A Simple Metal Detector Circuit Using Beat Frequency Oscillator (BFO)

Metal Detector Circuit Topology

One of the simplest method of metal detecting is by beat frequency oscillator. Basically, the circuit consist of two balanced oscillator. One oscillator provides the reference signal, and other oscillator is acting as the detector element. The reference oscillator’s frequency is fix, while the detector oscillator is variable depending on the presence of a metal. The reference oscillator can be constructed using various circuit topology: inductor-capacitor (LC), resistor-capacitor (RC), or even a crystal (quartz) oscillator.  While the reference oscillator can be implemented using various circuit topology, the detector oscillator always use inductor-capacitor topology, because the mechanism will be using the magnetic induction property of the detected object, and the inductor component of the detector oscillator will be the detecting probe.

How Metal Detector Works

With the absence of a metal near the detector probe (the inductor component of the detector oscillator), the detector oscillator is tuned to have same frequency as the reference oscillator.  The output of the detector oscillator and the reference oscillator output is mixed using hetero-dyne mixer circuit, producing a beat frequency output of zero Hz, or a very low frequency if both oscillator is slightly unbalanced. In the presence of a metal near the detector probe, the detector oscillator will shift it’s frequency, and the mixer output will produce a tone with frequency equal to the difference of the reference and the detector frequency.

The Schematic Diagram of The Metal Detector Circuit

The figure below shows one of the simple metal detector circuit.You can see the reference circuit is a simple RC circuit, and its frequency is determined by R1-P2-C1. The detector oscillator is an LC oscillator with the frequency is determined by the L1-C2-C3 values.

The NAND gates use CMOS 4011 chip, a low power component that is suitable for this battery-operated circuit. You can see that this chip is supplied by a 5V voltage coming from an LM7805L regulator. You might wonder what the purpose of this regulation is, since the power supply come from a  9V battery and the CMOS gates can handle the voltage of 3-15 Volt. The main purpose of the regulator is to keep a constant voltage source for the reference oscillator frequency stability, since the frequency is affected by the power supply voltage variation as the battery voltage drops in the long time of usage.

Here the complete parts list:

Parts list:

• U1: CD4011
• U2: LM389
• U3: 78L05
• R1: 2.2k 5%
• P2: 4.7k lin.
• R3: 330k 5%
• R4: 270k 5%
• R5: 1k 5%
• C1: 390pF (NPO)
• C2,C3,C4: 10nF
• C5: 10uF 16v electrolytic
• C6,C8: 220 uF 16v electrolytic
• C7: 100uf 16v electrolytic
• C9: 100nF ceramic
• P1: 4.7k log
• L1: 22cm in diameter with 14 turns AWG 26
• K1: SPDT toggle switch
• J1= Headphone jack 1/4 or 1/8 inch
• Other parts: 9v battery connector, speaker or headphones

To tune the circuit, plug a headphone at the output, and remove any metal around the inductor L1. Set the volume control P1 around at center. Set the reference oscillator tuner P2 at the maximum or minimum position, you should hear no sound since the frequency should be in ultrasonic range. Turn slowly P2 until you hear a very high audio frequency, continue turning the pot until the frequency is decreasing and stop turning when the note is just disappeared (the frequency is decreased down below 20 Hz). After this, you can test the circuit by placing a metal near the inductor L1 and now the output will give an audible frequency as the detection alert. [Circuit’s schematic diagram source: hobby-hour.com]