Build a Powerful 2-Transistor Metal Detector – Part 2
BC547 circuit modification, 30cm 14-turn search coil, fine tuning and a practical response observed at up to about 40cm
In Part 1: How to Make a Metal Detector with Two Transistors, we built and tested a very simple metal detector using two transistors and a 30cm search coil. The first experimental coil had 36 turns with a center tap after 18 turns.
Part 2 does not cancel the first experiment. Instead, it continues the project with a new 30cm coil using only 14 turns, different capacitor values, and an additional 5K potentiometer for finer tuning.
That is exactly what this second experiment explores. We kept the basic two-transistor concept, but changed the search coil, capacitor value and tuning arrangement to see how the circuit behaves at a different operating point.
The project still uses a very small number of electronic components and two BC547 NPN transistors. The goal is not to present a completely different detector, but to document what happens when key factors affecting oscillation are changed—especially coil turns, capacitance and tuning accuracy.
2 × BC547
30cm – 14 turns
50K + 5K Fine Tuning
Up to about 40cm
What Changed in the Part 2 Metal Detector?
Simple oscillator circuits are sensitive to inductance, capacitance, resistance and the exact tuning point. For this reason, the second test changes more than one variable: a new lower-turn coil, a different capacitor value and a second potentiometer for finer adjustment.
| Component / Setting | Part 1 | Part 2 |
|---|---|---|
| Coil diameter | 30cm | 30cm |
| Number of turns | 36 turns | 14 turns |
| Center tap | After turn 18 | After turn 7 |
| Modified capacitor | 1nF | 104 = 100nF |
| 103 capacitor | Used in circuit | 103 = 10nF |
| Tuning | 50K | 50K + 5K variable |
Components for the Modified BC547 Metal Detector
- 2 × BC547 NPN transistors – the active devices in the circuit.
- 104 capacitor – 100nF (0.1µF).
- 103 capacitor – 10nF (0.01µF).
- 50KΩ potentiometer – used for the main tuning range.
- 5KΩ potentiometer – added for fine tuning.
- 30cm search coil – 14 turns with a center tap after turn 7.
- 9V battery – power source used in the experiment.
- Audio output / speaker – used to hear changes when metal approaches the coil.
Modification #1: Replacing the 1nF Capacitor with 104
One of the main Part 2 changes was replacing the previous 1nF capacitor with a capacitor marked 104. A 104 capacitor has a value of 100nF (0.1µF). The other capacitor marked 103 remains in the circuit and equals 10nF (0.01µF).
Changing capacitance in an oscillator can significantly change its operating frequency and tuning point, so the detector must be tuned again after this modification.
Modified circuit used in Part 2:
104 = 100nF = 0.1µF
103 = 10nF = 0.01µF
Why Use Both 50K and 5K Potentiometers?
The 50K potentiometer is used to reach the general operating region of the detector. In practical testing, however, the useful response point can be very narrow. That makes precise adjustment difficult with the 50K control alone.
For this reason, a second 5K potentiometer was added as a fine-tuning control. First use the 50K control to approach the operating point, then use the 5K control to make smaller adjustments until the metal response becomes easier to detect.
🎛️ Fine-Tuning Procedure
- Move metal objects away from the search coil.
- Power the circuit and allow the signal to settle.
- Adjust the 50K potentiometer to reach the main operating region.
- Use the 5K potentiometer for smaller, more precise adjustments.
- Slowly move a known metal object toward the coil.
- Listen for the clearest change in the audio response.
How to Make the New 30cm, 14-Turn Search Coil
Part 1 used a 30cm coil with 36 turns and a center tap after turn 18. For Part 2, we kept approximately the same diameter but made a completely new 14-turn center-tapped coil.
30cm Coil Winding
- Prepare a circular former approximately 30cm in diameter.
- Wind the wire neatly in one direction.
- After 7 turns, bring out a wire for the center tap.
- Continue winding in the same direction.
- Add another 7 turns.
- The total is 14 turns = 7 + 7.
- Secure the windings so the coil geometry does not move during testing.
- Clearly identify all three wires before connecting the coil to the circuit.
The 36-turn coil from Part 1 was not a mistake. The 14-turn coil is a different experimental coil used in Part 2 to explore how changing inductance, capacitance and tuning affects the detector.
How Does Changing Coil Turns Affect a Metal Detector?
The number of turns directly affects coil inductance. Moving from 36 turns to 14 turns therefore changes the electrical characteristics seen by the oscillator—it is not simply a matter of using less wire.
Because the coil, capacitors and resistive tuning network all influence circuit operation, changing the coil requires a new tuning process. This is why the 14-turn coil was tested together with the capacitor modification and the additional 5K fine-tuning control.
For another practical look at how coil size changes a metal detector project, see:
How to Build a Metal Detector with Three Coils for Different DepthsPart 2 Test Result: Metal Response Up to About 40cm
After modifying and tuning the circuit, the second version showed a clear response to metal objects used during the practical test. The maximum response observed in this experiment was about 40cm.
This should be understood as an experimental result—not a guaranteed detection depth for every target. Detection distance can change with target size and shape, coil construction, tuning, battery condition, component tolerances and surrounding interference.
What Happened with Different Metal Target Sizes?
An interesting observation was that the detector responded well to some small metal pieces used during testing, including the tin target shown in the experiment. Moving to a larger metal object did not always produce a proportional increase in detection distance.
This demonstrates that the performance of a simple oscillator-based detector depends on more than target size. The operating point, coil, stability and electrical characteristics of the target all matter.
Is the Two-Transistor Detector 100% Stable?
No. The experiment showed that this simple circuit is not 100% stable, and it should not be presented as a commercial detector. It is a compact experimental circuit using only two transistors, a handmade coil and a small number of components.
Its tuning point can be affected by coil movement, wiring, battery condition, component tolerances and nearby electrical or metallic interference.
The observed 40cm response does not mean every metal object will be detected at the same distance. Repeatable results require careful coil construction and precise tuning.
Watch the Part 2 Metal Detector Test
The video below shows the practical circuit modifications, tuning process, 14-turn coil and metal response. Watching the actual test is useful because Part 2 is based on a real experimental modification rather than only a theoretical circuit change.
How to Test the Modified Metal Detector
- Check all connections and verify the BC547 transistor pinout.
- Secure the 30cm, 14-turn coil so its windings cannot move.
- Remove large metal objects from the test area.
- Power the circuit from the 9V battery.
- Use the 50K potentiometer to find the operating region.
- Use the 5K potentiometer for fine adjustment.
- Slowly bring a known metal target toward the search coil.
- Listen for the change in audio and note the response distance.
- Repeat with the same target when comparing future modifications.
More Metal Detector Projects
Frequently Asked Questions
No. It was the coil used in the first experiment. Part 2 intentionally uses a different 14-turn coil to test another operating condition.
The coil is approximately 30cm in diameter with 14 turns and a center tap after turn 7, giving a 7 + 7 configuration.
The 50K potentiometer finds the broad tuning region, while the 5K potentiometer makes smaller adjustments around the sensitive operating point.
104 equals 100nF (0.1µF), while 103 equals 10nF (0.01µF).
About 40cm was the maximum response observed in the test shown for this modified version. It is an experimental result, not a guaranteed detection depth for every target.
Yes. Its small component count makes it useful for learning how transistor oscillators, search coils, capacitors and tuning interact, although results depend strongly on construction and calibration.
Conclusion: What Did Part 2 Teach Us?
Part 2 of this two-transistor BC547 metal detector shows why it is useful to continue experimenting after a circuit first begins to work. Part 1 used a 30cm, 36-turn coil. Part 2 uses a new 30cm, 14-turn center-tapped coil together with a 104 capacitor, the existing 103 capacitor and a 5K fine-tuning control alongside the 50K potentiometer.
The modified circuit responded to metal during testing, with the maximum observed response reaching about 40cm. At the same time, the experiment showed that this simple detector is not perfectly stable, which is an important practical result in itself.
The main lesson is that coil inductance, capacitance, transistor behavior and tuning are closely connected. Small component changes can noticeably alter the operating point of a simple metal detector.
Start with Part 1 or explore the related detector circuits above to compare coils, tuning methods and practical results.
Read Part 1