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 A Simple Metal Detector that you can construct
 Preliminary Project Information

Making a metal detector was always my dream.  I was curious listening to those beeps at airport for which dad explained that they were metal detectors. Which did not mean anything to me. Last summer I had opportunity to find secrets behind this little stuff from my dad which turnout to be simple project one can make at home.

Recently I decided to make one for my school anyway. I spent some time with my dad & found that things were not very tough!!. And then, we found that we had everything in those component boxes that I have been arranging for dad for last few years. Finally, I understood that these boxes were "Useful" !!

 

Basic Principle:

Metal detectors have two oscillators. One is tunable and the other is "Fix" with a loop. Both are "Tuned" at same frequency. Outputs from this two oscillators are then mixed in a mixer which produces only the difference. Means, if both oscillators are at the same frequency, there is nothing heard in the speaker.

To start with, you need to "Zero beat" both oscillators or say "Null" them while keeping your detector in the free air, away from metal objects. Use the tuning control provided for this purpose. 

Once this is done, start placing detector around metal objects and you will notice that "Zero beating" is getting disturbed and an audio tone from the speaker is heard. Wow, the detector has found a metal & its working!!

 

The Circuit:

Both oscillators are built using MPF102 or J310 FETs. The Mixer is BF998 dual gate MOSFET. 

 

Audio IC amplifier is LM386 which drives a speaker. 

I have used a stabilized supply for two oscillators and mixer. U1, a 78L05 is doing this job nicely. 

Circuit is being designed on a Single Sided Glass Epoxy Printed Circuit Board. It will be using PCB mounted volume control and a variable capacitor to tune the oscillator. (or varicaps & a pot) However, final design may change.

You will have to wind the Loop Coil yourself. Contraction details are given in the project document given below for download.

I am happy that now I have a metal detector constructed by me and also learnt how it works, how to design simple oscillators and make PCB drawing for my projects. 

I hope you will enjoy this small kit


Picture of the Completed Metal Detector:
 

Not ready yet

 

 

 

 

 Metal Detector Document & Schematic:
 
Document
 
Schematic
     
Schematic

JPG 

 
Possible Loop Design (Not tested yet) JPG
 

 Datasheets of Parts used in this Project

 
LM386
BF998
J310
78L05
  
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