Temperature and Humidity Sensor Node Design
Thought for 4 sec
I’ll build the sensor node schematic end-to-end: choose the radio MCU, size the USB-C power path and protection, add the T/RH sensor and support passives, then wire and verify it.
Worked for 3 mins 1 sec
I’ve narrowed the architecture: ESP32-C3 module, SHT41 sensor, protected USB-C input, and a buck regulator to stay cool under Wi-Fi bursts. I’m verifying datasheet details and sizing the power path now.
Worked for 7 mins 54 sec
Architecture is locked and documented. I’m adding the full component set now in functional blocks, then I’ll wire power, USB, MCU support, and the sensor bus.
Worked for 13 mins 25 sec
Done: the USB-C Wi‑Fi/BLE environmental sensor node schematic is built, wired, and ERC-clean. I also added the supporting project files: Project Specification, Design Notes, and Firmware Starter — ESP32-C3-MINI-1-N4.
What’s in place: protected USB-C 5 V input with CC sink resistors, TVS + eFuse protection, 3.3 V buck rail, ESP32-C3 native USB, boot/reset circuitry, and SHT41 on I2C. Schematic is ready for layout; want me to go ahead?
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Design Notes
Chosen Parts
- U1: ESP32-C3-MINI-1-N4
- U2: SHT41-AD1B-R2
- J1: USB4215-03-A USB-C receptacle
- U3: TPS25940LRVCR eFuse / protected input switch
- U4: MP2122AGJ-Z synchronous buck regulator
- D1: TPD2E2U06DCKR USB D+/D- ESD protector
- D2: SMF5.0A VBUS TVS diode
Datasheet-Grounded Notes
ESP32-C3-MINI-1-N4
- Supply range: 3.0 V to 3.6 V, nominal 3.3 V.
- External supply should support up to 0.5 A.
- Wi-Fi TX peak current reaches 350 mA worst case at 802.11b 1 Mbps.
- Native USB pins: GPIO18 = USB_D-, GPIO19 = USB_D+.
- EN must not float.
- GPIO9 is a strapping pin with internal weak pull-up; pulling GPIO9 low during reset enters download mode.
- External decoupling is still required on the module 3V3 rail.
SHT41
- Supply range: 1.08 V to 3.6 V.
- Idle current: 80 nA typ at 25 C.
- Measurement current: 320 uA typ, 500 uA max while measuring.
- I2C address for SHT41-AD1B is 0x44.
- Pins: 1 SDA, 2 SCL, 3 VDD, 4 VSS.
- Typical application is direct I2C wiring with local decoupling and standard pull-ups sized per bus.
USB4215-03-A
- USB 2.0 Type-C receptacle, 16 active contacts.
- VBUS current rating: 5 A collectively across A4/A9/B4/B9.
- GND current rating: 6.25 A collectively.
- Good fit for a 5 V sink-only consumer node.
TPS25940L
- 2.7 V to 18 V operating range.
- 42 mOhm typical on-resistance.
- Adjustable current limit: 0.6 A to 5.3 A.
- Reverse current blocking.
- Programmable UVLO and OVP via resistor dividers.
- EN/UVLO must not float.
- Current-limit programming relation from TI search snippet: R_ILIM = 89 / I_LIM, with R in kOhm and I in A.
MP2122A
- 1 MHz fixed-frequency synchronous buck.
- Quiescent current: 45 uA typ.
- For 3.3 V output: use 806 kOhm upper FB resistor and 182 kOhm lower FB resistor.
- Input capacitor: 22 uF ceramic recommended.
- Output capacitor: 22 uF ceramic recommended.
- Inductor for 3.3 V output: 1 uH to 4.7 uH allowed; 1.5 uH is a standard recommended choice.
USB Protection
- TPD2E2U06: 1.5 pF typ capacitance, +-25 kV contact and +-30 kV air IEC 61000-4-2, suitable for USB 2.0 D+/D-.
- SMF5.0A: 5 V VRWM, 6.4 V min breakdown, 9.2 V max clamp, 21.7 A peak pulse current.
Power Budget
3.3 V Rail Loads
- ESP32-C3 Wi-Fi worst-case TX peak: 350 mA
- SHT41 measurement current max: 0.5 mA
- Status LED budgeted on-time current: 2 mA
- Marginal pull-ups / housekeeping allowance: 5 mA
- Total 3.3 V peak design load: about 358 mA
Reflected to 5 V Input Through Buck
Assuming 4.75 V worst-case USB input and 90% buck efficiency:
- Worst-case input current = about 276 mA
- Typical 802.11g/n active case = about 234 mA
- Light-sleep / low-activity case = about 108 mA including regulator losses and housekeeping allowance
Power-Path Sizing Decisions
- USB-C connector and input path are rated for up to 3 A capable sources.
- Actual node demand is intentionally kept below 500 mA worst case.
- eFuse current limit target: 0.6 A class to protect the board while allowing Wi-Fi startup bursts.
- Using R_ILIM = 89 / I_LIM, a 0.6 A limit gives about 148 kOhm, so use 150 kOhm 1%.
- UVLO target: about 4.1 V to reject brownout/droop on weak cables.
- OVP target: about 5.8 V to protect the downstream 5 V domain from abnormal adapters.
- Using the TPS25940 example thresholds inferred from the datasheet examples:
- UVLO divider target uses about 0.367 V at EN/UVLO.
- OVP divider target uses about 1.486 V at OVP.
- One practical set is Rtop_UVLO = 172 kOhm, Rbot_UVLO = 16.9 kOhm for about 4.1 V.
- One practical set is Rtop_OVP = 137 kOhm, Rbot_OVP = 47 kOhm for about 5.8 V.
Implementation Plan
- USB-C sink configuration with dual 5.1 kOhm CC pull-downs.
- VBUS TVS and USB data-line ESD at the connector.
- Protected 5 V path through TPS25940.
- 3.3 V generation with MP2122A buck.
- ESP32-C3 native USB, boot/reset support, and 3.3 V decoupling.
- SHT41 on I2C with a single pull-up pair for the bus.
- ERC/review cleanup and firmware starter file.
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