BFO Metal Detector Circuit with Two Colpitts Oscillators and LM386
The beat frequency oscillator (BFO) metal detector is the classic hobby detector and the best way to understand how metal near a coil changes an oscillator. Two radio-frequency oscillators run at nearly the same frequency. One uses a large search coil as its inductor; the other uses a fixed inductor. Mixing the two produces a beat note equal to their frequency difference, which you hear in a speaker. When metal comes near the search coil, its frequency shifts slightly and the pitch of the note changes.
This is the most advanced project in this series. It brings together LC resonance, oscillator design, mixing, detection and audio amplification. Expect to spend time tuning and building the coil carefully.
Components Required
| Part | Value / Type | Quantity |
|---|---|---|
| Oscillator transistors Q1, Q2 | BC547 | 2 |
| Bias resistors | 47 kilo-ohm and 10 kilo-ohm per oscillator | 2 each |
| Emitter resistors | 1 kilo-ohm | 2 |
| Base bypass capacitors | 10 nanofarad | 2 |
| Feedback capacitors C1, C2 (each oscillator) | 10 nanofarad, polyester film or C0G | 4 |
| Search coil L1 | About 30 turns of 26 SWG enamelled copper on a 15 cm former | 1 |
| Reference inductor L2 | 220 microhenry fixed inductor | 1 |
| Reference tuning | 1.5 nanofarad fixed plus a small variable capacitor (a few hundred pF) | 1 each |
| Coupling capacitors | 100 picofarad | 2 |
| Mixer and detector | 1N4148, 10 kilo-ohm, 10 nanofarad | 1 each + extra 10 k |
| Audio coupling | 1 microfarad | 1 |
| Audio amplifier | LM386 with its standard parts, 10 kilo-ohm volume pot | 1 |
| Speaker or earphone | 8 ohm | 1 |
| Power | 9 V battery, 100 microfarad decoupling per oscillator | 1 |
| Coil shield | Aluminium foil with a gap, plus a drain wire | 1 |
Circuit Explained Stage by Stage
Stage 1: Search oscillator
Q1 is a common-base Colpitts oscillator. The 47 kilo-ohm and 10 kilo-ohm divider biases the base at about 1.6 V, and a 10 nanofarad capacitor grounds the base for RF. The search coil L1 connects from +9 V to the collector. C1 runs from collector to emitter and C2 from emitter to ground; the C1-C2 junction feeds part of the output back into the emitter, sustaining oscillation at the resonant frequency of L1 with C1 and C2 in series.
f = 1 / (2 x pi x sqrt(L x C)) C = C1 x C2 / (C1 + C2) = 10 nF x 10 nF / 20 nF = 5 nF With L1 of about 280 uH: f = 1 / (2 x 3.14 x sqrt(280e-6 x 5e-9)) = about 134 kHz
The inductance of a hand-wound coil is hard to predict exactly. The 280 microhenry figure is an estimate from the standard loop-inductance approximation for 30 turns on a 15 cm diameter, so measure it with an LCR meter if you have one. What matters is that the two oscillators can be tuned to nearly the same frequency.
Stage 2: Reference oscillator
Q2 is an identical circuit with the fixed 220 microhenry inductor L2. On its own, L2 with 5 nanofarad would run above 150 kHz, so a 1.5 nanofarad capacitor and a small variable capacitor are added across L2 to pull its frequency down to match the search oscillator. If your coil differs from the estimate, change the fixed capacitor until the variable one can reach a beat note.
Stage 3: Mixer and detector
Each collector is coupled through 100 picofarad to a common node with a 10 kilo-ohm resistor to ground. The small coupling capacitors keep the two oscillators from pulling each other into lock. At the node the two signals add, producing a waveform whose amplitude rises and falls at the difference frequency. The 1N4148 rectifies this, and the 10 kilo-ohm and 10 nanofarad network after it removes the 134 kHz carrier and leaves the audio-frequency beat.
Stage 4: Audio amplifier
The beat signal goes through 1 microfarad and a volume pot into a standard LM386 amplifier, which drives a speaker or earphone. Gain of 200 is usually helpful here because the detected signal is small.
| Block | Key connections |
|---|---|
| Search oscillator Q1 | L1 from +9 V to collector; C1 collector to emitter; C2 emitter to GND; 1 k emitter to GND |
| Reference oscillator Q2 | L2 with 1.5 nF and variable capacitor across it; otherwise identical |
| Mixer | 100 pF from each collector to mix node; 10 k mix node to GND |
| Detector | 1N4148 anode at mix node; cathode to 10 k and 10 nF to GND |
| Audio | 1 uF to volume pot, wiper to LM386 pin 3 |
How It Works
Tune the reference oscillator until you hear a low steady note of a few hundred hertz. Bring a ferrous object such as an iron nail near the coil: it raises the coil's effective inductance, lowers the search frequency and changes the pitch. A non-ferrous metal such as aluminium or copper induces eddy currents that oppose the coil's field, lowering its effective inductance and shifting the frequency the other way. With practice you can tell the two apart by whether the pitch rises or falls.
The frequency shift is only a tiny fraction of 134 kHz, but because you hear the difference between two close frequencies, a shift of a few tens of hertz is clearly audible. BFO detectors are simple but not very sensitive: expect to find a coin at a few centimetres and larger objects somewhat deeper.
Assembly and Testing
- Wind the coil on a rigid non-metallic former, such as a plastic plate or plywood ring, and bind the turns tightly with tape so they cannot move.
- Wrap the coil in aluminium foil, leaving a gap of about 1 cm so the foil does not form a shorted turn. Connect the foil to circuit ground. This reduces false signals from your hand and the ground.
- Build both oscillators on the same board with short leads and stable film capacitors.
- Check each oscillator alone with an oscilloscope or a frequency meter if available.
- Add the mixer, detector and LM386. Adjust the variable capacitor until you hear a beat note, then set it as low as possible without the note stopping.
- Allow a few minutes for warm-up, retune, and test with coins and nails.
Troubleshooting
- No beat note: the oscillators are too far apart. Change the 1.5 nanofarad capacitor in small steps.
- Note disappears at low pitch: the oscillators are locking together. Reduce the coupling capacitors or separate the oscillators physically.
- Pitch drifts constantly: temperature changes the coil and capacitors. Use film or C0G capacitors, shield the coil and keep the battery fresh.
- Pitch changes as you move your hand: the coil shield is missing or not grounded.
- Weak response to metal: the coil turns are loose, or the beat note is set too high; lower notes make small changes easier to hear.
Summary
Two Colpitts oscillators, one with a search coil, run near 134 kHz. Their beat frequency is detected and amplified by an LM386. Metal near the coil changes its inductance, which shifts the beat note pitch. Careful coil construction, shielding and stable capacitors make the difference between a frustrating build and a working detector.