Overheated railroad journal bearings, wheels, and other wheel components on a moving or stationary railroad train are detected by amplifying the current signal from an infrared radiation sensor comprising a pyroelectric cell. A reference temperature is sensed by chopping the incident infrared radiation with an asynchronous shutter that momentarily closes at successive time spacings of shorter duration than the scanning period of the sensor. The amplified signal is converted to a digital signal and processed by a microcontroller and associated hardware and software. The detector automatically and periodically calibrates itself and compensates the temperature signals for any temperature difference between the ambient external temperature and the temperature inside the detector housing. The output signal may be digital or analog.