Simple deep searching metal detector circuit

The principle behind a metal detector is quite simple. This is demonstrated by the following circuit, which shows that a metal detector can be constructed quickly with a few readily available components. This metal detector circuit can detect a metal coin at a depth of 90 to 100 cm. Compared to many other detectors, this design is relatively stable and easy to use. Like many other detectors, this one utilizes the Beat Frequency Oscillator (BFO) principle. This principle involves creating a tone whose frequency is the difference between a variable oscillator and a reference oscillator. In the present circuit, the reference frequency is derived from a section of the circuit that functions as a medium wave receiver, while the oscillating oscillator is built around a six-gate Schmitt trigger inverter IC. The self-constructed detection coil is connected between the input and output of one of the six gates of the 40106 IC. According to theory, an induction opposes rapid changes in applied voltage. Therefore, any change in the logic level at terminal 2 of IC1 will be delayed at input terminal 1, resulting in circuit oscillation. The resulting signal is inductively coupled to the receiving circuit.

Although the receiving circuit is tuned to a frequency significantly higher than that generated at gate 40106, the BFO startup operates effectively with its harmonic oscillator.

When metal is present near the coil, the induction in L1 will either increase or decrease, leading to a corresponding drop or increase in the oscillator frequency. This change is transformed into an audio signal, which is amplified by the LM386 for headphones. The volume is adjustable using potentiometer R3.

Depending on the specific 40106 variant used, the oscillator operates at a frequency of 200-300 kHz. IC1 also serves as an isolator, ensuring that the oscillator experiences a light, relatively constant load, thus maintaining stability of the output frequency.

Coil construction involves 70 turns of insulated copper wire with a diameter of 0.35 mm, wound around a form with a diameter of 120 mm. The spirals can be stabilized with adhesive tape during winding. After winding, the coil should be fully insulated with insulating tape.

Shielding the coil is essential and can be achieved with aluminum foil strips. The insulated coil is first wrapped with bare copper wire, which is then electrically connected to the shield. The metal strips should cover the bare copper wire, ensuring that there is a small gap of approximately 10 mm where the ends of each metal strip do not touch. The shielded strips should be secured with insulating tape. The ends of the coil are connected to the circuit using a shielded microphone signal cable to enhance transport quality. For a more professional appearance, a disc and a suitable handle can be added to the detection coil.

Upon completion, the metal detector is powered using a 9V battery. The variable capacitor C1 is adjusted until a loud whistle is heard, indicating that the detector is ready to differentiate between metallic and non-metallic objects. In the presence of metal, the frequency increases, whereas it decreases in the absence of metal. There remains ample opportunity for experimentation with the oscillator output frequency and coil designs.

The components required for this metal detector include:
- R1 = 100K
- R2 = 10K
- R3 = 10K potentiometer
- C1 = 365pF variable capacitor
- C2 = 100nF
- C3 = 100nF
- C4 = 100nF
- C5 = 220μF/16V
- C6 = 100μF/16V
- D1 = AA119
- Q1 = 2N3904
- IC1 = 40106B
- IC2 = LM386
- L1 = Searching Coil (as described)
- S1 = On/Off switch
- B1 = 9V Battery
- H1 = Headphones

This schematic provides a foundational understanding of the components and their interconnections necessary for constructing a functional metal detector.The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the circuit of this metal detector, it is possible to detect a metal coin in depth of 90 to 100cm.

In contrast to many other detectors, the design of this, is relatively stable in coordination and therefore quite easy to use.

Like many other detectors, this one uses the Beat Frequency Oscillator (BFO) principle. According to this principle we have the creation of a tone, the frequency of which is the difference between a variable oscillator and a reference oscillator. In the present circuit, we get the reference frequency from the second part of the circuit that equates to a medium wave receiver, while the oscillating oscillator grows around an integrated six-way Schmitt trigger inverter.

The self-build detection coil is inserted between the input and output of one of the six gates of 40106. According to the theory, an induction is opposed to the rapid changes of each applied voltage. Thus, looking at the circuit diagram we have here, any change in the logic level at IC1 terminal 2 will be applied with delay to the input terminal 1.

This delay causes oscillation of the circuit and the resulting signal is coupled inductively to the receiving circuit.

Although the receiving circuit is tuned to a frequency that is much higher than that generated at the gate 40106, the BFO startup works equally well with its harmonic oscillator.

In the case now that there is some metal near the coil region, the induction of L1 will increase (or decrease), causing a drop (or increase) of the oscillator frequency. This phenomenon is transformed into an audio signal, amplified by the LM386 on the headphones. With the potentiometer R3, the volume is adjusted.

Depending on the construction of the 40106 we use, the oscillator operates at 200-300 kHz.

IC1 is also used as an isolator, ensuring that the oscillator will see a light, relatively constant load, and thus maintain the stability (within limits of course) of the output frequency.

 

Coil construction

The detection coil consists of 70 coils of insulated copper wire of 0.35 mm diameter, wrapped in a 120 mm diameter form. During wrapping, we can stabilize the spirals with adhesive tape. After the winding is completed, fully insulate the coil using an insulating tape.

 

Coil Shielding and connection

Then we will have to shield the coil, and for this we use some foil strips of aluminum foil.

First we wrap around the insulated coil, naked copper wire, which will then be electrically connected to the shield. The metal strips are now secured around the coil, covering the naked copper wire. Although shielding should cover the entire perimeter of the coil, we should leave a small gap around 10mm, or in other words, the beginning and end of each metal strip should not come in contact.

Shield the strips of the armor using an insulating tape. Then connect the ends of the coil to the circuit, where for better transport quality we use a shielded microphone signal cable. The whole presence of the detection coil can of course become more professional, for example by adding a disc and a suitable handle.

 

Metal Detection

Finishing the construction, we power up the metal detector feed with a 9V battery, and turn C1 until a loud whistle sounds.

Now the detector is ready and able to distinguish metallic from non-metallic objects. In the first case, the frequency is increased, while in the second drop. The truth is that there is enough room for experimentation about the oscillator output frequency, and coil designs. Happy searching ;)

 

Components

R1 = 100K
R2 = 10K
R3 = 10K potentiometer
C1 = 365pF variable capacitor
C2 = 100nF
C3 = 100nF
C4 = 100nF
C5 = 220μF/16V
C6 = 100μF/16V
D1 = AA119
Q1 = 2N3904
IC1 = 40106B
IC2 = LM386
L1 = Searching Coil (see text)
S1 = switch on/off
B1 = Battery 9V
H1 = Headphones