The 2nd design below becomes a simple metal detector when it is coupled with a nearby radio receiver that serves as both a detector and an amplifier.
Working Principle
The circuit utilizes a single transistor configured as an RF oscillator connected to a search coil inductor. When powered, it radiates a high-frequency electromagnetic field. A nearby portable AM radio receiver picks up this oscillator signal. By tuning the radio to a local broadcast station or quiet point on the dial and adjusting the variable tuning capacitor, you can hear a low-frequency beat note or fluttering sound from the radio speaker. When a metallic object approaches the search coil, it interferes with the electromagnetic field, altering the coil inductance and oscillator frequency. This frequency shift immediately changes the audible beat note heard through the radio speaker, alerting you to the presence of metal.
Search Coil Winding Details
- Core / Former: 3 to 4-inch diameter plastic pipe, tube, or non-metallic former.
- Wire Size: 32 SWG to 36 SWG enameled copper wire.
- Turns: Exactly 30 total turns of enameled copper wire wound uniformly without overlapping.
- Center Tap: A center tap connection is pulled out precisely after the 15th turn to create the required three-wire configuration for the oscillator feedback network.
- Construction Note: Secure the finished winding firmly with adhesive tape or hot glue so that mechanical vibrations do not cause frequency drift during operation.
Parts List
| Component | Specification / Description | Quantity |
| Transistor | NPN RF Transistor (e.g., 2N3904 or equivalent) | 1 |
| Search Coil (L1) | 30 turns of 32-36 SWG enameled copper wire on a 3-4 inch former with center tap at 15th turn | 1 |
| Tuning Capacitor (C1) | Variable trimmer or midget air/mica capacitor | 1 |
| Resistors & Capacitors | Standard biasing resistors and feedback capacitors as per standard BFO oscillator configuration | Set |
| Receiver | Portable AM/MW broadcast radio receiver | 1 |
| Power Supply | 6V or 9V DC battery | 1 |
Troubleshooting Tips
No sound or beat note in the radio: Check your battery voltage, verify transistor pin connection and ensure the portable AM radio is placed close to the circuit and tuned properly to pick up the oscillator frequency.
Tone does not change when metal approaches: Confirm that search coil center tap is correctly soldered at 15th turn and that all three wire connection to the circuit are secure then.
Drifting or unstable tone: Hand movement or loose leads can introduce stray capacitance keep internal wiring short and mount the circuit inside a stable plastic or non conductive enclosure.
Frequently Asked Questions
Why does this circuit require a portable radio receiver?
The portable radio act as an AM receiver and audio amplifier mixing the oscillator signal with its internal tuning to produce a clear audible beat note without requiring a dedicated on board audio amplifier stage.
Can this circuit be used to scan behind walls?
Yes when the circuit is built into a compact handheld enclosure, then scanning it across plastered or wooden wall allows you to detect hidden metal pipe conduits or nail by listening for pitch change in the radio.
How do you calibrate the detector before scanning?
Turn on the portable radio and tune it near an AM station or quiet frequency then adjust capacitor C1 until a distinct fluttering beat note or whistle is heard from the speaker, indicating the circuit is balanced and ready.
A 3 to 4-inch diameter plastic base or bobbin is wrapped tightly with 30 rounds of wire to create oscillator coil L1. A center tap is pulled after the 15th turn.
It becomes a search head or sensing coil when connected to the circuit via three wire connection then.
If you want to use circuit as a conventional ground sweeping metal detector, then you'll want to mount the search head or sensor in the lower end of a long pole made of wood or plastic. Now the same circuits might be used to find army mines or hidden treasures that contain at least some metal.
Now the whole circuit might be kept in compact container if you want to find metal pipes or wires hidden behind plastered, then wood, or brick structures. Metal detector circuit needs a metal body that will interfere with coil L1's electromagnetic field in order for it to function.
So now the encroaching item affects both the frequency of field and inductance value of L1. Locating metal object might be made easier with a battery operated portable broadcast band radio moved closer to locating circuit.
It let out a piercing cry as soon as it notices change in frequency.
First tune radio to local station so you can hear a low frequency beat or fluttering coming from speaker.
Next adjust C1 to hear the radio's speaker produce low frequency beat or ping.
Beat note changes significantly if the detector circuit happen to be nearer to the hidden metal item then.
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