Project Specification
Project Overview
Status: Draft — schematic design in progress.
One universal carrier PCB for a removable LilyGo TTGO T-Beam V1.2 (ESP32, SX1276 915 MHz, GPS). Optional sections are populated as needed for maple-sugar-camp monitoring and control.
Intended Use
Outdoor field nodes in Quebec maple forests. Remote vacuum nodes normally use one solar-charged 18650 and prioritize standby efficiency. Tank-level and relay/control nodes normally use external DC power.
What the Device Should Do
- Mount the T-Beam on removable 2.54 mm female headers.
- Accept a 3.0–4.2 V single-cell 18650 supplied by an external CN3065 solar charger.
- Accept protected center-positive 5–12 V DC barrel input with automatic external-power priority.
- Prevent source-to-source backfeed and prevent the barrel input from charging the 18650.
- Support optional vacuum, ultrasonic, DS18B20, and two relay sections on one PCB.
- Stop after a complete verified schematic. Do not place or route the PCB without approval.
Main Features
- Removable T-Beam V1.2, 915 MHz SX1276 variant.
- 0.5–4.5 V three-wire vacuum transducer input.
- 5 V TRIG/ECHO ultrasonic interface.
- 3.3 V DS18B20 interface.
- Two optional SRD-05VDC-SL-C relay channels with transistor/MOSFET drivers and dry-contact terminals.
- Expansion header with safe GPIO, I2C, 5 V, 3.3 V, and GND.
System Architecture
- Protected 5–12 V barrel input generates a high-current 5 V rail for externally powered configurations.
- Battery remains a direct low-loss source for the T-Beam power path.
- A low-IQ priority mux selects external-derived battery-equivalent power or the real 18650 for the T-Beam battery input, preventing backfeed and avoiding barrel charging.
- A separate low-IQ battery boost supplies the auxiliary 5 V rail when external power is absent; an output mux gives priority to the external high-current 5 V regulator.
- The battery boost is sized for normal T-Beam/sensor field use and intermittent optional loads, not continuous dual-relay operation. The external path provides the full approximately 2 A peak capability.
Hardware Subsystems
Power
- 18650 input: 3.0–4.2 V, externally solar charged by CN3065.
- Barrel input: 5–12 V DC, center-positive assumption, with fuse/PTC, reverse-polarity protection, transient suppression, and filtering.
- No electrical path may charge the 18650 from the barrel jack.
- Local 100 nF decoupling and suitable bulk capacitance, including at least approximately 100 uF on transient-prone rails.
T-Beam Interface
The official V1.2 schematic confirms GPIO27, GPIO32, and GPIO33 are occupied by the SX1276 LoRa interface; GPIO34 is occupied by GPS. They cannot be used as originally requested.
Provisional conflict-free mapping using exposed pins:
- VAC_ADC: GPIO36 (ADC1, input-only)
- ULTRASONIC_ECHO: GPIO39 (input-only)
- ULTRASONIC_TRIG: GPIO25
- DS18B20_DATA: GPIO4
- RELAY1: GPIO13
- RELAY2: GPIO14
- I2C expansion: GPIO21 SDA / GPIO22 SCL, shared with the onboard PMU bus
GPIO2 and GPIO15 are boot-strapping pins and will not be assigned to required functions. GPIO0 and UART0 pins remain reserved for programming/boot use.
Vacuum Interface
- 5 V / GND / SIGNAL screw terminal.
- Divider targets approximately 3.0 V ADC input at 4.5 V sensor output.
- RC low-pass filtering and external-input protection.
Ultrasonic Interface
- 5 V / GND / TRIG / ECHO field connector.
- 3.3 V TRIG is provisionally assumed acceptable; footprint/support options should permit later buffering if the selected sensor requires a 5 V VIH.
- ECHO scaled and protected to no more than 3.3 V.
Temperature Interface
- 3.3 V / GND / DATA connector.
- Approximately 4.7 kOhm pull-up to 3.3 V.
Relay Outputs
- Two optional 5 V SRD-05VDC-SL-C SPDT channels.
- Independent low-side drivers, boot-state OFF pull-downs, flyback diodes, and local decoupling.
- NC / COM / NO screw terminals.
- Contacts used primarily as dry-contact controls for external equipment/contactors.
Interfaces and Connections
Clearly labeled POWER IN, BATTERY, 5V, 3V3, GND, VACUUM, ULTRASONIC, TEMPERATURE, RELAY 1, RELAY 2, and EXPANSION sections.
Power and Runtime Expectations
Battery mode prioritizes low quiescent current and permits disabling auxiliary 5 V loads when asleep. Relay coils are not intended for continuous battery operation. External mode supports the T-Beam, sensors, ultrasonic interface, and both relay coils simultaneously.
Power Tree and Power Budget
Preliminary load assumptions pending selected-part verification:
- T-Beam transient demand is supplied through its battery-equivalent input path.
- Two 5 V Songle relay coils are approximately 0.36 W each, about 72 mA each nominal.
- Sensor and interface loads are small relative to the external 2 A design target.
- Battery-only auxiliary 5 V rail need not deliver 2 A continuously; it must support sensing loads and practical intermittent optional loads.
Manufacturing and Assembly Expectations
- One PCB with optional DNP sections.
- Two layers targeted.
- Approximately 60 mm x 70 mm desirable but not mandatory.
- Field wiring uses screw terminals; T-Beam remains removable.
- Select real orderable parts with manufacturer part numbers.
Firmware-Relevant Hardware Requirements
- Relay outputs default OFF at reset and during boot.
- Vacuum uses ADC1 because ESP32 ADC2 conflicts with active Wi-Fi.
- Shared PMU I2C expansion must avoid address conflicts and excessive bus capacitance.
Physical Design Expectations
Functional areas: POWER, T-BEAM, VACUUM, ULTRASONIC, TEMPERATURE, RELAY 1, RELAY 2, EXPANSION. Relay contacts must later be physically separated from RF, analog, and logic circuitry. No PCB placement or routing in the current phase.
Important Design Decisions
- External power is prioritized automatically.
- Barrel input never connects to the T-Beam USB/VBUS charging input.
- The T-Beam is powered through a battery-equivalent removable connection so the external supply cannot charge the real 18650.
- Separate external and battery auxiliary-5-V paths avoid burdening battery mode with a continuous 2 A buck-boost converter.
- Original requested GPIO27/32/33/34 assignments are replaced because they conflict with onboard LoRa/GPS.
Assumptions
- Barrel connector is center-positive.
- Exact ultrasonic model remains open; interface supports common 5 V TRIG/ECHO modules.
- The T-Beam battery-power connection will be removable in addition to its mechanical female headers.
- Battery cell and its external solar charger/protection are suitably rated for the selected battery-mode peak current.
Change Notes
- Replaced the earlier USB-C environmental-node scope with the universal maple-camp T-Beam carrier requirements.
- Added dual-source architecture and external-priority behavior.
- Revised battery optimization after clarification that relay nodes normally use external power.