The MotorCombo is a 6-channel signal conditioning add-on board for the BoatOpenIO gateway. It requires a BoatOpenIO system to work — it plugs into the input board and cannot be used on its own.

It connects the analog VDO sensors of a classic engine to modern systems — without replacing a single gauge. The original instruments keep working, the same values become available digitally via MQTT and Signal K.

One board covers what a typical engine needs: two voltage divider channels for starter and house battery, three channels for the resistive VDO senders (tank, temperature, oil pressure), and one RPM channel that reads the alternator W terminal through an optocoupler and an ESP-01 that counts the pulses and outputs a clean analog value.

All conditioning resistors are through-hole and user-populated. Different VDO instruments need different divider or pull-up values, so each channel can be configured for its sensor — voltage divider for parallel tapping alongside the existing gauge, or pull-up for a directly connected sender. Two solder jumpers per channel switch between the two modes.

Note on the BOM: there is intentionally no fixed bill of materials for the conditioning resistors. Their values depend on the specific VDO sender being connected — different instruments use different resistance ranges, so a single fixed value would be wrong for most builds. Only the fixed components (PC817 optocoupler, ESP-01, connectors) have defined part numbers, listed in the GitHub repository along with the calibration approach for the variable channels.

The board connects to the BoatOpenIO input board via JST connectors, one per channel. Ground is collected on a common rail routed to a single star point.

Requires: a working BoatOpenIO gateway (ESP32, multiplexer, ADS1115). See the BoatOpenIO project for the base system.

This is an early-stage prototype design. The circuit has been validated on a breadboard; the PCB is its first production run and will go into real-world testing on a 1989 motorboat.

Full documentation, schematics and Gerber files:

github.com/bigbrainlabs/BoatOpenIO

Open source, documented with all the failures included. Part of the "Logbook Without Posing" project.