Simple Beat Frequency Oscillator (BFO) Metal Detector Circuit Explianed

The post explains a simple metal detector circuit using the beat frequency oscillator (BFO) concept, the BFO technique is considered to be the most accurate and reliable methods of detecting metals.

The circuit functioning may be understood with the following points:

The proposed metal detector uses a 4093 quad Schmitt NAND IC and a search coil along with a switch and batteries for power. 

A lead from IC1d pin 11 connects to MW radio aerial, or another process would be to warp around the radio. The BFO switch if present in the radio must be turned on.

The resistance of rapid change in voltage – known as reactance, delays the logic level at ICI pin 10 back to its input pins 1 and 2, and is further delayed through propagation delays within 4093 IC. 

This entire process results into rapid oscillations of around 2 MHz, is picked up by a Medium Wave radio. 2 MHz is out of range for Medium Waves, but a MV radio can accept the harmonics of 2 MHz frequency.    

The process of winding of the coil is not complicated. The prototype uses 50 turns of 22 awg/30 swg (0.315 mm) enameled copper wire, wounded on a 4.7"/120 mm former, and then wrapped in an insulation tape. 

The coil is then connected to 0V.A Faraday shield which is a tin foil acting as a wrapper around the coil. This process leaves a small gap and care should be taken so that the foil does not wrap the entire circumference of the coil. 

An insulation tape is again used to wrap the Faraday shield. A connection can be established to the Faraday shield with a piece of stiff wire wrapper around the shield, before adding the tape. 

An ideal scenario would be to wire the circuit with twin-core or  microphone cable, and connect the screen to the Faraday shield. 

Setting up the metal detector involves switching on the MW radio to pick up a whistle on a harmonic of 2 MHz. 

However to note, not all harmonic works best, only the one which suits need to be used. With a suitable harmonic and the metal will alter the tone of a whistle. 

A metal detector detects a large coin at 80 to 90 mm, which is quote good for a BFO detector. It can even identify discrimination between ferrous and non-ferrous metals with the rise or fall in tone.

Submitted By: DhrubaJyoti Biswas

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