Metal Detector

Metal detectors generally operate on the basis of one of the following technical prin- ciples: BFO (beat frequency os- cillator). In this system, the inductor in the search head is part of an os- cillator, whose variable out- put frequency is mixed with another, fixed, frequency ob- tained from a second oscillator. The difference (beat) frequency falls within the audible range. When the search head is brought near a metal object, the variable oscillator causes an audible or otherwise detect- able change in the beat fre- quency. BFO-based metal de- tectors are relatively inexpens- ive and easy to operate. TR /IB (transmit-receive/induc- tion-balance). This system is based on the mutual-inductance coupling between a transmit coil and a receive coil. When a metal ob- ject is introduced in the vicinity of the inductors, the degree of coupling changes, and the resultant variation in the oscil- lator output level is detected. PI (pulse induction). A continuous pulse train is transmitted...