This instructables will give you one option to measure the inductance of coil using a few component and android application.
My major is computer engineering and my hobby is making small electric circuits like HAM receiver or headphone amp. Sometime, the coil that has specific inductance is needed (ex. low pass filter). But I don't have typical LC meter. So I decided to make my own with android application as programming practice.
And I'm not good at English. So if you feel difficult to understand, please write comments. Then I will reply as soon as possible.
Continuity Tester (Using Android Phone) by topstarcreations in life-hacks
give me motivation using android phone when making some electronic meter or tool. Thank you topstarcreations!
Step 1: Theory
(The picture is from http://www.rfdh.com/bas_rf/begin/reso.htm)
The principle of application is finding the resonance point of LC circuit.
Parallel LC circuit's frequency graph is decreasing then increasing shape and there is one peak. The peak is the resonance point. If we find the peak, then we can calculate inductance or capacity by using given capacitor or inductor.
Step 2: Hardware Part
2) 3.5mm 4 pole connector (earphone with MIC is also OK)
4) 100nF capacitor
5) 100, 1K ohm resistor
6) Jumper wire
I use both audio input and output of smartphone to feed sine wave to LC circuit and record the response. The switch is to calibrate the frequency response of smartphone. The audio input of smartphone is not flat because of low pass filter. So by using linear approximation, I manipulate the amplitude of signal of each frequency.
Step 3: Software Part
The link of source code: https://github.com/chosanghoon1118/CheapyLCMeter
The most important thing is peak search algorithm. I made my own algorithm because existing one is not working properly (I guess reason is the noise of audio input).
The basic principle of my algorithm is finding the change of slope's sign (minus to plus).
First, take samples of frequency response that divide frequency range (1000 ~ 10000 Hz) into three equal intervals. And find the slope between samples next to each other. If slope's sign is changed, that point is estimated peak.
Second, repeat first step in two intervals that contain estimated peak of previous step.
The error between actual peak and estimated peak would be decreasing whenever running introduced procedure. And time complexity is O(logN).
Step 4: Demonstration
I used 4.7mH inductor as example(green one in video).
The range of measurement is about 3 ~ 100mH (range is determined by given capacitor).
1st and 2nd pictures are before and after of calibration. And 3rd and 4th pictures are actual result and simulated graph using LTspice.
I hope that you can build simple LC meter at urgent situation by following this instrucables!