We have observed the He I D3 λ 5876 Å line as a chromospheric activity indicator in 36 stars (34 F-type and 2 G-type) belonging to the Hyades open cluster, the Ursa Major group and the field. Complementing our data with measurements taken from the literature, we have correlated the equivalent width and absorbed energy flux of the He I line with other chromospheric and coronal indicators across the F-K range of spectral classes. We have confirmed that D3 is a particularly suitable chromospheric activity indicator for the early F range, and have used it to set limits to the onset of chromospheres (B - V ˜ 0.3) and the onset of magnetic activity (B - V ˜ 0.42), as well as to assess the kinds of mechanisms which heat chromospheres in that range, finding acoustic waves to be the most probable dominant energy source.