Digital Engine Tachometer with ESP32 and Optocoupler

🎬 Video Transcript

Introduction

In this video we’ll set up a digital tachometer for our engine. This is another engine-monitoring project using the super-useful ESP32 microprocessor. In other projects we set up engine-temperature monitoring, oil pressure, and tank-level monitoring with an ESP32. For this tachometer, we’ll use the same ESP32.

The ESP32 functions as a remote hub with numerous ports to connect a wide variety of sensors. It collects data from these sensors and transmits it wirelessly to our central system, a Raspberry Pi running Home Assistant. If you haven’t set up an ESP32, don’t worry—I have a separate video guide dedicated to that with a quick and easy setup.

For this project we’ll use an optocoupler module that costs around two dollars and is straightforward to implement. We connect two wires from the optocoupler to the engine’s alternator, link the optocoupler to the ESP32, and perform some basic configuration in Home Assistant. The optocoupler detects electrical pulses from the alternator stator in an isolated circuit using light, enabling RPM measurement. It’s all very easy to set up.

Integrating tachometer monitoring into the smart-boat system offers several advantages: alerts when engine RPM exceeds thresholds, counting engine hours, and historical data for insights into fuel-consumption patterns based on RPM. The tachometer can also serve as a binary entity for other automations to indicate whether the engine is running. For example, the low-oil-pressure automation can check if the engine is running before deciding if low oil pressure is actually an issue. You’ll also benefit from a sleek digital gauge on your dashboard.

Wiring up ESP32 and Optocoupler

Here we have the optocoupler module. One end connects to the alternator; the other end connects to the ESP32. There’s a small LED that illuminates when there’s an input signal.

My ESP32 is already provisioned as in my ESP32 video. The red cable goes to 3.3 V, the white cable goes to pin 25, and the black cable goes to ground. On the optocoupler: the red 3.3 V cable plugs into VCC, the white cable from pin 25 plugs into OUT (the data terminal), and the black ground plugs into GND.

On the input side, the black cable comes from your engine or alternator ground. The red cable goes to the + terminal on the optocoupler and then to your alternator. On the alternator, use either the W terminal, the stator terminal, or the tacho terminal—it depends on the alternator. If you have a smart regulator, connect to its tacho output. If you need to share that connection with an existing analog tachometer, use a double spade adapter.

Normally you power the ESP32 with a buck converter (12 V to 5 V). For this demo I’m using a USB connector. That’s all that’s needed to connect the alternator tachometer to our smart-boat system.

Add to Home Assistant

In Home Assistant, we need an ESP32 already provisioned. I’m using ESPHome, and the ESP32 here is flashed with the basic YAML that ESPHome provides. To add the tachometer, scroll to the end of the YAML and paste in the tachometer configuration from my code page. I’ve included the YAML there to configure the optocoupler with the ESP32. I’ve also added some optional YAML I’ll explain in a moment.

Make sure the logger level starts at debug so we can calibrate the tachometer. Check that a logger block isn’t already present; if it is, change its level instead of adding another. The tachometer uses a pulse counter platform in ESPHome, which we’ll calibrate to become RPM. Leave everything the same: pin 25 is the pin we used in the wiring. If you changed the pin on your ESP32, this is the only thing you need to change.

For calibration, we’ll connect the optocoupler to the alternator, turn the engine on, and collect data points. For my engine, at 1000 RPM I was getting about 15,200 pulses per minute. At 1200 RPM I was getting about 18,200. You can add more points. ESPHome will calibrate a line that converts alternator pulses to engine RPM. The additional YAML I added includes engine hours and a binary sensor that indicates when the engine is active. They’re optional—remove them if you don’t need them. Save, validate, and install over-the-air.

RPM Calibration

When the logs start, look for the line showing the engine tachometer’s “received counter pulses per minute.” That’s the value we’ll calibrate against actual engine RPM. Start the engine and set it to known RPM values using your analog gauge or a strobe app with reflective tape on the flywheel. For each setpoint, note the pulses per minute and the corresponding RPM.

As a quick example at idle, I see around ~13,000 pulses per minute and about 860 RPM. For my calibration I used 0→0, 1000→15,200, and 1200→18,200. I took four points and found it reasonably accurate, but you can keep going if you like. After editing the calibration, compile and upload again. Once you finish calibration, change the logger level back to info so it doesn’t spam the logs and slow the ESP32.

Add Gauges to Dashboard

To visualize the entities, go to Settings → Integrations → ESPHome, open the device, and add the entities to a dashboard. I created a dashboard called “Tacho” for this demo. Add a Gauge card for RPM. My engine maximum is 2400 RPM, and I set severities with green around idle (~800), yellow at 2000, and red at 2200 and above. Turn the engine on and it should show the correct RPM. You’ll also see the engine_active binary entity and engine_hours, which you can use in automations.

Outro

That’s it—tachometer monitoring integrated into our smart-boat system. If you found this helpful and informative, please like the video and consider subscribing for more boating and technology projects.