April 2026 — on building a survival loop for a car that spends its summers in a greenhouse, and why ‘propping it up’ is an engineering challenge

When you store an Audi A8 for the summer, you face a classic air-suspension paradox. The car wants to stay level, but the leveling system needs power. If you leave it on the battery, it eventually drains itself trying to keep its posture. If you disconnect the battery, it slowly sinks.

This is the real-world narrative behind Project Sentinel. It’s about a car standing idle in a greenhouse, catching enough sun through a flexible PV panel to keep the main battery topped up and the telemetry alive while it waits for the next drive.

The Problem: The Summer Sink

The A8 is a masterpiece, but it’s a power-hungry one. During long periods of storage, the air suspension requires periodic adjustments. Without a maintainer, the car will eventually bottom out.

The goal of the Sentinel is to act as a low-power watchman. By using a flexible PV panel positioned to catch the sun entering the greenhouse, we can keep the primary car battery in a healthy state, powering the suspension and our monitoring loop without the risk of a “black start” scenario.

Part One: From Breadboard to Custom Silk

Engineering often starts with a mess of jumpers, but for a car that spends its life in a greenhouse, “messy” is another word for “failure point.” Damp environments and temperature swings are the natural enemies of loose connections. To honor the A8, I moved away from the breadboard and into a custom-designed PLC.

There is a specific, tactile reward in hardware that’s hard to find in pure software. Remember the feeling of buying a physical music album? Taking off the plastic wrapping, smelling the fresh ink on the liner notes? That’s exactly what it feels like to open a package of fresh, custom silk-screened PCBs.

Moving the trigger and echo pins to GPIO 32 and 33 settled the logical conflicts, but the Masterboard PCB settled the physical ones. Clean traces, patient solder joints, and a dedicated circuit are what turn a prototype into a watchman you can actually trust when you’re kilometers away.

Part Two: Designing for Storage (Power Modes)

Building for storage means every milliamp counts. I implemented a tiered power strategy designed to protect the A8’s primary battery:

• Online Mode (>12.6V): Fresh sun is hitting the panels. The system is fully awake, syncing high-resolution telemetry every 10 minutes.
• Eco Mode (12.2V - 12.6V): The sun has gone down or the sky is overcast. We drop to hourly syncs to conserve power.
• Survival Mode (11.5V - 12.2V): Total modem shutdown. To save the car battery, we stop all cellular communication. All height and voltage data is logged silently to the SD card.
• Limp-Home Mode (<11.5V): Deep sleep emergency cycle. The Sentinel shuts down completely to ensure there is enough juice left in the A8 to prevent total battery failure.

Part Three: The OTel Watchtower

How do I know the car is still level while I’m kilometers away? That’s where OpenTelemetry (OTel) comes in.

The Sentinel isn’t just a logger; it’s a first-class citizen in my monitoring cluster. After carving out the right Ingress paths and aligning the service discovery, I finally saw the “Moment of Truth”—the first OTLP packet arriving at the gateway.

The trophy isn’t just a log line; it’s this JSON record sitting in the Prometheus database:

{
  "status": "success",
  "data": {
    "resultType": "vector",
    "result": [
      {
        "metric": {
          "__name__": "otel_a8_sentinel_battery_voltage",
          "service_name": "a8-sentinel",
          "instance": "13.5V-fixed-test"
        },
        "value": [1776527248.123, "13.5"]
      }
    ]
  }
}

By centralizing metrics like Suspension Height (via Kalman-filtered ultrasonic pings) and Main Battery Voltage, I now have a live, redundant watchman. If the voltage drops below 11.5V or the height deviates in the greenhouse, the cluster-shepherd alerts me instantly.

The Result: Rewarding Effort

To keep the internals safe, I designed a dedicated enclosure in Onshape. It’s a custom-fit home for the PLC, the modem, and the sensors, complete with IP67 glands to keep the greenhouse humidity at bay.

There is an immense sense of reward in this kind of multi-disciplinary effort. It takes work to align the 3D model, the PCB traces, and the OTel ingestion paths, but the result is a functional solution built with accessible tools—calipers, KiCad, and some patience with a soldering iron.

The entire hardware design and firmware are now open-source. For a project that started with a few concerns about a sinking suspension, seeing the data finally ticking away on a production dashboard—knowing every wire is secured on a custom board—is the satisfying conclusion of the build phase.

The greenhouse watchman is officially on watch.

🚀 Explore the A8-Sat Sentinel Archive on GitLab

Mission complete. The Sentinel is synced.