I want some bright lights to flash in the studio when the VFR extension rings (rather than bells sound). The analogue telephone circuit uses a ringing signal of 80-90V AC at 25 Hz (it is quite strong; if you accidentally hold on to bare telephone wires it can definitely be felt!) – and all sorts of random signals can come down a phone line.
So I had to build a circuit to isolate the phone line from any other equipment; not let through any DC electricity (or the phone line will be permanently engaged) and reduce the voltage from the line to trigger an optoisolator.
https://www.youtube.com/watch?v=IvYd8DZkugk
https://www.youtube.com/watch?v=HZkoP1ESsag
I tried to add a sensitive relay to the output of this; but it “chatters” at 25Hz because of the ringing signal; which would knacker it fairly quickly. In any case it will mostly be triggering LED displays, and the 25Hz flicker reminds you these lights have something to do with telephones. So I just added another transistor stage using a PNP transistor; and 6 hi brightness blue LEDs on 12V raaa
https://www.youtube.com/watch?v=64QWMATyhw8
https://www.youtube.com/watch?v=bBtYtBLwalc
circuit is here (blocking capacitor was 470n 250V rating (often found in old power supplies) I used 10V Zener diodes and a CNY17 optoisolator, and BC177 PNP transistor to drive the LEDs. The whole lot only uses about 10mA when all the LEDs are on!
(a full size copy is on my flickr)
