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AC measurement circuit question

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Tom18991:
Sorry, forgot to answer that. The sensor has but this one purpose in life -- nothing else is connected to it.  I've never dealt with ac (at all) so thanks for the on-going schooling about zener diodes and all else. Will report back with parts in hand. :)

Ian.M:
If the purpose is to detect that the shaft speed is above a certain minimum, measuring the amplitude is *NOT* a good idea - it will vary with the tolerance of the sensor to rotating object clearance, from sensor to sensor, and possibly with temperature.   

Instead I recommend measuring the pulse rate.   If your MCU can do input capture, or can time between interrupts that's pretty easy.   Here's one approach using a 555 timer as a dual comparator + flipflop to detect the pulse.   It wont fire till the pulse goes over +/- 1V which should give it reasonable noise immunity.

I've simplified the sim model for the sensor a bit and given it a floating output.  Sorry, its not dynamically variable speed yet (i.e. with a control voltage for speed like the amplitude one), if you need that I might be able to hack something together , but not within the next 24H.

Tom18991:
No, no, not trying to test speed with amplitude.  I’ve watched real time how half a mm of sensor positioning can create big swings in the output.  I have the speed available via other hardware, really just to confirm it’s moving.  I’m confirming the amplitude for the sake of comforting the amplitude, to confirm it is strong enough for legacy hardware to detect.

Ian.M:
OK, so it would actually be desirable to *MEASURE* the amplitude so you know how close to the 3V pk-pk threshold you are and don't pass any that are marginal.  One approach to that would be a  combo of the 555 circuit to detect when the pulse occurs and fire an interrupt on its leading edge,  and an OPAMP to buffer the input, already offset to 1/2 Vcc, and feed the ADC.   When the interrupt fires, you'd repeatedly do an ADC conversion, saving minimum and maximum values,  and stopping when the minimum and maximum values were at least two volts apart, either side of midrange and the input had returned to near midrange.   however that would need an ADC that could do >10KHz sample rate to avoid missing the peaks.  You'd probably need to mod the clamping for the 555 circuit to reduce the load on the sensor and keep the waveform symmetrical, and  maybe stick a potential divider in front of the OPAMP if the normal pulse is over 5V pk-pk.

radiolistener:

--- Quote from: Tom18991 on March 10, 2019, 05:43:16 pm ---My actual project involves a low-current ac sensor, and the waveform is not a sign wave.

--- End quote ---

That schematic with diode will not works for your case. It is intended for clean sine wave AC and for voltage higher than diode open threshold.

If your sensor produce not sine AC, then you're need to measure RMS value. It can be done with RMS-to-DC converter chip. Then the output of RMS-to-DC converter can be measured with low speed ADC. For example, you can use LTC1968: https://www.analog.com/media/en/technical-documentation/data-sheets/1968f.pdf



Another way is to use high speed ADC and perform math calculation of RMS value in the microcontroller.

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