This, will detect the ring signal, energize the relay which latches up, and the LED comes on and stays on till you push SW.
____
tip o--CC---RR----o-D<-------o-----o------>D----^ ^----o-----+
| | | SW | |
| | | | |
\-/ |- R|| B- LED
Z C L|| ............ A |
| |+ Y|| __.__ T+ R
| | | +--o o---+ |
| | | | |
ring o------------o----------o-----o---------o----------------+
CC=.47 uF 200 V. capacitor
RR= 3k (depends on relay)
D = 200V diode ( > < direction od diodes)
Z = 12 zener
RLY= any small relay
SW= normally closed switch
K = relay's contacts
BAT= 9 V. battery
R = 500 ohm (for LED)
C = some (10) uF capacitor
Components are not critical. It should latch on first ring, if not reduce RR. If it took too long to deenergize, reduce the C.
Sources
- Massoud Ajami (massoud[at]chemteca.sdsu.edu)