coilinductorne555
Please explain to me the working of this metal detector circuit.
Also,I have constructed this circuit but the problem is that I am getting the output(speaker) even when coil is disconnected from the circuit.
Where would the problem be occurring?
Additional information:The two coil terminals are having high impedance(from continuity check).
C4 with much lower impedance(is that normal)
Use the simplest LC generator, where the inductor is implemented as a searching coil.
Use frequency in the range of 100..200kHz
Measure the frequency, using the MCU. If the metal is ferromagnetic (iron, nickel, etc) it will cause decreasing of the generated frequency. If the metal is not ferromagnetic (copper, aluminum, gold, etc.) it will cause increasing of the frequency.
Notice, that some temperature drift of the frequency is unavoidable, so provide the proper (automatic) zeroing procedure by the software.
4 pins would most likely be a bipolar stepper motor. Two pins per coil (often two coils in series, but seen as one coil), two coils (or coil pairs):
Grab a small battery (AA) and see which pairs make the stepper twitch. Those are your two coil pairs.
How do you know if you have the coils connected the "right" way? Well, what is the "right" way? With a bipolar stepper you drive the coils in both directions at different times, so it's just a question of which way you drive them when. If you don't drive them in the right way in the right sequence then the motor won't "step", it'll just shake.
Best Answer
T1 forms a colpitts oscillator: -
Image taken from this article.
T2 is a buffer amplifier in common-collector mode - it feeds an amplitude detector formed by D1 and D2.
There will be a varying DC amplitude across C7 as the search coil has energy stolen from its magnetic field by metal objects it encounters. This voltage when it rises will activate T3 and reset the 555 timer.
The 555 timer looks like it's operating as an astable multivibrator.
So, it seems that when you encounter a piece of metal, either the oscillator stops or starts depending on the type of metal - ferrous will do the opposite to non-ferrous.
BTW it's a really ineffective design unless you are looking to detect large objects.
If the coil has a high impedance then it's probably gone open circuit. Try measuring the DC resistance.