Metal Detector Circuit – Using Beat Frequency Oscillator (BFO)

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.

How it Works

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.

Coil Winding Specifications

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.

How to Set up the Circuit

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

Circuit Diagram

IC 4093 pinouts

Metal Detector using Magnetic Absorption

Behind the detection technology of this metal detector is a sensor that identifies the existence of ferrous and non-ferrous metals by absorbing the magnetic energy.

This magnetic field is produced by an inductor which a part of a modified oscillator circuit. The moment a metal object is approached to the magnetic field, sufficient magnetic energy is absorbed to halt the oscillator.

Figure below depicts the Colpitt’s oscillator that fires around 70 kHz. Inductor L₁ functions as a sensor due to the emitter resistor’s (R₁) large value and eventually, the oscillator just works.

This is favourable because alternatively the losses in the regulated circuit will be reloaded by the transistor. D₁ and D₂ will rectify the oscillating output and the subsequent direct voltage is directly applied to the inverting input of Schmitt trigger IC₁.

Once the voltage dips below the value at pin 3 which is represented by P₁, the output will switch to a high, energizing the relay. We recommend constructing the detector on a PCB as shown in the figure below.

The actual purpose of inductor L₁ was not to mount in on the PCB. In case the oscillator does not immediately start at whatever setting the P₁ was engaged, you must bring down the value of R₁.

Alternatively, if the oscillator continues detecting even when a metal object is held close to L₁, the R₁ value must be increased.

You need to start with the wiper of P₁ to earth and control the preset so the relay does not operate at all. When you need a little more sensitivity, increase the wiper a tad more.

The energising of the relay principally dictates the current consumption and for most cases, it is not more than 50 mA.

LC Tuned Metal Detector

Unlike the metal detectors discussed above, this one works under the rule that the frequency of an LC oscillator varies when there is modified inductance. To make that happen, the inductor is approached with any type of metal detector.

The frequency change rate depends on the properties of the metal and on the frequency itself. If the latter is too high, a metal component will act like a shorted turn that brings down the inductance so that the frequency elevates.

In the event the frequency is substantially low for eddy-current losses to be neglected, we can then differentiate between ferrous and non-ferrous metals.

It will be quite challenging to make an oscillator frequency below 200 Hz. Due to that, the oscillator in the current circuit operates around 300 kHz. To make its inductance is quite simple and all you need is a single turn of a coaxial cable a depicted in the following figure.

How it Works

The LC tuned metal detector circuit is made up of an oscillator T₁, a frequency-to-voltage converter IC₁ and a BiMOS operational amplifier IC₂. By employing a detector coil diameter of 400 mm, the values of capacitors C₁ and C₂ guarantee an oscillator frequency of 300 kHz. When smaller diameter coils are used, you will need more turns.

To supply the 4046B adequately, the oscillator signal strength must be around 400 mV_ππ. The phase comparator warrants that the internal phase-locked loop always locks at that level. At pin 10, the source follower input is supplied to a CA3130 where it is sufficiently amplified.

How to Set up

Conveniently, P₁ sets the centre frequency of the phase-locked loop and the zero of the centre-zero microammeter. Using P₂, you can make fine adjustments if the sensitivity of the opamp is high.

Moreover, P₃ sets the sensitivity in the discussion which is attached in a negative feedback loop to the inverting input. Notice there is a positive feedback through the microammeter and R₁₀ to the non-inverting input. When you choose a different resistance, it is important to modify the values of R₉, R₁₀ and R₁₁ appropriately.