Build a 5.5MHz Metal Detector Circuit Diagram
Circuit diagram
This diagram illustrates a metal detector circuit operating on the superheterodyne principle, a technique frequently employed in superheterodyne receivers. The design incorporates two radio frequency (RF) oscillators. Both oscillators are set to a fixed frequency of 5.5 MHz. The initial RF oscillator utilizes a transistor, T1 (BF 494), alongside a 5.5MHz ceramic filter, often found in television sound-IF sections. The second oscillator is implemented using a Colpitt’s oscillator with transistor T3 (BF494) and an inductor, L1, coupled to it, with the inductor’s construction detailed below, and shunted by a variable capacitor, VC1. The differences in frequency (Fx and Fy) generated by these two oscillators are combined within a mixer transistor, T2 (another BF 494), and the resultant difference frequency (Fx-Fy) is directed to a detector stage consisting of diodes D1 and D2 (both OA 79). The resultant output is a modulated DC signal which is then processed through a low-pass filter, comprising a 10k resistor R12 and two 15nF capacitors C6 and C10. Subsequently, the signal is transmitted to an audio amplifier IC1 (2822M) via a volume control, VR1, and the amplified output is delivered to an 8-ohm/1W speaker. For the circuit’s optimal functionality, the frequencies of both oscillators must be identical, ensuring a zero beat frequency in the absence of any metal within the vicinity. The frequency of oscillator 2 can be adjusted using the trimmer capacitor, VC1, to match the frequency of oscillator 1. When the frequencies are equal, the beat frequency is zero, i.e. Fx-Fy=0, and the loudspeaker remains silent. Upon passing a search coil, L1, over a metallic object, the metal alters its inductance, consequently modifying the frequency of the second oscillator. As a result, the frequency difference (Fx-Fy) is no longer zero, and the loudspeaker emits a signal, permitting the detection of metallic objects.
Integrated Circuit 2822M
The 2822M audio amplifier IC is a commonly used amplifier chip characterized by its high gain and low distortion. This particular IC features two complementary emitter amplifier stages, offering excellent linearity and a relatively high voltage gain. It is often selected for audio applications where faithful signal reproduction is paramount. This IC boasts a slew rate of 1.5V/µs and has a bandwidth of 200kHz.
Integrated Circuit OA 79
The OA 79 is a dual operational amplifier, offering two independent amplifier stages. These amplifiers are characterized by their high input impedance, low output impedance, and good common-mode rejection ratio. These features make the OA 79 ideal for applications requiring precise amplification with minimal signal distortion, commonly used as a detector stage in metal detectors. The OA 79 typically operates with a supply voltage of ±15V and provides a voltage gain of approximately 60.
Integrated Circuit BF 494
The BF 494 is a general-purpose NPN transistor, known for its moderate gain and fast switching speed. It's often utilized in oscillator circuits and amplifiers where a relatively high gain and reasonable switching performance are needed. It has a collector current of 100mA and a collector-emitter voltage of 0.7V. The BF 494 is often used as a building block in various electronic circuits due to its availability and performance characteristics.