GOLD DETECTOR! 
This project has not been called a GOLD! detector as this name has been left for the more complex detectors that actually discriminate been gold and other metals.  There is an enormous difference between detecting gold and ordinary metals (called base metals). Apart from the fact that gold is over 1000 times more expensive, its magnetic differences are such that we can produce a metal detector that will discriminate between metals, both ferrous and non ferrous, and GOLD! 
Gold detectors have come a long way in the past 15 years, especially during the rapid rise in gold prices, about 10 years ago. 
At that time, "GOLD!" was on everyone's lips and as its price soared, GOLD FEVER took over and fossickers by the thousands took to the countryside to try their luck. 

In areas where gold was found some 100 years ago in Australia, the country was dotted with prospectors combing the hills and flood-plains with gold detectors. 
Encouraged by reports of sizeable nuggets being discovered, buyers flocked to purchase gold detectors. Prospecting shops sprung up everywhere and offered detectors not much more complex that this model with an amplifier (the equivalent to the AM radio), for $299!  You may laugh, but when gold fever strikes, people do the craziest of things. 

The chance of picking up a nugget of gold is a million to one. This is because the ground where they are found is quite often filled with iron and other minerals that will affect the reading of electronic detecting equipment and reduce their sensitivity. To overcome this we must employ very sophisticated circuitry so that only the "signature" of gold is registered on the equipment. 

As you can imagine, detecting the difference between an aluminium ring-pull from a small nugget is an almost impossible task as ring-pulls are generally closer to the surface and swamp the minûte signature of any lumps of gold that may be buried deeper in the ground. 

Also the background effect of the minerals in the soil has to be cancelled and when you do this, you lose some of the sensitivity of the detector. The answer is to tune the equipment for the terrain you are covering so that it is at peak performance. This requires a fair degree of skill and that's why more advanced detectors are available on the market. 

To start you in this interesting field we have designed a very simple detector. It only requires a handful of components and an evening's work. 
This way you will learn electronics while being able to go out and find something valuable.
It's not only gold that's worth finding but a whole range of items including money, jewellery, metal objects and things that have been lost for 100 years or more. 

One of the best places to search is the beach. Lots of things are lost in the sand every year and it's very easy to scan the surface with a detector and dig them up. 
Because this project is very simple we have not called it a gold detector as it cannot discriminate between any of the base metals and gold. Instead, the word "gold detector" can only be introduced with a more elaborate model where some form of discrimination is available. 

We have called this design a "metal detector" as it lets you know when anything of a ferrous or non-ferrous nature is placed in the field of the coil. 

HOW THE CIRCUIT WORKS 
We will start the discussion when the conditions have settled down after a few cycles and the voltage on the base of the transistor is stable (fixed by the "holding" or "resisting" action of the 10n capacitor). 

The circuit is an oscillator and the way it keeps oscillating is due to positive feedback. This is the case with all oscillators and the component that provides the feedback is the 1n capacitor between 
the collector and emitter of the transistor. It may seem unusual that the transistor can be turned on via the emitter to keep it oscillating, but in fact it does not matter if the emitter or base receives a signal as the important factor is THE VOLTAGE DIFFERENCE between these two terminals. 

If the base is kept fixed and the emitter voltage is reduced, the transistor sees a higher voltage between the base and emitter and it is turned ON harder. If the voltage on the emitter increases, the transistor turns OFF as the difference between the two is reduced. 
This is exactly what happens in this circuit. The 1n capacitor between the collector and emitter influences the voltage on the emitter to turn the transistor on and off. It does this by constantly monitoring the voltage on the tuned circuit and passing the change to the emitter. 

In this project, the TUNED CIRCUIT is the parallel components consisting of the inductor (the search coil) and the 1n capacitor across it. This is called an LC circuit in which the L is the inductance of the inductor in Henries (or mH or uH) and C is the capacitance of the capacitor in Farads (or uF or nF or pF). 

We start when the transistor turns ON and allows a pulse of energy to enter the tuned circuit (later you will see how the transistor turns on). 

The pulse of energy (current) starts by trying to entering both the coil and capacitor. You would think the coil has the smallest resistance but the capacitor is uncharged and presents a theoretical zero resistance and begins to charge. When a small voltage appears across it, you would think the coil would become the least resistance as it consists of only a few turns of copper wire. 

But the wire is wound in a coil and forms an inductor (it has inductance). When a voltage is applied to it, the low resistance of the inductor allows a current to flow but this current produces magnetic flux that cuts the turns of the coil and produces a back-voltage that opposes the incoming current. It works like this: Suppose you supply 200mV to the coil. The back voltage it produces may be as high as 199mV and thus you only have 1mV with which to push current into the coil. 
If the resistance of the coil is 100milli-ohms, the current will be about 10mA. The capacitor will accept more than this and so it gets charged first.

As the voltage on the capacitor increases, it presents its voltage to the inductor and allows a current to flow (at a rate which the coil will accept) to produce magnetic flux. This flux is called electromagnetic lines of force and creates an expanding field. The capacitor cannot provide energy for very long and after a short time the current reduces and this causes the magnetic field to begin to collapse.

The collapsing magnetic field produces a voltage that is opposite to that originally supplied to it and the bottom of the coil becomes positive with respect to the top. 
If we think of the coil as being a tiny battery we see it adds its voltage to the 9v of the supply and the collector end of the coil becomes higher than 9v. 

This voltage is detected by the 1n feedback capacitor (between the collector and emitter) and it passes the voltage to the emitter where it increases the emitter voltage. The base of the transistor is kept stable and fixed by the holding action of the 10n capacitor and the transistor turns off slightly. This action continues and eventually the collector can be considered to be removed from the circuit so that it puts no load on the tuned circuit. When an inductor is not loaded like this, the collapsing magnetic field will produce maximum voltage. 

This is the case in the circuit above and as the magnetic field collapses, it produces a voltage (about 25v) that is considerably higher than that applied to it. This voltage is passed to the "C" component of the tuned circuit (the 1n capacitor connected across the coil) and the capacitor charges up.