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Using a timer switch for power, how to wire two lights so that when one is on the other is off. In the dull and cold winter days I am trying to grow seedlings. The light is insufficient during the day and so is enhanced with grow lights, which should be off at night, but the nights are cold so I want pad heating on then.

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    Need some clarification: 1 - Are you trying to make so that "something" is always (24/7) on, but alternates automatically between "lights" and "heating"? Or something else? 2 - Total power requirements of "lights" and "heating"? 3 - What model timer do you currently have? Jan 13 at 15:20
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    It's possible with a contatctor as well, with NO and NC contacts, but far simpler with another timer.
    – Tim
    Jan 13 at 15:21
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You could use a smart plug like a Shelly Plug which will allow you to set up time with built-in tools to automatically turn on and off at dawn and dusk and be controllable with a smart phone. The advantage is that the Shelly is smart enough to automatically adjust to changing daylight hours, which a wall timer will not usually do.

It's hard to argue with the simplicity of a simple timer that @Tim suggests comments for your question.

Edit

There are timers which will allow you to control two different devices from the same power outlet. An example would be the NearPow Outlet Timer, which allows two different devices to have nine on/off settings per day and seven day programmable.

Good luck with the plants.

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    General advice: For anything plugged in to 120V or 240V, make sure it has a UL or ETL listing. The NearPow makes no mention of that (most product listings will make it clear if they are properly listed) and there are some real horror stories of fires. A properly designed product will almost always "fail safe". Jan 13 at 19:01
  • @manassehkatz-Moving2Codidact Sage Advice and I appreciate you pointing that out. The NearPow is CE listed and I could evidence that it is UL listed, but I do not have a url. Shelly products are UL listed, CE and RoHS compliant. I think if anyone is looking at home automation, it is a great way to go. But the NearPow seems like it fits the task.
    – gwally
    Jan 13 at 19:20
  • You have to learn what the different terms mean. CE doesn't actually mean very much when it comes to safety and for a number of reasons it can be faked without getting sued. RoHS is about hazardous materials - nothing to do with electrical safety. In a quick search, I didn't see anything on NearPow about UL (or ETL or anything similar). Jan 13 at 19:31
  • Now you're being insulting.I know what the symbols mean. You might want to educate yourself to the penalties of faking CE certification. This discussion is off-topic.
    – gwally
    Jan 13 at 20:36
  • Not trying to be insulting. My understanding (limited, but based on some personal experience plus plenty of wonderful messages from Harper (who has some creative acronyms)) is that CE, real or fake, doesn't guarantee electrical safety. Same for RoHS. Or FCC. UL and ETL do. Jan 13 at 21:48
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Power the heat pad through dusk to dawn sensor.

Most are for bulb sockets, which you can convert with a socket-to-plug adapter, but there's also plug-only switchers with photocells.

if ambient light isn't enough to trigger cleanly, get an extension cord from the timer's output and plug in both the grow light and a nightlight to it. Then tape the nightlight onto the front of the dusk to dawn so that when timer has power, the sensor doesn't, and vice-versa. I use these photocell-based consumer electronics "logic inverters" with LED nightlights in several places in my smart home setup to avoid modifying electronics.

To be clear:

  • the wall powers the timer and dusk to dawn switcher
  • the timer powers the heating pad (and optionally a nightlight to extend the light)
  • the dusk to dawn powers the heating pad.

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