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I'm making my own two way intercom for my house. There will be a speaker and microphone above the front door of my house (as well as a camera). When someone rings my doorbell, I get a notification on my phone and am able to see the video.

The issue is that when I enable the microphone on my phone to talk to the person, I want to disable the outside microphone automatically to prevent feedback.

I'd like to do this with hardware. Can I do this with an NC relay to cut power to the mic; having the speaker voltage controlling the relay coil?

EDIT: Below is what I'm trying to accomplish. I ordered a few sound activated switches, but they will not work for my application. After some more research I think I found what I'm looking for...line audio detectors. Problem is the few for sale are audiophile devices upwards of $100. So I'm going to make my own. Just need some advice on the questions in my schematic.... (https://i.stack.imgur.com/o5nma.png)

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  • \$\begingroup\$ Try googling voice operated switch. \$\endgroup\$ – Andy aka Dec 16 '18 at 10:07
  • \$\begingroup\$ Thanks. At first I thought this was the answer, but then I realized that it wouldn't work for my application because the switch would keep the mic off when the person outside starts talking. I need the switch to be dependent on the outside speaker voltage/current only. \$\endgroup\$ – ILikeWaffles Dec 17 '18 at 14:07
  • \$\begingroup\$ It can take an electrical signal from the speaker to do the functionality you want. \$\endgroup\$ – Andy aka Dec 17 '18 at 19:01
  • \$\begingroup\$ I found what I was looking for on ebay... ebay.com/itm/… \$\endgroup\$ – ILikeWaffles Dec 20 '18 at 3:23
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The standard way to deal with this situation is to use something called a "two to four wire converter". This is ancient terminology dating from early days of the telephone industry but the techniques are still in use today.

What this circuit does is to subtract the local microphone signal from the speaker. There are fully-analog versions of this but modern implementations (cell phones, etc) use digital techniques.

Google will teach more than you want to know about this technology but you should easily find an analog implementation using only a couple of op-amps.

Most analog versions require you to manually find the null. However, the null is normally stable and you should need to adjust it only once.

Using this technique allows you to have a fully duplex link (talk and listen at the same time).

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  • \$\begingroup\$ A more common terminology is “hybrid”, hybrid coil, or telephone hybrid. \$\endgroup\$ – Edgar Brown Dec 20 '18 at 2:07
  • \$\begingroup\$ Yes, indeed. I'm thinking more in Intercom terms, where the 2 to 4 terminology is most common. \$\endgroup\$ – Dwayne Reid Dec 20 '18 at 3:40
  • \$\begingroup\$ Interesting. This is good info, thanks. Although for my application, I am not sending and receiving on the same link. The mic and speaker are processed separately. I just want to cut power to the mic when there's activity on the speaker line. \$\endgroup\$ – ILikeWaffles Dec 20 '18 at 3:44
  • \$\begingroup\$ Thanks for the help guys. I found a Chinese PCB on Ebay that will help me do what I need. They're calling it "Audio/ Video Signal Monitor AV Detection Tester". I can't make out the chip on there, but it's probably a dual/quad opamp. For 5 bucks I'll try it. Otherwise I'll make my own opamp/comparator/switch circuit. \$\endgroup\$ – ILikeWaffles Dec 20 '18 at 3:52

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