I have two corded phone lines that I want to connect both of them to one telephone device. The phone must be connected to line 1 by default, and it switches to line 2 when it rings. I made a simple circuit to detect line 2 ring signal that turns on a 2-contact relay to switch lines. It works well when the relay powered from an external supply, But I want it to be a small box beside the telephone device and I don't want to uses an external power supply. The problem is that powering the relay from the phone line draws about 35mA that causes the phone to be answered immediately as it rings. Is there any idea to solve this problem? Like electronic switch replacement for the relay. Note that I don't want to get power from line 1.

(I will add a timer and end-of-call detection circuit to this circuit to avoid switching back to line 1 while using line 2 later.)


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Use a latching relay on the ring current circuit. \$\endgroup\$
    – Andy aka
    Commented Apr 23, 2020 at 13:07

1 Answer 1


The basic part for this would probably be a two coil latching relay like this one: -

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I've put red boxes close to the two coils on the lower part of the above picture.

One coil being activated latches (sets) the relay and the other coil de-latches (resets) the relay. To activate or deactivate you momentarily drive the relevant coil with voltage but where to get that voltage from?

Well, you have two incoming lines and you could, on both lines independently charge up capacitors via 1 Mohm resistors. If both lines are inactive then both charge up to about 48 volts. So that gives you some stored energy to drive the coils.

Then you need to have ring detector circuits as shown in the diagram in the question - it's the opto-coupler part and is quite a conventional ring detection circuit for remote bells and buzzers. Between the ring detectors and the relay coils you need to operate a MOSFET or BJT that dumps the energy into its respective relay coil.

Sure, there are going to be details that need ironing out but I got one of these to work back in the 1980s when I designed POTs.

  • \$\begingroup\$ Thank you, I'll check it. \$\endgroup\$
    – Vahid
    Commented Apr 24, 2020 at 17:27

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