AVR LC Meter With Frequency Measurement

Posted on Feb 5, 2014

Building an LC meter for a while since I do not have a multimeter that is capable of measuring inductance and while the multimeters I have can measure capacitance, they are not able to give accurate readings for small capacitance in the range of several pF`s. There are quite a few good articles on how to build LC metersusing PIC MCUs (like the ones here: 1, 2, 3 ), but instructions on how to build one with an ATmega

AVR LC Meter With Frequency Measurement
Click here to download the full size of the above Circuit.

MCU are few and far in between, although the basic principle is largely the same. So I decided to write this article on how to build an LC meter using an ATmega328p chip and Arduino libraries. A typical LC meter is nothing but a wide range LC oscillator. When measuring an inductor or capacitor, the added inductance or capacitance changes the oscillator`s output frequency. And by calculating this frequency change, we can deduce the inductance or capacitance depending on the measurement. The following schematic shows the comparator based LC oscillator I used in the LC meter. The oscillator portion is quite standard. Most of the other designs I have seen use LM311 comparator. But for this type of application, any comparator capable of oscillating up to 50kHz should be more than sufficient. I happen to have some spare LM339 ²s lying around so I used it in the oscillator circuit. Because what we are really meausring is the frequency of the oscillator, we can build a frequency meter using the same circuit at almost no additional cost. As you can see in the circuit above, a reed relay is used to switch the measurement from LC mode to frequency mode. In the schematics above, the second comparator forms a Schmitt trigger to condition the input waveform so that the frequency measurement can be made more accurate. When in the LC mode, the frequency output from the first comparator is simply feed through the Schmitt trigger. The...

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