rpi-seism — Open Source 3-Axis Seismograph

Hardware design

Two-Board
Architecture

All analog electronics co-located with the geophones to eliminate interference. Only clean digital RS-485 data travels over the Cat5/6 cable back to the host.

Sensor side

Sensor PCB

01

Co-located with the geophones to keep analog paths as short as possible. Differential instrumentation amplifiers feed a 24-bit ADS1256. An MCU samples all channels and streams framed packets over RS-485.

3x differential inst. amp — gain x100
ADS1256 — 24-bit, PGA up to x64
Arduino / ESP32 / RP2040 MCU
THVD1406 RS-485 transceiver
12 V input, onboard regulation
JST XH2.54 geophone connectors

ADS1256 INA(TBD) gain x100 THVD1406 12 V → 3.3/5 V

Host side

Host Interface

02

Compact interface board sitting between the Cat5/6 cable and the Raspberry Pi. Handles RS-485 conversion, power injection, and remote sensor-board control via DTR/RTS signals.

RS-485 → UART at ≥250k baud
MOSFET — remote power switch
DTR/RTS control lines
RJ45 + 12 V barrel jack input
USB-C to host (Raspberry Pi)

RS-485 → UART DTR/RTS MOSFET PWR RJ45

Validation

Test Log

Incremental hardware validation from first prototype to best configuration.

T-01 ✕

AD620 Module + Oscilloscope

Geophone connected to AD620 module, powered from Pi 5 V rail. Offset trimmed to 2.5 V. Output clipped at ~3.8 V (non-rail-to-rail swing). Signal significantly noisier than raw geophone output due to flying wires and single-ended measurement.

FAIL

T-02 ✕

AD620 + ADS1256 Single-Ended

AD620 Vout → AIN0, GND → AGND. PGA=1, 30k SPS. Signal clipped below 0 V — AINCOM tied to GND via PCB via, not modifiable without desoldering. MiniSEED recording confirmed working. Clipping makes data unusable.

FAIL

T-03 ✓

Geophones Directly into ADS1256 — Differential

AD620 bypassed. Two geophones differential on AIN0/AIN1 and AIN2/AIN3. PGA=64, 2000 SPS internal, read via Arduino Nano at 100 SPS/ch, 250000 baud. Significantly cleaner signal. Negative values correct. Heartbeat watchdog implemented. PGA=64 alone insufficient for distant events — external gain stage required on final PCB.

BEST

Firmware protocol

Data Link
Protocol

The serial protocol between the MCU and the Raspberry Pi acquisition daemon.

Baud rate

250,000 bps

RS-485 over Cat5/6, reliable up to 50 m. Half-duplex, differential signalling.

Packet frame

HDR · CH · DATA · CRC

Header, channel ID, raw 24-bit ADC value, checksum. Ensures integrity over the RS-485 link.

Heartbeat watchdog

≤ 1 s timeout

Host sends a heartbeat each second. If absent, MCU halts the data stream automatically.

Runtime config

Sent at connect

PGA, ADC data rate, MUX assignments, and output SPS sent by host at first connection — nothing hardcoded in firmware.

Sample rate

100 SPS / ch *

Default, software-configurable at connect time. ADS1256 internal rate up to 30k SPS, decimated in firmware.
* no reflashing required

Archive format

MiniSEED / SDS

SeisComp SDS archive on host. StationXML with GD-4.5 PAZ instrument response. ObsPy / SeisComP compatible.

ADS1256 — VREF = 2.5 V

Input Range
by PGA Setting

External amplifier gain x100 applied before the ADC. Absolute pin range: -100 mV < AINx < AVDD + 100 mV.

PGA x 1

± 5 V

PGA x 2

± 2.5 V

PGA x 4

± 1.25 V

PGA x 8

± 625 mV

PGA x 16

± 312 mV

PGA x 32

± 156 mV

PGA x 64

± 78 mV

max

Source code

Repositories

Five repositories covering the full stack from MCU firmware to web frontend.

stack