On Friday, August 12, 2016 at 10:38:23 AM UTC-4, wrote:
On Fri, 12 Aug 2016 20:09:04 +0800, "Mr. Man-wai Chang"
wrote:


Is there a simple circuit that does that?

Pretty easy to do with chips. Get a ring detector chip and an SCR.
Connect the ring detect to the gate of the SCR and reset it by opening
the circuit to the LED on the SCR anode.


A quick google produced this, which is cheap and should serve the
purpose:

https://www.vellemanstore.com/en/vel...h-relay-output

That's first thing I saw, probably more on Ebay, etc.