“Relay” function from shoebox KSU

[rturner]

I've done it before with one of these old 4-line shoebox KSU's where you can rewire it in a way so that when a call comes in and triggers the KSU to ring the phones (flash the lamp, buzz the buzzers etc) that it opens a circuit as long as it's ringing (and closes again when the line is answered or calling party hangs up from no answer) that can be used as a low voltage relay. I used this function to control a light in a radio studio that would come on while the phone was ringing using the KSU to open and close the 24v circuit to the light. (I'm not 100% sure as it was almost 10 years ago since I did that wiring job but it may have also involved moving jumpers around on the line card.) I need to replicate this function on a different system to hold a circuit open to control an external system while the line is ringing.

Does anyone have an idea of what I'm talking about and how I can do it again as I forgot the steps.

Thanks in advance!

[dsk]

If the relay just shall close the contact set when the bell normally sounds a capacitor of max 1 uF and a relay with a dc resistance of at least 1000 ohms is all you need. Just put the relay and the capacitor in series, and connect it across ring and tip wire.
If you want it to give signal between the rings too I will suggest a delaying circuit.

LineA--------------capacitor--------------relay windings------------LineB