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TL;DR

I want to send a signal between 2 isolated DC-powered circuits using only single wire connecting them. If possible, I'd like to avoid wireless solutions.

Details

I have a broken doorbell installation which I'm trying to fix. Doorbell itself operates on rather low voltage (around 6V). This is a weird custom installation, which most likely doesn't work, because something happened years ago to the initial power supply. I wasn't able to trace the cables to any useful culprit.

What I was able to deduce so far:

  1. There's a switch outside the apartment with 2 dead cables coming from inside of the wall,
  2. There's a doorbell inside the apartment with different 2 cables coming from inside the wall,
  3. I know for sure that 1 cable is going through the wall directly to the doorbell.

In the simplest form, the circuit must have looked in the past somewhat like that:

schematic

simulate this circuit – Schematic created using CircuitLab

What I want to do is to make 2 separate circuits with 2 separate power sources. First one for the swtich outside the apartment and the second one for the bell inside the apartment. I want the first circuit to notify the second circuit that someone pressed the button via 1 existing cable. I guess it is possible, but I'm not sure what components do I need to use to make it work.

At first I thought about 2 microcontrollers (even something arduino–like like DigiSpark USB). The first microcontroller would send a HIGH signal through that 1 wire to the second microcontroller — then the second one would turn on a doorbell.

Then, I though that maybe I can use less dark magic and make use of basic components like... transistors?

In the end I want to have something like the following:

schematic

simulate this circuit

What magical components can I use in a place of Magic 1 and Magic 2?

PS. Yes, I marked a doorbell as an LED.

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    \$\begingroup\$ Maybe a wireless device? Kirchoff's Current Law is pretty clear, one wire isn't going to deliver any current between two isolated circuits. \$\endgroup\$ – The Photon Apr 18 '17 at 22:24
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    \$\begingroup\$ Just out of curiosity, when you write "cable" do you mean "conductor" or "wire?" Or do you mean two wires in each cable? Or more wires? What exactly is a "cable" here? \$\endgroup\$ – jonk Apr 18 '17 at 22:38
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    \$\begingroup\$ I imagine some capacitive to earth detection might be possible with a metal touch plate where your switch is but it'd take a lot of tuning and probably false trigger like nobody's business. Go for a wi|elress doorbell for a tenner \$\endgroup\$ – TonyM Apr 18 '17 at 22:56
  • \$\begingroup\$ I'm sure a 10 dollar wireless doorbell kit will make this a purely academic endeavour. \$\endgroup\$ – Passerby Apr 18 '17 at 22:59
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    \$\begingroup\$ If both circuits had some connection to ground (via an AC outlet ground, actual earth ground, or a "common" ground wire), the a simple Hi/Low signal on your "signal" wire could very well operate a MOSFET in the doorbell circuit. The switch circuit would simply connect the gate of the MOSFET to either +9V, or GND (and use virtually 0 power doing it), while the doorbell would be activated every time the button was pressed. ... If that doesn't work for you, then I'll have to vote "wireless doorchime" too. \$\endgroup\$ – Robherc KV5ROB Apr 18 '17 at 23:10
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If you want to wrestle a little and not to go over, where the fence is lowest(=ready to use wireless system), you can try the following:

enter image description here

M1 and M2 are the magical units. You can probably connect capacitively because there surely is some big metallic parts - for example electric cables - that are quite near both magical units. P1 and P2 that make the connection are insulated metal plates or long enough insulated wires.

You need quite high frequency, at least 50 kHz. Also you must modulate something detectable to avoid false alarms. So, let M1 be a 50 kHz oscillator + a pulser that amplitude modulates the oscillator by 500 Hz. M2 is the following:

  • a resonant bandpass filter that is tuned to 50 kHz
  • an amplifier
  • a diode detector
  • a tone decoder for 500 Hz, can be also a bandpass filter + level detector
  • proper buffering for your indicator

Note: 1) The receiver must be on continuously 2) You must have quite pure transmitter signal to stay off from radio bands. An easy to make square wave transmitter can disturb AM radios at several hundred meters.

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