Re: A/D on telephone line
Dear DavidP,
If you want to know is the hook is on or off,
you can use a simply optocoupler as a switch.
If you want to measure the actual voltage,
you can use a simple phototransistor based
optocoupler (NPN for example).
Look for a graph in the data sheet of
CURRENT TRANSFER RATIO vs. FORWARD CURRENT.
This graph will describe the ration between
the LED forward current and the transistor
gain.
Find a linear zone and use it.
As the LED forward current will change, the
current through the phototransistor will
change. Use a small resistor at the collector
(or emitter) to measure this current.
Be aware that calibration might be needed per
circuit.
It should work
Warning:
Be careful when you deal with the DC componenet.
Ususally none of the lines has any relation to the
common AC network ground.
This means that when you deal with the telephone
lines, you have to well isolate grounds and power
supplies.
Regards,
Pazam