How to sense mains AC?

Question

Hey, I need to know if a latching SPDT relay is on the A or B side. The relay has a 5V control, but it will have 120VAC on it. I was planning to use a half wave rectifier and RC filter with large R to tap off ~5V into a gate. Am I crazy? Is there a better/simpler way? Thanks, John 
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jw0752 · Accepted Answer

Hi John,

I recently had a circuit that sensed AC mains using an AC optocoupler. This would allow you to sense the mains without a direct connection to the rest of your circuit. On my board they used a 120K resistor in series with the input to the optocoupler and the output of the optocouple was tied into a 5 volt circuit that interpreted it. Here is a link to my blog.

https://www.element14.com/community/people/jw0752/blog/2020/02/22/ac-optocoupler-ltv-814

John

shabaz · Answer

Hi John, Could you explain your actual goal / use-case? The trouble is, with the small bit of information, any answer may not be appropriate or may not help you, because the answer can be different based on context. Some example things that can usually help (not all may apply): 1. My Goal in detail (e.g. climb a mountain next year) 2. What things I'm trying to solve (e.g. establish how high I am) 3. What constraints I have (e.g. cost, software experience, hardware experience) 4. What hardware, tools and software I'm using (e.g. part number, link to the tools and software) 5. Diagram or sketch to show the scenario, and what I've attempted 6. Where I believe the problem lies, or which part I need help on (e.g. unexpected response from the part I'm using) 7. Non-blurry photographs of the set-up (zoomed out for context, and detail), and video if needed Non-fuzzy, focussed Overview Photo (for context)

johninjax · Answer

Added a pic like what I am talking about.

How to sense mains AC?

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