# MH-DAQ instrumentation carrier

Hardware revision is printed on the board. This is a **bench prototype measurement board**, not an engine controller or a tested production instrument. No fuel-valve or generator power connection is provided. Native KiCad, BOM, schematic and Gerbers accompany this file.

Fit a **Raspberry Pi Pico (RP2040, non-wireless)** with headers to J1/J2, USB connector toward the board title. J1 is Pico pins 1–20, J2 is Pico pins 40–21, both counted from the USB end. Power through Pico USB. Do not connect external power to its reserved VSYS/VBUS pins. Use an isolated USB connection when the engine power ground is attached. Keep electronics away from the hot zone.

| Header | Pin sequence, starting at square pad 1 | Destination |
|---|---|---|
| J3 | 3V3, GND, SCK, MOSI, MISO, CS-hot, DRDY-hot, FAULT-hot | Adafruit MAX31856 #3263, **labeled wires**, not a direct plug-in module |
| J4 | 3V3, GND, SCK, MOSI, MISO, CS-cold, DRDY-cold, FAULT-cold | Second MAX31856 #3263, labeled wires |
| J5 | 3V3, GND, SDA, SCL | Adafruit INA228 #5832 logic header; bus/shunt current stays on that separate module |
| J6 | Piston sensor 0–5V, GND | Buffered position sensor output; sensor powered separately |
| J7 | Displacer sensor 0–5V, GND | Buffered position sensor output; sensor powered separately |
| J8 | Pressure sensor 0–5V, GND | Fast transducer with range above maximum cycle pressure; sensor powered separately |

MAX31856 wiring: board 3V3 → module **VIN**, SCK → SCK, MOSI → SDI, MISO → SDO, CS → CS, DRDY → DRDY, FAULT → FLT. Leave module **3Vo** unconnected. Use electrically insulated thermocouple junctions. The pin ordering on J3/J4 is a harness specification, not the order on the Adafruit board.

Analog inputs use 15 kΩ / 20 kΩ 0.1% dividers and 4.7 nF C0G capacitors: 5V becomes 2.857V at the Pico ADC. The nominal pole is 3.95 kHz, appropriate for an initial 70–90Hz waveform study; it is **not a sufficient anti-alias filter for precision PV work**. Inputs lack industrial overvoltage/isolation protection: never connect 24V sensors, a thermocouple directly, generator terminals, or a 0–10V output. Buffer outputs with source impedance ≤100Ω; calibrate divider and ADC reference.

The firmware exports raw ADC values and timestamps. A capture burst records actual sampling times, marks late samples and reads channels sequentially. Calibrate position, pressure zero/span and channel timing before calculating indicated power. No sampled rate, accuracy or fault response is claimed without a populated board.

Assembly: inspect Gerbers/drills and the BOM; fabricate FR-4 1.6mm, 2 copper layers, 1oz, plated through holes; fit resistors/capacitors then headers; clean; inspect for bridges; verify ground and 3V3 continuity with the Pico removed. First power from a current-limited isolated USB source; verify supply polarity and 3V3 before adding sensor modules. Compare the three channels at known 0/1/2/3/4/5V bench inputs. Check both thermocouples at ambient and a controlled reference temperature, and the INA228 against a reference meter and an electronic load. These bench checks are outstanding, not claimed to have happened.

Firmware: install the **RPI_PICO** MicroPython UF2 from the official download page, then copy `firmware/main.py` onto the Pico. Ctrl-C pauses slow telemetry; `capture()` records an ADC burst. Firmware version `1.1-reed` prints on every telemetry record. No Pico or populated board was attached to VH155 during this work.

Sources: [Pico hardware documentation](https://www.raspberrypi.com/documentation/microcontrollers/pico-series.html), [MAX31856 pinouts](https://learn.adafruit.com/adafruit-max31856-thermocouple-amplifier/pinouts), [INA228 module](https://www.adafruit.com/product/5832), [MicroPython RPI_PICO](https://micropython.org/download/RPI_PICO/).

Firmware 1.1-reed checks MAX31856 configuration readback and enables periodic open-wire detection for source resistance below 5kΩ. Open-wire detection occurs once every 16 conversions, so it is not an instantaneous trip. These diagnostic paths still require physical bench verification.