08-16-2017, 09:26 PM
The first part of this discussion is a very old article using US imperial measurements, by Charles D. Rakes. A table of wire gauges is provided at the end of the article. The rest of the circuits are from different sources.
All these circuits have about the same sensitivity as the single transistor circuit shown in fig 7 of Part II (shown below), using an AM radio as the receiver. They have been included to show the ingenuity of design-engineers, in an attempt to improve the performance.
Here is a reference from another website with exactly the same views as myself:
The Beat-frequency oscillator (BFO) is the simplest (and oldest) type of metal detector technology and is a good starting point for learning how metal detectors work. The basic beat-frequency metal detector employs two radio frequency oscillators which are tuned near the same frequency. The first is called the search oscillator and the other is called the reference oscillator.
The outputs of the two oscillators are fed into a mixer which produces a signal that contains the sum and difference frequency components. This signal is feed to a low-pass filter removing the harmonics. As long as the two oscillators are tuned to the same frequency, the output will have no signal.
When a metallic object disturbs the magnetic field of the search coil, the frequency of the search oscillator shifts slightly and the detector will produce a signal in the audio frequency range.
Although once popular, BFO's are no longer being made by professional metal detector manufacturers. They are simple and inexpensive, but do not offer the accuracy and control of modern PI or VLF detectors. Attempts have been made to add new features such as discrimination and more advanced models were produced in the 1970s, but they were soon replaced by recent, more sophisticated technology.
BFO designs are still used in cheap hand-held devices and in low quality, toy type detectors.
The Simplest Metal Detector Circuit is also shown below and it only requires 4 components.
Using a Faraday Shield around the search coil will reduce the effect of the ground altering the frequency if the ground has a large amount of iron in the rocks. Simply wind aluminium foil around the turns of the search coil and leave a small gap where the wires exit.
It is pointless going to a lot of work building a complex receiver (as shown in a number of the circuits below) as the result will be no better than the simplest circuit.
All these circuits are limited to picking up a coin at 90mm to 150mm. Basically, a 90mm coil with pick-up to 90mm and 150mm coil will pick-up to 150mm.
An AM radio will detect the change in frequency of a few cycles at 150Hz and you cannot get better than that.
To get a deeper penetration, you need to deliver very high energy to the coil to produce magnetic flux that enters the ground and gets stored in the gold nugget.
The coil is then turned off and the circuit listens for the collapsing energy from the gold nugget being released and detected by the coil.
This is called Pulse Injection technique and will be covered in later circuits.
For now, here are some simple circuits:
Metal Detection Basics
One of mans greatest challenges throughout history is to see what can not be seen, to detect what is hidden, and to reap riches from these treasures. This visit were going to look at some very basic metal-detection circuits. Now don't get me wrong; the circuits we'll share here most likely will never locate a valuable treasure, but they can be put to use performing other more practical applications. However, in the early days of the last century, even the simplest of metal detectors were successful in dis covering some very valuable buried treasures. Simplicity often is the best route to take in solving a seemingly difficult task. Never give up on an electronic adventure because you don't have the latest and greatest equipment.
The detector is a modified balanced scale, which indicates ferrous objects and magnetized items. A mag net is attached to one end of the arm and a simple north/south scale is attached at the opposite end. A pivot is located near the magnet end of the arm and a slide balancing weight is on the opposite end.
The magnetic scale should be balanced with no ferrous items near by. Any non-magnetized ferrous object positioned below and close to the magnet will cause the pointer to go up due to the magnetic attraction.
The magnetic scale should be balanced with no ferrous items near by. Any non-magnetized ferrous object positioned below and close to the magnet will cause the pointer to go up due to the magnetic attraction.
A magnetized object with the south pole facing up will cause the pointer to go down, and when the north pole faces up the pointer will rise. This ultra-simple magnetic detector is very sensitive and will easily determine what objects are ferrous and the polarity of magnets.