Temperature and Humidity Sensor Design
0:00
- Updated project description to “SDI-12 Open-Collector Interface on Swan GPIO D5”
- Added and placed NPN transistors for TX and RX stages:
- Added and connected resistors:
- Made signal connections:
- Q1:E → GND
- Q2:E → GND
- R5:P1 → U1:D5; R5:P2 → Q1:B
- R6:P1 → SDI12_BUS; R6:P2 → 5 V
- R7:P1 → SDI12_BUS; R7:P2 → Q2:B
- R8:P1 → U1:D5; R8:P2 → 3.3 V
- Renamed nets:
- SDI-12 bus net → SDI12_BUS
- Swan GPIO D5 net → SWAN_D5
- Updated design notes with open-collector operation, logic inversion, and pull-up values
- Ran ERC/DRC checks without violations
- ✅ TX stage isolates MCU and drives SDI-12 bus with 5 V pull-up via Q1
- ✅ RX stage buffers SDI-12 to 3.3 V with Q2 and proper resistors (100 kΩ, 10 kΩ) and inversion noted
- ✅ Integration preserves L1 EMI choke and D1 ESD diode on SDI12_BUS
- ✅ ERC/DRC validations passed and design notes reflect implementation details
Reviews
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ag telemetry prototype
Properties
3.3
Latest ERC/DRC checks have been run via @review; see Reviews panel for any remaining violations or warnings related to SDI-12 interface changes.
flowchart TD Charger["LiIonSolarChargerBlock"] Gauge["BatteryGauge"] Reg3V3["Regulator3V3"] Reg5V["Regulator5V"] BW["BluesWirelessNotecard"] MCU["SwanMCU"] BLE["BLEProvisioningModule"] SDI["SDI12Interface"] Loop4_20["FourToTwentyMilliampInterface"] DI["DigitalInputInterface"] PC["PulseCountingInterface"] TB["TerminalBlock12Position"] Charger --> Gauge Charger --> Reg3V3 Charger --> Reg5V Reg3V3 --> BW Reg3V3 --> MCU Reg3V3 --> BLE Reg3V3 --> SDI Reg3V3 --> Loop4_20 Reg3V3 --> DI Reg3V3 --> PC Reg3V3 --> TB Reg5V --> TB BW --> MCU BLE --> MCU Gauge --> MCU MCU --> SDI MCU --> Loop4_20 MCU --> DI MCU --> PC SDI --> TB Loop4_20 --> TB DI --> TB PC --> TB
50000
100
Agriculture
Ultra-low-power solar agricultural telemetry node with duty-cycled sensing and cellular uplink
Solar + Li-Ion Battery
Mermaid
BOM Component Selections and Power Budget Solar Input: - Panel: 1.2 W, 6 V nominal (I_max ≈ 0.2 A) - Use single-panel input to MCP73871 solar-friendly Li-ion charger (U4). - Front-end protection: TPS25210 (U8) for reverse-polarity and inrush control, LTC4365 (U9) and TPS25962 (U10) for OV/UV and eFuse-style protection. Battery and Gauge: - Cell: Single-cell Li-ion/LiPo, nominal 3.7 V, capacity TBD (e.g., 2000–5000 mAh depending on deployment). - Charger: MCP73871-2CCI/ML (U4), configured for solar input via VPCC and PROG pins. - Fuel gauge: BQ27441-G1 (U5) on I2C with pull-ups (R1–R3). Regulators: - 3.3 V rail: TPS62840DLCR (U6), high-efficiency buck, selected for ultra-low quiescent current to minimize standby drain. - 5 V rail: TPS61030PWP (U7), boost converter for 5 V sensor/IO needs, duty-cycled to reduce average consumption. Protection and I/O Conditioning: - ESD: 0603ESDA-MLP7 (D1–D4) on SDI-12, 4–20 mA, Digital Input, and Pulse Counting lines. - EMI filters: 10 uH generic inductors (L1–L4) inline with each sensor channel. - Current sense: 500 Ω shunt (R4) for 4–20 mA loop, instrumentation amplifier INA333AIDGKR (U11). Terminal Block: - Phoenix Contact 1875522 (TB1) 12-position, 3.81 mm pitch for SDI-12, 4–20 mA, Digital Input, Pulse Counting, and expansion. Core Compute and Connectivity: - MCU: Blues Swan Feather 3.0 (U1) as main microcontroller. - Cellular: Blues Note NBNA (U2) for cloud uplink. - BLE: Raytac MDBT50Q-P1MV2 (U3) for provisioning. Power Budget (from 1.2 W, 6 V Panel): - Panel electrical limits: P_max = 1.2 W, V_nom = 6 V, I_max ≈ 0.2 A. - Assuming MPPT/charger efficiency ~85% and Li-ion battery voltage ~3.7 V: usable battery charge power ≈ 1.0 W. - Resulting average charge current into battery: I_charge_avg ≈ 1.0 W / 3.7 V ≈ 270 mA (best-case full sun). System-Level Budgeting Notes: - Daytime: allow higher system activity (MCU + Notecard + radio + sensors) while surplus panel power is available. - Night / low-irradiance: system primarily runs from battery; MCU and radios heavily duty-cycled with regulators kept in low-IQ modes. - Design target: average system consumption over 24 h kept well below the panel’s daily energy harvest at deployment site (weather and latitude dependent). - Panel and battery sizing margins to be revisited once detailed per-mode current profiling is available (sleep, sample, transmit, fault conditions). SDI-12 Interface: - SDI-12 bus is implemented as an open-collector bus on net SDI12_BUS. - TX path: Q1 (MMBT2222A NPN) has emitter at GND, collector on SDI12_BUS, and base driven from Swan GPIO D5 (net SWAN_D5) via R5 = 1 kΩ. When SWAN_D5 is driven high, Q1 sinks current and pulls SDI12_BUS low; when SWAN_D5 is low or high-Z, SDI12_BUS is released. - Bus pull-up: R6 = 4.7 kΩ from SDI12_BUS to 5 V defines the idle high level for the SDI-12 line. - RX path: Q2 (MMBT2222A NPN) has emitter at GND, base connected to SDI12_BUS via R7 = 100 kΩ (net SDI12_RX_BUF), and collector tied to Swan GPIO D5 (net SWAN_D5) with R8 = 10 kΩ pull-up to 3.3 V. - Logic behavior: When SDI12_BUS is high (idle), Q2 base is biased, Q2 conducts, and pulls SWAN_D5 low against R8, so the RX stage inverts the SDI-12 line. When SDI12_BUS is pulled low, Q2 turns off and R8 pulls SWAN_D5 high. - Firmware considerations: Swan D5 is shared for TX and RX. Firmware must account for the inverted polarity on SWAN_D5 when receiving SDI-12 (e.g., by inverting bits in software or configuring an inverted UART/bit-bang routine) and must ensure direction control so D5 never actively drives the bus high. - Open-collector operation: Only open-collector (or open-drain) devices are allowed to pull SDI12_BUS low. No device may drive SDI12_BUS high; the bus high level is solely defined by R6 to 5 V (on the field side) and R8 to 3.3 V (on the MCU side via Q2’s transistor action). This preserves SDI-12 multi-drop compatibility and prevents contention with external sensors’ open-collector drivers. - Voltage domains: SDI12_BUS is at 5 V logic high for compatibility with SDI-12 sensors, while Swan D5 operates at 3.3 V. The Q1/Q2 transistor stages and resistors (R5, R6, R7, R8) provide level shifting and protect the MCU from the 5 V bus.
Battery
Cellular
Arduino
Pricing & Availability
Distributor | Qty 1 |
|---|---|
Arrow | $21.25–$23.22 |
Digi-Key | $81.26–$85.25 |
LCSC | $177.63 |
Mouser | $27.41 |
TME | $6.22 |
Verical | $18.39–$23.26 |
Controls
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