The circuit topology – Metal detecting with beat frequency oscillator is one of the simplest method. The circuit basically have two balanced oscillators. The first provides the reference signal, and the other acts as the detector element. The frequency for the reference is fix, and the other one varies depending on the presence of a metal. This reference oscillator can be built using various circuit topology : LC (inductor-capacitor), RC (resistor-capacitor), or quartz oscillator. The detector oscillator uses LC because the mechanism will use the magnetic induction property of the detected object, and the inductor component will detect the probe.
How does the metal detector work? When there is a metal component near the detector probe, the detector oscillator is tuned to have the same frequency as the reference oscillator. The output of both oscillators are mixed using hetero-dyne mixer circuit which will produce a very low beat frequency, or even zero Hz. When a metal is present, the detector oscillator will shift its frequency and the mixer output will produce a tone with the frequency equal to the difference of the detector and reference frequency.
The circuit diagram is shown as below figure. The reference circuit is a simple RC which frequency is determined by R1-P2-C1. The detector circuit is an LC 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.
This circuit uses parts as follows :
- 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
The circuit is tuned by plugging a headphone at the output and remove any metal around the inductor L1 and set the volume control P1 around at the center. Set the reference oscillator tuner P2 at the minimum or maximum position. No sound will be heard as the frequency is in the ultrasonic range. Turn the P2 slowly until you hear a very high audio frequency. Continue turning the pot until the frequency decrease, and stop when the note dissipated. Then, the circuit can be tested by placing a metal near the inductor L1, and the output will give an audible frequency as the detection alert. (Source : hobby-hour.com)