The principle of operation of the proposed metal detector circuit is quite basic yet very interesting.
How the Circuit Works
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.
More technically the operation 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 detections.
Article Courtesy - https://www.datasheetcatalog.org/datasheet/on_semiconductor/CS209A-D.PDF