Telephone line current sensing method

The most straightforward way of adding telephone controlling function to telephone recording unit is to add a relay which senses the telephone line operation current and controls the recorder. That relay coil is simply connected in series with telephone in on of the wires going to telephone. The basic idea is very easy and works reliably. All you have to find is suitable relay, which will pull at normal telephone operation current, can handle the maximum line current, has good enough insulation to withstand telephone line overvoltages and low enough resistamce to not to disturb normal telephone operation. The relay should also be selected so that it does not trip at normal telephone ring currents. There is usually a few microfarad capacitor (usually bibolar electrolytic) in parallel with relay coil to allow audio to pass easily the high inductance relay coil. One commercial circuit used a relay with 150 ohm coil resistance and there was 47 uF 25V bipolar electrolytic capacitor in parallel with relay coil. Here is a schematic of one simple telephone controlling circuit: To <----------------------+
Tape <==== Relay Contacts
Recorder <------------------o/ o
mmmmm DC 3v 20 mA Relay Coil

Line >------------------------+---+----------> To Phone
47 uF Capacitor
Line >-----------------------------------------> To Phone