This Sensor Will Never Run Out of Power

The IoT (Internet of Things) is great, but power is always a challenge. If there is a mains outlet nearby, then great! If not, things go downhill fast. Replacing batteries every few months is a huge inconvenience and solar only works during the day. To eliminate the worst of both worlds, Juan Flores built the everAliveSensor that will never run out of power.

The name may be a tiny bit misleading, because this build isn’t about any specific sensor. It can work with a wide range of sensors, including a BME680 temperature/humidity sensor, as shown in the demo schematic.

The point of this device is achieving “infinite runtime” with any suitable sensor and a network-connected microcontroller. In an ideal real-world scenario, the user would be able to put an everAliveSensor somewhere with even slight sunlight and then never think about it again.

It overcomes the traditional issues of both solar and batteries, because it has a solar panel and a supercapacitor. If you aren’t familiar, a supercapacitor is basically just a really big capacitor with almost instant charge/discharge and none of the cycle degradation of traditional rechargeable batteries. The charge and discharge rates aren’t particularly important for this application, but the longevity certainly is.

The solar panel recharges the supercapacitor when there is enough sunlight and the supercapacitor keeps everything going through the night.

But we’re still talking about very little energy the device can count on at any given time, so most of Flores’ effort went into efficiency and optimization.

The microcontroller, an ESP32-C6, is normally in a deep sleep. Every five minutes, it wakes up, samples the BME680 (or whatever sensor is in use), puts together a very simple BLE advertising packet with a 6-byte payload, fires that off four times over a 20ms period, then shuts down the peripherals and goes back into deep sleep. Because that BLE advertising doesn’t adhere to normal conventions, Flores used a second ESP32 as a receiver and it looks for that specific pattern.

The entire wake period should only last about 200ms and should only consume 21.68mA on average during that time. The five-minute interval is 300,000ms, so power consumption is very nearly 0A for about 99.93% of the day. With so little energy needed, a full supercapacitor (23.61mAh) would let the everAliveSensor run for more than 19 days in absolute darkness. With even occasional sunlight, it can run indefinitely until the hardware itself fails.