help understand operation of LM358 based circuit

[sherazi]

this is the circuit for amplifying the signals that are generated in the inductor by a vibrating magnet, used in vibration sensors. the signal is amplified and then fed to a relay,and an led.. as my knowledge of op-amps is limited, i cant understand it fully,if any one could explain the operation in the Op-amp section and d2,r8,&c3 and use of C4 I would be really grateful.. thanks in Advance

 

[mister_rf]

D2. C3, R8 -There’s a simple envelope detector.

An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal.

 

[atripathi]

Best way to use a circuit simulator like PSpice to simulate the circuit. The result at various node in circuit would help you understand the behavior an circuit it self.

 

[mister_rf]

C4 = coupling capacitor
A coupling capacitor is used to couple or link together only the ac signal from one circuit element to another. The capacitor blocks the DC signal from entering the second element so only passes the AC signal.

 

[FvM]

For reasonable circuit operation, you would want to bias the first OP stage as a linear amplifier.