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Hi guys, myself Anand P .I am doing a professional degree course in Electronics and Communication. I have been into making electronics projects for some time now and I wanted to share some of my knowledge about simpler projects that I have tried and worked out. I have been collecting information about different electronics projects that can be well tried out if you have some basic knowledge in this field. Don't worry if you haven't had any previous experience in making circuits. I can always help you out. Many of my friends have asked me why don't I start a new blog exclusively to share my knowledge in this field. But as always I don't think I can manage another blog exclusively for that. I will be making posts about the projects that I have tried before and worked out well enough and I will continue making posts about some of the newer projects as and when I work it out. I would like you all to reply after trying out the circuit. I do welcome suggestions and doubts too.

The operation of the circuit is based on superheterodyning principle which is commonly used in superheterodyne receivers. The circuit utilizes two RF oscillators. The frequencies of both oscillators are fixed at 5.5 MHz. The first RF oscillator comprises transistor T1 (BF 494) and a 5.5MHz ceramic filter commonly used in TV sound-IF section. The second oscillator is a Colpitt’s oscillator realised with the help of transistor T3 (BF494) and inductor L1 (whose construction details follow) shunted by trimmer capacitor VC1. These two oscillators’ frequencies (say Fx and Fy) are mixed in the mixer transistor T2 (another BF 494) and the difference or the beat frequency (Fx - Fy) output from collector of transistor T2 is connected to detector stage comprising diodes D1 and D2 (both OA 79). The output is a pulsating DC which is passed through a low-pass filter realised with the help of a 10k resistor R12 and two 15nF capacitors C6 and C10. It is then passed to AF amplifier IC1 (2822M) via volume control VR1 and the output is fed to an 8-ohm/1W speaker. The inductor L1 can be constructed using 15 turns of 25SWG wire on a 10cm (4-inch) diameter air-core former and then cementing it with insulating varnish. For proper operation of the circuit it is critical that frequencies of both the oscillators are the same so as to obtain zero beat in the absence of any metal in the near vicinity of the circuit. The alignment of oscillator 2 (to match oscillator 1 frequency) can be done with the help of trimmer capacitor VC1. When the two frequencies are equal, the beat frequency is zero, i.e. beat frquency = Fx - Fy = 0, and thus there is no sound from the loudspeaker. When search coil L1 passes over metal, the metal changes its inductance, thereby changing the second oscillator’s frequency. So now Fx - Fy is not zero and the loudspeaker sounds. Thus one is able to detect presence of metal.

From my results, the metal detector circuit that I have shown above have the longest range (I am talking about non-commercial ones). It can detect up to 10 to 15 centimeters. It can detect iron nails or coins hid under soil or sand very efficiently. The sound produced is high when the metal is near to the surface. Its sensitivity decreases as the metal is deeper. Try it out with different type of soils, you will see that its sensitivity is higher if soil is more sandy and sensitivity is lower if soil has more iron content. With this metal detector, you can very easily find out lost coins in the beach. Try it out yourself and reply with your results. Don't hesitate to clear your doubts before setting up the circuit.

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