Apparatus, systems, and methods for detecting the presence of a metallic surgical instrument. A metal detector for detecting insertion of a metallic surgical device into a hollow tube may include a switch, resonant circuit and a controller. The resonant circuit has a capacitor and a coil mounted to the hollow tube. The controller turn on the switch for a preselected time to temporarily provide a current to the resonant circuit and analyzes a resulting decaying voltage waveform originating from the resonant circuit when the switch is turned off in order to determine the presence and longitudinal depth of the metallic surgical device in the hollow tube.