Are there home-brew photometers for amateur observing of variable stars and are they widely used? Comments on Why do these photometric observations of Betelgeuse look “quantized” in 0.1 magnitude steps? suggest that these are visual estimates, mostly (but not all) to 0.1 magnitude resolution.

Are there any designs for simple photodiode-based amateur photometers for stellar brightness measurements? 

Just as an illustration of the kind of thing I'm asking about, I'm imagining a pair of binoculars with a 1 mm photodiode suspended by its wires the focal plane of one eyepiece, and a Raspberry Pi recording the photocurrent (with a small amplifier) and audio simultaneously ("I'm now measuring xxx... mark!") The data would be jumpy but presumably would reach a plateau whenever the star fell on the active area of the diode.

Or is this too impractical and not backwards-compatible with existing data?
It should be doable with a standard photographic setup, a star catalog with distances and measurements for a some standard stars and a bit of software.

With the usual "ritual" for obtaining images from the sky with a given setup, chip and temperature (obtain dark frames, flat field frames and ground noise frames for division and subtraction) one can calibrate the brightness of an area (subframe around a star) in a photography by comparing it to known stars. The capture software or one of the many post processing packages for amateur astronomy can calculate the brightness of a region, if it is not totally saturated, i would guess if it is well away from any saturation.

A more detailed description: http://www.bellatrixobservatory.org/cvaaI/17/

Edit: An add-on script for one of the amateur software packages for doing photometry: https://pixinsight.com/doc/scripts/AperturePhotometry/AperturePhotometry.html.