In this current monitoring circuit we use a Reed switch with a LED and a resistor to indicate if is current through a circuit. A Reed switch usually requires between 10 and 100 AT (Ampere-turns = corresponding to the current in a coil multiplied by the number of turns). The lower the AT, the more sensitive the reed switch.
For example a car headlight (both light bulbs) has a current consumption of 7 A or 8 A at 12V so a 50 AT Reed switch requires 7 or 8 turns in order to monitor the current.
As soon as one bulb is malfunctioning the current is dropping below half and the switch opens its contact.