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  • Showing only topics in ~comp with the tag "time". Back to normal view / Search all groups
    1. Can you set a clock using a light sensor to detect sunrise and sunset?

      While pondering an off-grid microcontroller project, I got to wondering: A light sensor can obviously detect day vs night. So it could be used as a very cheap way to set a device's clock - but how...

      While pondering an off-grid microcontroller project, I got to wondering: A light sensor can obviously detect day vs night. So it could be used as a very cheap way to set a device's clock - but how accurately? To within an hour? A few minutes? How would you do it?

      Questions that arose from this include:

      • Should it detect dawn/dusk (light <-> dark transition), or noon/midnight (brighest/darkest time) ?
      • How do dawn/dusk times relate to clock time? Does it depend on lat/long?
      • If using dawn/dusk, what light level threshold to use?
      • The same threshold for dawn & dusk, or different ones?
      • Better to detect a darker threshold (start of dawn, end of dusk) or a lighter one?
      • Some days will be lighter/darker than others, so how to manage averaging of times?
      • How accurate could it be made?

      My naïve first stab at this would be: Pick a light threshold. Record the dawn/dusk times according to that threshold. Average them, call that "noon", and gradually tweak the clock time over several days to bring it into line with the sensed/calculated "noon" - but a searching for graphs of sunrise/sunset times quickly showed that the midpoint of sunrise & sunset is not noon.

      Googling threw up lots of results for sensor lights combining a clock and a photocell, but I couldn't find anything about using the photocell to set the clock. So does anyone know if this has been tried before? Is it a non-starter for some reason?

      Edit:

      Perhaps it's worth sharing the project I had in mind, which is a rain alarm so I can rush out and get the washing in from the line when it starts to rain. I was thinking how annoying it would be if I left it switched on and it rained in the middle of the night and the alarm woke me up. So I decided should automatically avoid triggering during the sleeping hours of night (say 10pm to 8am). My first thought was a photocell so it wouldn't trigger when it's dark. Then I remembered that it gets light at 3am at the moment, which wouldn't work. So it needs a clock. How to set the clock:

      • Manually - Needs a user interface with buttons and a display. Seems overkill just for a clock.
      • Serial port - Clunky to plug a laptop in just to set the clock.
      • WiFi - Needs a username and password or WPS, and an ESP32 or similar - again seems overkill just to get the time.
      • GPS - also overkill and expensive.
      19 votes