Metal Detector Circuit using Single Transistor – Tutorial

The post explains a simple one transistor metal detector circuit which is very sensitive and and can detect any metal from a significant distance.

We are going to begin the topic assuming situations have settled following a few cycles and the voltage within the base of the transistor is steady (fixed by the “retaining” or “resisting” the activity of the capacitor 10n).

The circuit is really an oscillator and the technique it keeps oscillating is a result of positive feedback.

This is actually the situation with all oscillators and the component providing you with the feedback is the capacitor in between 1nF the collector and emitter of the transistor.

It might appear inquiring that the transistor could be triggered through the transmitter to keep it oscillating however in reality it is not important if the transmitter or base gets a signal that the important factor is the voltage difference between these two boundaries.

When the base is fixed and the voltage of the transmitter is decreased, the transistor recognizes a higher voltage between the base and the transmitter and is more challenging to light. When the voltage on the transmitter boosts, the transistor switches off as the difference between the two is lowered.

This is just what occurs within this transistorized metal detector circuit. The capacitor 1nF between the collector and the emitter affects the voltage on the emitter consequently switches the transistor on / off. It does this by continuously checking the voltage on the tuned circuit and passing the change to the transmitter.

In this project, the tuned circuit consists of the parallel elements of the inductor (the search coil) and the capacitor 1n by means of this.

simple metal detector using single transistor

This really is called an LC circuit where L is the inductor of the inductor in Henries (or mH or UH) and C is the capacitance of the capacitor in farads (or uF or nF or pF).

Let’s commence when the transistor activates and enables a pulse of energy to get into the tuned circuit (afterwards you will notice how the transistor activates).

The energy pulse (current) starts by attempting to input both the coil and a capacitor. You possibly can think about the coil the smallest resistance, nevertheless the capacitor is discharged and has a assumptive zero resistance and starts to charge.

Whenever a small tension shows up via this, you might believe that the coil could end up being the least resistance since it includes just a few turns of copper wire.

But the wire is wound in a coil and forms an inductor (it has an inductor). If a voltage is put on, the low resistance of the inductor enables a current circulation, however this current generates magnetic flux which reduces off the turns of the coil and constitutes a voltage feedback which clashes the incoming current. It functions this way: Assume you supply 200mV to the coil.

The voltage feedback it generates could be as high as 199mV and for that reason you merely get 1mV with which it drives current into the coil.

When the resistance of the coil is 100mohms, the current is going to be about 10mA. The capacitor will acknowledge in addition to that and so it charges first.

As the voltage on the capacitor boosts, it shows its inductor voltage and enables current flow (at a level which will take the coil) to create magnetic flux.

This flux is known as electromagnetic force lines and produces an increasing sector. The capacitor cannot supply the energy for very long and after a brief period of time the current diminishes, evoking the magnetic field to start to break down.

The magnetic field created collapses a voltage which is reverse to this initially delivered to it and the lower section of the coil turns into positive based on the top part.

If we consider the coil being small battery we see that this contributes to its voltage to the 9v of supply and the collector’s terminate of the coil gets greater than 9V.

This voltage is noticed by the feedback capacitor 1n (between the collector and the emitter) and it moves the voltage to the transmitter, where it boosts the emitter voltage.

The base of the transistor is held steady and constant by the activity of the capacitor 10n retaining and the transistor turned off somewhat.

This process carries on and ultimately the collector may very well be as withdrawn from the circuit in order that it does not place any load on the tuned circuit. Whenever an inductor is not loaded on this kind, the magnetic field of collapse may generate the maximum voltage.

This is actually the case in the circuit above and as the collapse of the magnetic field, it constitutes a voltage (about 25v) that is substantially greater than that put on it. This voltage is handed down to the “C” component of the tuned circuit (the capacitor 1n connected across the coil) and the capacitor charges as much as.

Whenever all the magnetic flux has become transformed into the voltage the capacitor is charged and it starts to offer this charge back to the coil. Along the way, the voltage across the capacitor is decreased

The frequency of the circuit is approximately 140 kHz and it is fixed by the inductance of the coil and the capacitor through it.
Once we put an article of metal in the magnetic field of the coil, many of the flux lines go through the metal and so are changed into an electrical current known as eddy current in the metal.

Which means that we eliminate a few of the magnetic flux and for that reason it is less accessible to return to the coil as soon as it begins to break down.

Because of this the reverse-voltage created by the coil is going to be reduced and then the capacitor is going to take much less time to charge to its optimum value. Therefore, the transistor is going to be turned on sooner and thus the frequency of the circuit increases.

The flux created by the coil is electromagnetic radiation similar to radio waves having the identical frequency. If we place a radio close to the coil and tune this to a harmonic, both frequencies will “beat” together and develop a “null spot” on the radio.

If a piece of metal gets into the field of the coil, the frequency varies a bit and a low-frequency tone is imparted from the loudspeaker.

A change in the frequency of only a few hertz is going to be distinctly heard and this is the reason why the circuit is so efficient.

The sensitivity of the coil is determined by the frequency of alter energising the circuit at the smallest insertion of a metal item.

This involves the transistor to work at an amplitude that is not saturated, in order that the tiniest penetration of a part of metal inside the field will probably affect the frequency.

You will need to remember that the amplitude of the wave is additionally decreased as soon as a piece of metal is brought near, nevertheless the radio is not really setup to identify this. Various other metal detectors identify the drop in amplitude and afterwards you will notice the way the two circuits compare and contrast.


All parts fit on a small PC board with two coil wires and two of the battery.

PCB for metal detector circuit

1 – 220Ω (red-red-brown-gold)
1 – 47k (yellow-violet-orange-gold)
2 – 1n
1 – 4nF7
1 – 10nF
1 – 47uF
1 – BC 547
1 – slide switch
1 – 9V Battery connector
1 – battery 9V
6.5 m of winding wire (noncritical gauge)

Search Coil Winding Details


metal detector search col construction

The search coil for this one transistor metal detector circuit is created by winding 16 turns around a diameter of 12cm spherical subject. This is often a bottle of juice or perhaps a square object which the coil could be built rounded later on. Make use of 4 pieces of tape or tape round the winding turns to keep them in position and glue the coil to the base board of silicone sealant.

The base includes a wooden handle screwed into the angle of 60 °. Additionally, you will require a small transistor stuck on the stick close to the base in order that it could take the field of the coil and identify when the frequency of the oscillator alters. The picture listed below displays the most effective layout.

Give it a try:

Hook up the battery and switch on the transistor radio. Tune on the dial and you may receive several points in which the radio produces a whistle due to its local oscillator beat with the detector coil output.

you might get the best outcome at about 1400kHz that is certainly the location where the tone could possibly be set at a really low frequency.

Once the detector is searched on a 20cm to about 10cm piece, the enhancements made on tone could possibly be easily recognized.
The frequency of the oscillator of the metal detector changes somewhat the battery voltage falls and the temperature of the circuit increases on a hot day.

This is often reimbursed by adjusting the frequency of the radio in order that the tone is kept as minimal as you can.
You are now all set to go out and try your chance.