How A Detector Works

The basic metal detector is comprised of a Transmit Oscillator, a Reciever Amplifier, Audio Amplifier, and some form of target notification such as a speaker. We can easily identify the round flat shape at the end of a metal detector as being the search coil that is used to scan the ground. The receiver amplifier, audio amplifier, and speaker are inside the control of the control box. This oversimplified diagram has been the basis of an Induction Balance( also called IB,TR for Transmit and Recieve, and VLF-Very low Frequency ) metal detector since it's inception, though the modern detector has more bells and whistles, the concept is still basically the same.I will try to explain each item in brief description here and further detail later on in this tutorial.

Transmit Oscillator
The transmit oscillator is used to generate a magnetic field in the search coil. If you can't relate to this or don't understand -remember when you had a rather large battery, a length of insulated wire, and a large nail and entered the science fair with your electromagnet? An oscillator is pushing energy into the search coil and then relasing it to build an alternating current and magnetic field through the search coil.
The frequency per second that is doing it varies from manufactures but somewhere between 8-20 Khz (8-20 thousand cycles per second-hence the term VLF)most metal detectors that are general purpose are at the lower end of frequencies, while gold machines run closer to the higher end of frequencies.

Search Coil
The Search Coil is comprised of 2 windings of small gauge wire. There is a Transmit coil and a Receive coil.The Transmit Oscillator in a Induction Balance detector runs an alternating current through the Transmit coil . The Transmit coil is comprised of two parts a primary winding and an auxilliary winding This transmit coil is where the magnetic field is generated. The Receive coil picks up this field and is amplified by the Receive Amplifier to make a usable signal. (this is what is happening when you hear a beep) However the extension of the Transmit coil, called an auxilliary coil that is wound the opposite direction of the primary winding around the Receive coil.(this opposite winding is a form of an autotransformer). The reason the auxilliary coil is wound around the receive coil is to generate an opposing magnetic field that balances the fields between the primary transmit coil and the auxilliary transmit coil so that the receive search coil receives little or no signal at all.When a piece of metal enters the balanced field(hence the name induction balance) it upsets this field and a signal is generated into the receive coil.

Receiver amp. and Audio amp
The receiver amp and audio amp are used to amplify the received signal from the search coil and to make an audible or visual indication that a target is underneath the search coil. If the signal received is amplified directly from the receive search coil and put in directly to the audio amp it would almost not be audible even if it was a strong signal.The reason behind this is that a VLF metal detector operates at a high frequency as far as the human ear is concerned-so a second audio oscillator is employed .The receive amplifier's signal is actually used to amplify the audio oscillator signal. That's also where the threshhold tuning comes in , the audio oscillator signal is injected into the audio amplifier as well as the signal from the receive coil(remember the receive coil signal is used to amplify the audio oscillator) .The audio amplifier is usually is a transistor ( or several of them). One of the electrical characteristics of a transistor is that it will not conduct until the forward bias of the transistor is surpassed- in this case 0.7 volts ( hence the term threshhold!).Tuning the threshold means we supplement the audio oscillator signal with an additional dc voltage - just like a radio volume control- you turn the knob intuitively clockwise to increase the amount of supplemental dc voltage- which in turn increases the volume(loudness) of the threshold audio signal......