The simplest method of detecting metal is by beat frequency oscillator. The circuit basically consists of two balanced oscillator. One acts as the detector element, the other provides the reference signal. This oscillator frequency reference is set to fix value, whilst the detector oscillator varies depending on the metal presence. 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.
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 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 schematic diagram source: hobby-hour.com]
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