telephone activated power supply circuit is a very unique design. The
clips clip on to the telephone pair, red and green wires, anywhere
along the pair. When the phone is on the hook, there are 0 outputs at
the positive and negative terminals labeled (Output Voltage).
When the telephone is turned on - or activated, the device is turned on
thus producing an output voltage that originates from the 9V battery.
The actual output voltage depends on the loading of the device or
circuit that is connected to the terminals. A 50 Ohm load will produce
5 Volts @ 100mA and a 300 Ohm load will produce 6.7 Volts @ 22mA. The
higher loads will produce more voltage.
Basically it is an electronic switch that will power up whatever the
output voltage terminals is connected to. The X points marks the spot
where you can cut-in a double pole double throw switch as a failsafe
switch, if desired to switch the unit on and off. Basically it is a
trigger that will turn a connected circuit on or off depending on the
status of the telephone on hook and off hook.