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Comment by jrflo | Hacker News Reader

I was curious about timing accuracy too, has any testing been done on this? Is it accurate to 1s, 0.1s, 0.01s? How dependent is it on the smart switch hardware?

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I played with development photos in a darkroom a long time ago. I don't think one second actually makes much difference.

Depends on how powerful of a light you’re using. 1 second on a 7 second exposure is too much. One second on a 20, not.

I don't think 1 second on a 7 second B+W exposure is that noticeable.

14% is a lot.

I think you can work around it very easily by stopping down the lens and using filters.

But I’m sure if the phone has direct connection to the device and not working through hubs it’s not anywhere near 1s of delay.

Using slow motion recording, I measured 0.1s accuracy. I'd say it's more than good enough. You can see a print on the website I made with the app and a smart plug. The average print needs between 4-60s of exposure.

As for how dependent it is on that specific type of hardware (Shelly plugs), it is 100% dependent as the app directly controls the smart plug (with the help of the router ofc). There is no way around it, no other smart switch family is currently supported.

Repeatability is more important than accuracy.

Note also that some of the best printers used metronomes to count exposure time and turn the enlarger light on and off manually.

I'd probably put the typical smart switch hardware/software stack at around 0.25 seconds. Maybe you can but I don't think I'd try to rely on anything much faster than that.

If you did, I would go in the custom firmware for the smart switch direction. A lot of them are just an ESP32 inside, and you could probably get to 0.001 seconds that way (but you are switching AC which cycles on the order of 0.02 seconds, so whatever)

If I were to tackle this project, I’d use an ESP32 and a relay with 120V contacts. That way you can use a GFCI receptacle and be code compliant if your darkroom is in a basement or the enlarger receptacle is near your darkroom sink.

If you buy something like the Sonoff S31 they have an 8266 in them that can be easily flashed

Many such projects were actually made, some of them are better documented then others. For this application I wanted something that is as off the shelf as possible.

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