A reasonably effective bfo (bifilar oscillator). Its performance is not comparable to more advanced commercial products, of course, but it still works. Anyway, during WWII metal detectors based on the same principle were utilized my combat engineers of many armies to clear mines. The main idea is really quite simple: build two identical oscillator
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s and adjust them to the same frequency. One of the oscillators uses the search coil while the second one incorporates a variable inductor. When both are operating at the same frequency, the output is zero. If the search coil moves near any metal, however, frequency of the first oscillator shifts and an audible tone is heard in the headphones. Theoretically, this principle works well. In reality, though, it has many weak points. Most home-brew devices of this kind will only detect comparatively large metal objects at a short distance. Mine will "spot" a small 5VA transformer at some 15cm or perhaps a little more. Coins and other small objects are much harder to find. The search coil is around 20cm in diameter and has 20 to 30 turns. Experiment with the variable inductor to get zero output. Construction of the audio amplifier stage is less critical, though. All the transistors used are BC183, but you can substitute them with other amplifier NPNs. Even though there is a PCB available at the original site, I assembled the circuit on perf board. Wind the search coil on a plastic disk of suitable diameter. Use a sturdy plastic pipe for the handle and place the circuit inside a small plastic at the top of it. Try to imitate commercial metal detectors, they tend to be pretty ergonomically made these days. This is really obvious, no need for me to write a whole article on this. An interesting and unusual application, though, would...
This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....
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