Telephone-operated night light

In most telephone networks, a direct voltage of 48-60 V exists at terminals a and b. When the call signal comes in, a 50-60 V alternating current at a frequency of 25--27 Hz is super-imposed on the direct voltage. This open-circuit voltage is more than halved when the receiver is taken from the hook (which closes the current Ioop). The current in the loop is 20-100 mA.