Hello Johannes,

you may know that already, but you could shrink the whole device considerably by using a brightness sensor like TSL2591 and a µC like RasPi Pico which drives then two or more 7-Segment-Displays.

Expenditures for such a device would be between 15€ - 20€, so, yes, probably more expensive than your system if you are using recycled parts.

Grüße aus Nürnberg

Thank you for your tip!

Indeed my costs were very low, I think I only ordered opamp and diode for <5€, everything else including wood for the case was reused from scrap or old leftovers (which of course doesn't mean they are free).

One thing you do not want in your darkroom is a glowing display bringing more light into the dark which is why I opted for a big moving coil instrument. But I get your point, it does have benefits to not reinvent the wheel and use of the shelf sensors, one of them being calibration.

Grüße aus Frankfurt

Ah yes of course. Then analog instrument is the best solution. 

That's cool. 👍 Many decades ago, a light meter on the same principles increased my proportion of correctly exposed paper. Now all you have to do is couple it to a enlarger lamp timer.

Thank you!

For now it will be done by hand. But how could the light meter be coupled to the timer? It had to be in two steps I guess, measuring light intensity and calculating exposure time through a empirically selected factor. Then in the second step exposure the photopaper with the calculated time.

Send an analog signal to the MCU controlled timer and then it's just a matter of software to do the computation.