Abstract: An alarm circuit for detection of an overheated condition comprises a bridge rectifier or a battery to provide a d.c. input, and a detector bridge including a thermistor and Zener diode in adjacent arms. The diode is set to operate along that portion of the thermistor response curve which is slowly changing. Hence, the Zener diode will be activated if the thermistor detects at the very onset of an overheated condition. Bridge unbalance is fed to an operational amplifier which activates a transistor to turn on an alarm such as an audio indicator, LED, telemetry system, etc. The alarm circuit has application for detection of a line overload, as a monitoring device for patients and for persons undergoing physical activity such as joggers and the like. By sounding an early alarm, overheating in a line wire caused by say, an overheated appliance, can be corrected with a consequent energy saving and minimizing damage to the appliance.