Abstract: A Drill Bit detector for use with numerically controlled drilling machine capable of sensing missing or broken Drill Bits in the range of 1 mil to 30 mils in diameter, or larger, is described. An infrared emitting diode is driven at a predetermined frequency with the pulsed light being directed through fiber optic cable directed at a portion of the Drill Bit to be detected. A change of light intensity occurring when a Drill Bit is missing is detected and the pulsed light detected converted to a pulsating electrical signal at the input frequency of the pulse light source. A frequency responsive circuit converts the pulsating electrical circuit to a digital signal indicative of the absence of the sensed Drill Bit. A timer determines whether or not the sensed digital signal occurs for a predetermined threshold time period, and causes it to be ignored as spurious if less than the predetermined time interval, or causes the drilling machine operation to be halted if occurring longer than the threshold time period.