How to Make a Metal Detector with Two Transistors
A simple DIY metal detector circuit powered by a 9V battery and using a 30 cm search coil with 50 turns. This project is ideal for learning the basic principles behind simple metal detector circuits.
Yes. A simple experimental metal detector can be built using only a small number of electronic components. The project explained on this page uses two transistors, a homemade search coil, several capacitors and resistors, and a 9V battery.
The interesting part of this project is not the number of components, but the way the circuit uses the properties of an inductive search coil and an oscillator circuit. When a metal object approaches the electromagnetic field around the coil, the electrical characteristics seen by the oscillator can change. This can affect the frequency or behavior of the signal, providing a simple method for detecting nearby metal objects.
This design is especially useful for anyone who wants to learn how to make a simple homemade metal detector and understand the function of the search coil, transistors, capacitors and tuning potentiometer before experimenting with more complicated detector circuits.
If you would like to compare this simple design with another detector project, you can also read our metal detector circuit with a detailed explanation . It provides another useful reference for understanding homemade metal detector electronics.
9V Battery
2 Transistors
30 cm Diameter – 50 Turns
50kΩ Potentiometer
Components Required for the Metal Detector Circuit
The circuit uses relatively simple and commonly available electronic components. This makes it a useful educational project for experimenting with the basic principle of metal detection and understanding the relationship between the search coil and the oscillator circuit.
- Two transistors: BC547 transistors can be used. A 2N3904 may also be tested, but always verify its pin configuration before connecting it.
- Search coil: Approximately 30 cm in diameter with a total of 50 turns.
- Center tap: Wind 25 turns first, bring out a center-tap wire at the beginning of turn 26, and then continue winding another 25 turns.
- Potentiometer: 50kΩ for circuit adjustment and tuning.
- Capacitors: Used as part of the oscillator and signal circuit.
- Power supply: 9V battery.
- Audio output: A speaker is used to hear changes in the circuit response.
If one of the required electronic components is difficult to find, our guide to replacement parts for metal detector circuits provides additional information about alternative components that may be useful in DIY detector projects.
Why Is the Search Coil So Important?
The search coil is one of the most important sensing elements in this homemade metal detector. In this experiment, the coil has a diameter of approximately 30 cm and consists of 50 turns of wire.
First, wind 25 complete turns. At the beginning of turn 26, bring out an additional wire to create the center tap. Continue winding in the same direction for another 25 turns.
The completed coil therefore consists of: 25 turns + center tap + 25 turns.
It is important to maintain the same winding direction and connect the three coil wires correctly according to the circuit diagram. Incorrect coil connections or an incorrectly positioned center tap can affect the oscillator and may prevent the circuit from operating as expected.
Search-coil design can vary significantly between detector projects. For comparison, see our metal detector with three search coils , which demonstrates another approach to experimenting with different coil configurations.
Important Electronics Principle
The search coil is not simply a long piece of wire. Electrically, it acts as an inductor with an inductance represented by the letter L.
When an inductor is used with a capacitor of value C in a resonant circuit, the approximate resonant frequency can be described by:
This relationship helps explain why the characteristics of both the search coil and the capacitor influence the operating frequency of the circuit. Changes in the electrical behavior of the coil near a metal object can therefore affect the oscillator.
How Does the Search Coil Detect Metal?
When the circuit is operating, an alternating current in the search coil produces a changing magnetic field around it.
If a conductive metal object enters this changing magnetic field, electrical currents known as eddy currents can be induced inside the metal. These currents generate their own electromagnetic effects that interact with the original field produced by the search coil.
This interaction can change the effective impedance, inductance and losses associated with the coil. As a result, the behavior of the oscillator can change, producing a signal that can be detected or heard through the audio output.
In other words, the detector does not work because the coil physically attracts metal. Detection occurs because of the electromagnetic interaction between the search coil and the nearby metal object.
What Do the Two Transistors Do?
A transistor is an active electronic component capable of controlling and amplifying electrical signals. In an oscillator circuit, the coil and capacitors alone cannot maintain oscillation indefinitely because resistance and other electrical losses remove energy from the circuit.
The transistor stage helps provide the gain required to compensate for part of these losses and maintain oscillation. The remaining parts of the circuit help convert changes in oscillator behavior into a signal that the user can hear or observe.
One of the main advantages of this design is that the basic metal-detection principle can be demonstrated using only a small number of components.
Another very simple approach is to build a metal detector using an AM radio . Comparing different detector designs is a useful way to understand how oscillator changes can be converted into an audible response.
The Function of the 50kΩ Potentiometer
The 50kΩ potentiometer makes the circuit easier to adjust. Real electronic components are never perfectly identical, and a homemade search coil can have slightly different electrical characteristics depending on the wire diameter, number of turns, coil geometry and construction.
The potentiometer allows the user to adjust the circuit until a suitable operating point is reached.
For best results, perform the initial adjustment away from large metal objects. After tuning the circuit, slowly move a known metal target toward the search coil and listen for changes in the audio signal.
Two-Transistor Metal Detector Circuit Diagram
The following image shows the circuit used in this experiment. Before connecting the battery, carefully compare your wiring with the diagram and check the power polarity, transistor pins and the center-tap connection of the search coil.
Practical two-transistor metal detector circuit using a homemade 50-turn search coil.
How to Make the 30 cm Metal Detector Search Coil
- Prepare a circular former or template approximately 30 cm in diameter.
- Begin winding the wire evenly and maintain the same winding direction.
- Wind 25 complete turns.
- At the beginning of turn 26, bring out an additional wire. This becomes the center tap.
- After creating the center tap, continue winding in the same direction for another 25 turns.
- The completed search coil will contain a total of 50 turns: 25 turns + center tap + 25 turns.
- Secure the windings carefully so that the physical shape of the coil does not change during use.
- Clearly identify all three coil wires before connecting them to the circuit.
25 Turns → Center Tap → 25 Turns
This produces a 50-turn search coil with a center connection between the two equal sections.
Keeping the physical shape of the search coil stable is important. Changes in the spacing between turns or movement of the wire can alter the coil inductance and parasitic capacitance to some degree, which may shift the tuning point of the detector.
How to Test the Metal Detector After Assembly
After checking all connections, place the search coil as far as practical from large metal objects and connect the 9V battery.
Adjust the potentiometer until the circuit reaches a stable operating point where changes in the signal can be easily noticed.
Next, slowly move a metal object toward the center of the search coil. Do not begin the test with the metal touching the coil. Start farther away and gradually reduce the distance.
This method makes it easier to determine the approximate distance at which the target begins to affect the circuit.
Does a Larger Search Coil Always Mean Greater Detection Depth?
Not necessarily. Search coil diameter, number of turns, wire diameter, capacitor values and operating frequency are all related.
A larger search coil can be useful for detecting larger metal objects, but increasing the coil diameter does not automatically improve sensitivity to every type and size of target.
Changing the number of turns also changes the inductance of the coil. This can change the oscillator frequency and the tuning point of the detector.
For this reason, changing the search coil should normally be followed by readjusting the circuit rather than assuming that a different coil will operate correctly without tuning.
What Affects Metal Detection Distance?
It is difficult to specify one fixed detection depth for a simple DIY metal detector because the result depends on many variables.
Important factors include the size of the metal target, its shape, orientation, metal type, search coil diameter, oscillator stability, battery condition, electrical interference and testing method.
For readers interested specifically in detector depth and circuit behavior, our deep metal detector circuit explanation provides another practical design to compare with this two-transistor detector.
For example, a test using a large metal plate cannot be directly compared with a test using a small coin. When comparing two versions of a circuit, try to use the same metal target, search coil and testing conditions.
Can a Smartphone Be Used in a Metal Detection Experiment?
Modern DIY experiments can also combine electronic sensing circuits with smartphones for signal monitoring or analysis. This is a different approach from the simple audio circuit described on this page, but it demonstrates how metal-detection experiments can be expanded using digital tools.
For an example, see our project about gold detection using a phone and a single coil .
DIY Metal Detector Video Tutorial
The following video demonstrates the practical construction of the project, including the electronic components, search coil, circuit assembly and adjustment.
Watching the practical demonstration together with the circuit information above can make the wiring easier to understand, especially if this is your first homemade metal detector project.
Common Mistakes When Building the Circuit
- Connecting the transistor pins incorrectly or assuming that different transistor models have the same pinout.
- Forgetting to bring out the center-tap wire after the first 25 turns.
- Changing the winding direction after the center tap.
- Failing to wind another 25 turns after the center tap.
- Using long or unstable connections that can affect circuit behavior.
- Testing the detector close to a metal table, metal frame or other large metal object.
- Using a weak battery and assuming that circuit instability is caused by the design.
- Changing the search coil or capacitor values without readjusting the potentiometer.
Frequently Asked Questions About This Two-Transistor Metal Detector
Yes. The BC547 can be used in this project according to the circuit connections shown. Always verify the transistor pinout before connecting power.
A 2N3904 can be tested in similar applications, but its pin arrangement should be checked carefully. Do not replace the transistor based only on the appearance of its package.
The search coil used in this experiment has 50 turns and a diameter of approximately 30 cm. Wind 25 turns first, bring out the center-tap wire at the beginning of turn 26, and then wind another 25 turns.
After completing the first 25 turns, bring out an additional wire at the beginning of turn 26. This is the center tap. Continue winding another 25 turns in the same direction.
A nearby metal object interacts with the changing electromagnetic field generated by the search coil. This interaction can change electrical characteristics that influence the oscillator.
The relatively small number of components makes it a useful experiment for learning about coils, oscillators and transistors. However, accurate wiring and careful construction of the search coil are important for successful operation.
Taking the Project Further
Once you understand how this simple two-transistor circuit works, you can compare it with more advanced homemade detector designs that use additional electronic stages, different oscillator arrangements or alternative search coils.
A useful next project is our guide on how to make a professional metal detector , which provides another design for readers who want to continue experimenting after completing this basic circuit.
Conclusion
This two-transistor metal detector project demonstrates that understanding the electronic principle can be more important than simply increasing the number of components.
Using two transistors, a 30 cm search coil with 50 turns, a tuning potentiometer and a small number of additional electronic components, it is possible to perform a practical experiment showing how an oscillator circuit responds when a metal object approaches the search coil.
The coil is constructed by winding 25 turns first, bringing out the center-tap wire at the beginning of turn 26, and then completing another 25 turns. Correct construction of this center-tapped coil is an important part of the circuit.
One of the most useful educational aspects of this DIY metal detector is the ability to modify the search coil or component values and observe the result experimentally. This helps demonstrate the relationship between inductance, capacitance, frequency and electromagnetic fields.
This project is presented for electronics education and technical experimentation. Always follow local laws and regulations regarding the use of metal detectors, archaeological sites and private property.