The principle of operation of the proposed metal detector circuit is quite basic yet very interesting. The detecting function is triggered by sensing the decrease in the Q level of the LC network associated with the circuit in the presence of a metal at a specified proximity level.
Basically the built-in oscillator of the IC CS209 is made functional with the inclusion of a parallel resonant LC tuned network in conjunction with a feedback resistor wired up with the OSC and RF pin outs.
The impedance of the tuned resonant network may be expected at the maximum level as long as the driving source frequency is equal to the resonant frequency of the LC circuit network.
On detecting the presence of a metallic object at a close proximity to the inductor sensor, the voltage amplitude of the LC network gradually begins to fall corresponding to the closeness of the metal to the inductor.
Due to the above factor when the oscillation frame of the chip drops and reaches a certain threshold level, triggers the position of the complementary outputs such that they change states.
The precise technical the operations may be understood as follows:
Referring to the figure, as soon as a metal object is detected at the inductor input, the capacitor connected to the DEMOD gets charged through an in built current source of 30 uA.
However during the detection process the above current gets deviated away from the capacitor proportionately with the generated negative bias on the LC network.
Therefore the charge from the capacitor is removed attached to DEMOD with every negative cycle generated across the LC network.
The DC voltage with ripple over the capacitor of the DEMOD is then directly referenced with an internal fixed 1.44 voltage level.
When the procedure forces the internal comparator to trip, it switches the transistor which introduces a 23.6 K Ohms in parallel to the given 4K8 resistor.
This resulting reference level then equals near about 1.2 volts which introduces some sort hysteresis in the circuit, and becomes ideally suited for preventing wrong or false triggering.
The feedback pot connected across the OSC and the RF is used for setting the detection range of the circuit.
Increasing the resistance of the pot, in course increases the range of detection and subsequently the tripping point of the outputs.
However the detection and the trip points may also be dependant on the LC configuration and the Q of the LC network.
How to Set up the Metal Detector Circuit
The proposed metal detector circuit may be set up initially by following the below described steps:
Position a metal object at relatively larger distance away from the inductor, assuming the Q of the LC to be at the maximum sensitivity and the distance to be within the allowable range provided by the Q factor of the inductor.
With this set up adjust the pot such that the outputs just shift states indicating the detection of the metal object.
Repeat the adjustment procedure by gradually increasing the distance until a suitable maximum sensitivity of the circuit is optimized.
Removing or displacing the metal manually should make the output of the circuit to revert states, confirming the perfect working of the circuit.
Though the circuit is able to detect metals within a range of 0.3 inches, the range may be suitably increased by increasing the Q of the inductor.
The Q factor is directly proportional with the sensitivity of the circuit and the degree of detection.
Metal Detector using Ordinary Components
This metal detector simply employs all of the common components, as shown below. It utilizes a 2N2222 transistor and a couple of 741 ICs.
Even the detector coil is as simple as it can be! You just need to wind 8 turns of 22 SWG super enameled copper wire over a 9 inch diameter former.
After finishing the winding, secure the coil using strapping or a strong adhesive and gently pull it off and remove from the former. Transistor Q1 works like the main component of a Colpitts oscillator. Diode D1 rectifies the frequency from the Colpitts oscillator to a certain varying DC.
Op amp U1 works like as a differential amplifier to zero the varying DC, and U2 is used to boost the signal over a 200 µA meter. To use the simple metal detector circuit, fine-tune the potentiometer until the meter M1 reaches at the midscaleof the dial.
As soon as a metal object such as gold, tooth fillings, etc come to a close proximity of the coil's field of view, small alterations in the amplitude of the frequency waves trigger changes on the meter reading. Switch S1 works like an attenuation or sensitivity selection switch.