Батарея CR2032 3V In default mode (alarm mode) you put it into the soil near a plant and it emits a tiny chirp when the soil is dry, reminding you to water the plant. But we go to switch it to sensor mode to capture measurement data. You need to switch Chirp from Alarm mode to Sensor mode by reset (small button on Chirp). After reset Chirp reads capacitance and light levels. That can take from 1 up to 9 seconds if dark. If any I2C communication is received during that time, Chirp will switch into sensor mode - it will not chirp, just respond to I2C requests. See wemakethings dot net/chirp/
Really cheap and work's nicely