String collar detector

The utility model relates to a detection device for downhole tools used in oil fields and oil wells, in particular to a string collar detector. The string collar detector comprises two parts, namely a detection probe and a signal conditioner, and adopts an excitation coil and a detection coil which are connected with a filter magnification unit, a distinguishing unit, an alarming unit and a contact output sequentially, and are connected with a power management circuit; a DC constant-current source drives the excitation coil to generate a magnetic field signal; the detecting coil measures the signal; when the collar of the string passes the magnetic field, the magnetic flux is caused to change; the detecting coil transmits the signal to the signal conditioner after detecting the signal by inducing voltage; a weak signal output from the detecting coil is transmitted to the distinguishing link after being magnified and filtered during the filter magnification link; and when a collar enters the magnetic field, an LED is lighted for alarm. The system has the advantages of simple structure, easy realization, convenient application, accurate detection, and high anti-interference capability.