This is a practice project to make an Arduino-based incident light meter for old meterless mechanical film cameras, probably more useful for portrait photography, since you'd have to measure direct light on subjects. And the results may not be as accurate or reliable as real light meters.
(Usually I just use an app on my phone, which is reliable enough for my amatuer film photography. I only own two or three meterless cameras which I use occasionally, so why bother to buy a real one that cost more than any of these cameras?)
The device is powered by a small power bank (connected to USB port of the Arduino); light meter info are updated on a 0.96" SSD1306 OLED each time you press the single button. The measured light level from the BH1750 light intensity sensor will be calculated into Exposure Values (EV). I set the incident-light meter calibration constant to 340 (reference). Aperture and ISO value can be set via two potentiometers. If you plug and Arduino on your computer you can get more detailed serial port data via serial port.
I used two mini breadboards glued back-to-back. But you can reproduce this on regular breadboards. The BH1750 model I used is called BH1750FVI/GY-30. GY-302 is practically the same with slightly different pin arrangement.