A simple metal detector on two transistors. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

A simple metal detector on two transistors. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / metal detectors

Comments on the article

Without a doubt, many novice radio amateurs will be interested in the design of a simple metal detector, the basis for which was a circuit that was repeatedly published in domestic and foreign specialized publications in the mid-70s of the last century. Using this metal detector, made with just two transistors, you can detect metal objects that are several tens of centimeters away from the search coil.

Schematic diagram

This design is one of the variants of metal detectors of the FM (Frequency Meter) type, that is, it is a device based on the principle of measuring the frequency deviation of the reference oscillator under the influence of metal objects that have fallen into the search coil coverage area. In this case, the assessment of the change in frequency is carried out by ear (Fig. 2.4).

Rice. 2.4. Schematic diagram of a simple metal detector on two transistors

The device circuit is based on a high-frequency generator and a receiver, which registers changes in the frequency of the generator when approaching metal objects.

The high frequency generator is assembled on the transistor T1 according to the capacitive three-point circuit. The oscillatory circuit of the reference oscillator consists of a chain of capacitors C1, C2 and C3 connected in series, to which the coil L1 is connected. The operating frequency of the RF generator is determined by the inductance of this coil, which is also a search coil.

One of the features of this device can be considered that it uses a heterodyne type receiver, which is made on only one transistor, as an analyzer. In this case, the cascade on the transistor T2 combines the functions of a local oscillator and a detector. The local oscillator is assembled according to the capacitive three-point scheme. The advantage of such a circuit is the possibility of using an inductor without taps, which, although slightly, simplifies the design. The oscillatory circuit of the local oscillator contains an inductor L2 and a capacitance made up of capacitors C4, C5 and C6 connected in series. The frequency of the local oscillator can be changed by rotating the tuning core of the L2 coil.

From the collector of transistor T2, the detected signal is fed to headphones BF1.

If there is a metal object near the coil L1, then its inductance will change. This will lead to a change in the frequency of the reference oscillator, which will be immediately registered by the metal detector receiver. As a result, the tone of the signal in BF1 phones will change.

Details and construction

All parts of a simple metal detector on two transistors, with the exception of the search coil L1, the local oscillator coil L2, connector X1 and switch S1, are located on a printed circuit board measuring 70x40 mm (Fig. 2.5), made of one-sided foil getinax or textolite.

There are no special requirements for the parts used in this device. It is advisable to use any small-sized capacitors and resistors that can be placed on a printed circuit board without any problems. As can be seen from the circuit diagram, this metal detector uses outdated RF transistors such as P422, P401 or P402. Instead, you can use any modern pnp RF conduction transistors designed to work in the input stages of radio receivers.

The search coil L1 used in the reference oscillator is a rectangular frame 175x230 mm in size, on which 32 turns of PEV-2 wire with a diameter of 0,35 mm or, for example, PELSHO with a diameter of 0,37 mm are wound.

Segments of a 400НН or 600НН type ferrite rod with a diameter of 7 mm are placed in two paper cylindrical frames. The length of the first of them, fixed permanently, is about 20-22 mm. The second rod is movable and is used to adjust the inductance of the coil. Its length is 35-40 mm. The frames of the rods are wrapped with paper tape, on which 55 turns of PELSHO wire with a diameter of 0,2 mm are wound. You can also use wire type PEV-1 or PEV-2.

Coil L2 (Fig. 2.6) should be installed at a distance of 5-7 mm from the plane of the coil turns L1.

As a source of sound signals, you can use headphones with a resistance of 800-1200 ohms. The well-known phones TON-1 or TON-2 are also suitable, however, when using them, both capsules must be connected not in series, but in parallel, that is, connect the plus of one capsule to the plus of the other, and the minus to the minus. In this case, the total resistance of the phones should be approximately 1000 ohms.

Simple metal detector on two transistors

Simple metal detector on two transistors
Rice. 2.5. Printed circuit board (a) and arrangement of elements (b) of a simple two-transistor metal detector

A simple metal detector on two transistors is powered from a source B1 with a voltage of 4,5 V. As such a source, you can use, for example, the so-called square battery of the 3336L type or three elements of the 316, 343 type connected in series.

The printed circuit board with the elements located on it and the power supply are placed in any suitable plastic or wooden case. A switch S1 and connector X1 for connecting headphones BF1 are installed on the housing cover.

Coils L1 and L2 are connected to the board with a flexible stranded insulated wire.

Establishment

Adjustment of the metal detector should be carried out under conditions when metal objects are removed from the search coil L1 at a distance of at least 1,5 m.

Simple metal detector on two transistors
Rice. 2.6. Construction of the coil L2

After turning on the power, check the voltage at the emitters of the transistors. At the emitter of transistor T1 there should be a voltage of -2,1 V, and at the emitter of transistor T2 - about -1 V.

Further, slowly moving the tuning core of the L2 coil, it is necessary to achieve the appearance of a loud, clean low-frequency signal in the phones. If the generator is initially set, for example, to a frequency of 465 kHz, then a signal with a frequency of about 500 Hz will be heard in the phones.

When the L1 coil approaches a metal object, which can be used during the tuning process, for example, a tin can, the tone of the low-frequency signal in the headphones will change. The beginning of the change in the tone of the signal must be at least approximately fixed. After that, moving the core of the L2 coil to fine-tune the local oscillator frequency, you should achieve the highest sensitivity of the device.

This completes the process of setting up a simple metal detector on two transistors.

Operating procedure

Carrying out search operations using this device does not have any features. If a metal object is in the coverage area of the search coil L1, then the pitch in the headphones will change. When approaching some metals, the frequency of the signal will increase, and when approaching others, it will decrease. By changing the tone of the beat signal, having a certain experience, one can easily determine what metal, non-ferrous or so-called black, the detected object is made of.

Author: Adamenko M.V.

See other articles Section metal detectors.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Artificial leather for touch emulation 15.04.2024

In a modern technology world where distance is becoming increasingly commonplace, maintaining connection and a sense of closeness is important. Recent developments in artificial skin by German scientists from Saarland University represent a new era in virtual interactions. German researchers from Saarland University have developed ultra-thin films that can transmit the sensation of touch over a distance. This cutting-edge technology provides new opportunities for virtual communication, especially for those who find themselves far from their loved ones. The ultra-thin films developed by the researchers, just 50 micrometers thick, can be integrated into textiles and worn like a second skin. These films act as sensors that recognize tactile signals from mom or dad, and as actuators that transmit these movements to the baby. Parents' touch to the fabric activates sensors that react to pressure and deform the ultra-thin film. This ... >>

Petgugu Global cat litter 15.04.2024

Taking care of pets can often be a challenge, especially when it comes to keeping your home clean. A new interesting solution from the Petgugu Global startup has been presented, which will make life easier for cat owners and help them keep their home perfectly clean and tidy. Startup Petgugu Global has unveiled a unique cat toilet that can automatically flush feces, keeping your home clean and fresh. This innovative device is equipped with various smart sensors that monitor your pet's toilet activity and activate to automatically clean after use. The device connects to the sewer system and ensures efficient waste removal without the need for intervention from the owner. Additionally, the toilet has a large flushable storage capacity, making it ideal for multi-cat households. The Petgugu cat litter bowl is designed for use with water-soluble litters and offers a range of additional ... >>

The attractiveness of caring men 14.04.2024

The stereotype that women prefer "bad boys" has long been widespread. However, recent research conducted by British scientists from Monash University offers a new perspective on this issue. They looked at how women responded to men's emotional responsibility and willingness to help others. The study's findings could change our understanding of what makes men attractive to women. A study conducted by scientists from Monash University leads to new findings about men's attractiveness to women. In the experiment, women were shown photographs of men with brief stories about their behavior in various situations, including their reaction to an encounter with a homeless person. Some of the men ignored the homeless man, while others helped him, such as buying him food. A study found that men who showed empathy and kindness were more attractive to women compared to men who showed empathy and kindness. ... >>

Random news from the Archive

Bilateral solar panels 22.01.2021

An international team from the King Abdullah University of Science and Technology in Saudi Arabia (KAUST) and the University of Toronto Engineering (Canada) has developed a solar cell that combines the best features of perovskite and silicon technologies.

Today there are tandem solar cells that combine perovskite and silicon. Compared to conventional silicon batteries, these cells are more efficient at converting sunlight into electricity by absorbing additional spectrum.

Specialists offer a two-way tandem configuration, which not only absorbs the widest possible spectrum of direct sunlight, but also captures scattered reflected light (albedo). This design allows the battery to generate a record amount of energy.

The new batteries are double sided. Their front part captures direct sunlight, and the back part captures the light that is reflected from the ground or other surfaces. It is important that such batteries can be produced on existing equipment without complicating the technology.

The potential for capturing indirect sunlight has been studied before, but so far it has not been possible to verify it experimentally. Now, researchers have finally been able to solve the scientific and engineering challenges required to absorb indirect light. Tests have shown that the bifacial tandem solar array generates nearly 25% more power than any commercially available solar panel.

Other interesting news:

▪ The capacity of hard drives will dramatically increase

▪ Found stars of the second generation

▪ New Switching Regulators

▪ Neural noise helps you learn

▪ Protein from algae, sugar and light

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the Electrician website. Article selection

▪ article The roof of your house. Tips for the home master

▪ article Which country invented Chinese fortune cookies? Detailed answer

▪ article Digital Integrated Circuit. Radio - for beginners

▪ article Amplifier on a TDA1905 chip, 5 watts. Encyclopedia of radio electronics and electrical engineering