environmental exposure risk measuring device based on ESP32, ultraviolet light (UV) and CO2 gas sensor modules. It has a voltage booster based on MT3608 chip and a solar panel lithium battery charger with MPPT based on CN3791 chip.

Properties

Properties describe core aspects of the project.

Pricing & Availability

Distributor

Qty 1

Arrow

$0.46–$0.79

Digi-Key

$1.53–$9.74

LCSC

$6.21–$6.31

Mouser

$0.60

Verical

$0.35–$0.65

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