In post #3 I said you need a rectifier as shown in the video.
Will you replace your simple amplifier circuit with the circuit in the video?

 

I see the schematics looks fine, just one thing is not clear to me. The output of peak detector will keep the voltage high - if the input goes low, capacitor will still keep the higher voltage as peek detector does. Who discharge the capacitor to measure lower voltage then ?

In the video's schematic R11 and R12 combine to discharge capacitor C3 down towards ground with a 1.5 second time constant. It will not get to ground as U1B will not let it go below the Vref voltage of about 0.8V. (With no input signal R1 also discharges C3 some as well, so its more like 1.3 seconds time constant, and maybe about 1 second for the final output to go from full scale to zero.)

 

Still say a precision full-wave rectifier would likely be the best for you needs.
If you answer my questions in post #26 I can give you a circuit example.

 

Still say a precision full-wave rectifier would likely be the best for you needs.
If you answer my questions in post #26 I can give you a circuit example.

The circuit shown in the video IS a precision rectifier and peak detector.
It uses a virtual ground to allow handling the bipolar input signal, then uses a differential amp to remove the ground offset- resulting in a 0 referenced positive DC output.

 

So what's the impedance of the coil?

What power supply voltages do you have available?

How fast does the circuit need to respond?

A precision rectifier circuit should do what you want.

I don't know inductance of the secondary coil. The voltage at secondary does not change very fast lets say can be in 3-4 seconds. I have 24VDC, 5VDC - whatever I can provide them but I prefer 5VDC.

 

In post #3 I said you need a rectifier as shown in the video.
Will you replace your simple amplifier circuit with the circuit in the video?

Yes, I will

 

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The circuit shown in the video IS a precision rectifier and peak detector.

Okay.
I didn't watch the video because I don't have the patience.
But I prefer to average a precision rectifier signal rather than use a peak detector, since it is less sensitive to noise and distortion on the signal.

I don't know inductance of the secondary coil.

Do you know it's approximate resistance?

 

In the video's schematic R11 and R12 combine to discharge capacitor C3 down towards ground with a 1.5 second time constant. It will not get to ground as U1B will not let it go below the Vref voltage of about 0.8V. (With no input signal R1 also discharges C3 some as well, so its more like 1.3 seconds time constant, and maybe about 1 second for the final output to go from full scale to zero.)

Ok, I see now and C3 must be adjusted according to input signal - how fast he changes his amplitude , I suppose 10uF will be ok. Anyhow I must try all that staff in the field. Thanks for all your suggestions !