this post was submitted on 28 Jul 2024
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Simple light-sensitive light switch? (self.askelectronics)
submitted 4 months ago by leisesprecher to c/askelectronics@discuss.tchncs.de
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I'm trying to build a very simple, stupid light switch for my grow light. Essentially, I want to turn on the light, if it gets too dark outside, so that my plants can survive the northern winter.

Since I'm a software guy, my first thought was an ESP32, but that seems excessive.

My current approach would be something like this: https://www.ebay.com/itm/313561010352 In conjunction with a relay, both powered by a USB-PSU.

If the light level is low enough, the logic DO pin should send a signal and that should be enough to trigger a small relay, so that the relay then closes the circuit to switch on the lights.

Is that idea completely stupid? With electronics, I'm usually missing something very obvious.

The lights themselves are already just usb powered and only draw 5W, so that shouldn't be problem.

What I'm concerned with is the actual switching. Is the logic signal "strong" enough to activate a relay? Would simple transistor maybe sufficient?

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[–] nilclass@discuss.tchncs.de 6 points 4 months ago (1 children)

It's not completely stupid. One pissible issue: when the light level is around the threshold, slight variations (think: cloud passing by) will cause the relay to switch back and forth quickly. This can be solved with some additional parts (keyword: hysteresis).

I would recommend using a microcontroller, it makes this problem easy to solve, plus you can have more logic to trigger the lights (eg time based).

Another thing is turning off: if it's purely based on light level, you need to make sure the sensor does not "see" any of the LED light, otherwise it thinks the sun is back up

[–] leisesprecher 3 points 4 months ago (1 children)

At least the last problem could probably be solved by simply a simple housing so the sensor "looks" outside - at least I hope so.

[–] litchralee@sh.itjust.works 2 points 4 months ago

If you're looking to do this project without a microcontroller -- it's an exercise that every SW engineer should consider doing at least once -- then the venerable 555 timer may be of aid here:

https://elonics.org/light-sensor-circuit-using-ldr-555-timer-adjustable/