Simplest Metal Detector Using a Single CMOS IC 4093

In this post I have explained a simple metal detector circuit having a decent sensitivity using a single CMOS IC 4093, as I have explained below.

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

Parts List

How to Set up the Circuit

Power on the circuit and tune a nearby Medium Wave (MW) radio to pick up the oscillator whistle on a harmonic of 2 MHz. Adjust the radio tuning until a clear tone is heard. Bring a coin or metal object close to the search coil; the pitch of the whistle will change noticeably. A larger coin can typically be detected at 80 to 90 mm, and you can observe a rising or falling tone variation depending on whether the metal is ferrous or non-ferrous.

Troubleshooting Tips

No whistle heard on radio: Check battery voltage, verify IC pin connections and ensure the radio is tuned to the correct MW frequency band where the harmonic falls.

Tone drifts or triggers from hand proximity: Make sure the Faraday shield is properly wrapped around the search coil and connected to 0V ground.

Very weak metal detection: Verify that 50 turn coil is wound neatly on 120 mm former and that connection are secure.

Frequently Asked Questions

Can I use any other NAND gate IC instead of CD4093?

CD4093 includes built in Schmitt trigger inputs which are important for clean, stable high frequency oscillation in this BFO setup. Standard NAND gate without Schmitt trigger may not oscillate reliably,

Does this circuit require a speaker connected directly to it?

No, this design acts as a miniature transmitter whose harmonic signal is picked up and heard through a nearby Medium Wave radio speaker.

Can it distinguish between ferrous and non ferrous metals?

Yes, depending on target's magnetic and conductive properties, frequency shift can cause the audio whistle to rise or fall differently.