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Turn Android Phone to an A.C. Spectrum Verion 2


(Dangerous test ... Explosion may occur)

voltage divider

The First DIY A.C. Spectrum was made in 17th Jan 2017.

This is version 2¡Athe fixed FUSE was changed, the values of R1 and R2 were changed.

A.C. Spectrum Version 1

Voltage Divider Circuit

The value of R2 was changed because I would like to make the operation of the A.C. Spectrum more convinent ... Just Plug and Play.

The value of R2 matchs the impedance of the microphone of the handfree which is about 2.4 k ohm when measuring with a multimeter.

R2 is constructed by one 2k ohm resistor and one 0 - 5 k ohm potentialmeter. Therefore, I can change the output voltage of the voltage divider.

Since the R2 was increased, R1 should be increased too.

This time, R1 is 100 M ohm.


R1 should be large

R2 should be small


Vt / R2 = (V1-Vt) / R1

V1 is known

Vt is determined

The Impedance of earphone is about 33 ohm.

I used 47 ohm resistor to simulate the earphone.

Mic connector

The most tricky part is to make a cable to cheat the Andriod phone that there is a microphone and two earphones.

You can buy a connector like the one as shown Or you can use the "Rubbish" grade hand-free ... only use the connector and cable.


This time, the input cable is plug and play.

There are many Free Spectrum Apps in the Google Play Store.

I found that different apps show different reading, you may try yourself.

I used the "Spectrum Analyzer" and "Spectroid"

You may have to try different Andriod devices or Apps to show the correct readings because some Apps cannot properly run on some devices.

Click to View Original Size

Power Off (Reading from "Spectroid"

Click to View Original Size

Power On

If you view the Original Size Photos, you may find Peak to Peak Difference is about 100 Hz

The Fundenmental Frequency of A.C. in Hong Kong is 50 Hz.

The 2nd Harmonicsis 100 Hz, 3rd Harmonics is 150 Hz ...

Reading from "Spectrum Analyzer"