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Turn Android Phone to an A.C. Spectrum


(Dangerous test ... Explosion may occur)

voltage divider

First, you have to make a Voltage Divider in order to get a very low voltage for the microphone input of your Android phone.

The safe microphone input is about 0.01V.

Voltage Divider Circuit

For better result, I guess, I chose to use the Carbon composition resistor which does not have coil inside.

If there is a coil, the Resistance (impedance) of the resistor will change with the frequency of the input voltage V1.

R1 should be large

R2 should be small


Vt / R2 = (V1-Vt) / R1

V1 is known

Vt is determined

voltage divider

Since I do not know the Impedance of the Microphone input of my Andriod Phone, I chose to use a small value R2 and tried as the followings:

I used ONE resistor as the R1: 100 k ohm

Five resistors in series connection as the R2: 0.5 x 5 = 2.5 ohm

V1 is about 228V at my home

Vt is about 0.006 V

Mic connector

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

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.

After finding out the correct cables, you have to connect those cables to a microphone and plug the connector to the Andriod Phone.

If correct ...

The Microphone indicator will be shown on your phone.

If not, un-plug the connector and change the polarities of the connection among the crocodile clips and the microphone.

Then plug the connector to the phone again ... ...

voltage divider

When the Microphone Indicator is ON, you can disconnect the Microphone and connect the crocodile clips to the Voltage Divider's Vt output.

Be careful ... double check the Vt is very very low voltage ... if not ... your phone may explode.

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"

Click image to enlarge

Before getting the reading, turn the phone to the "On the Plane" mode is recommended. This can redue the interference.

When the A.C. Power is OFF, the reading is as left.


The video shows the spectrum reading.

The green line keeps changing even the Power is OFF or even I only make the microphone input "short circuit".

The green line keeps moving, it may be caused by the high sensitivity of the APP and environmental interference.

Since I do not have any professional equipment to compare the results from this Free A.C. Spectrum, the results from this DIY project is only for reference.

However, the result is reasonable ... it shows there are Harmonics in the A.C..

AC Harmonic Frequency
Click image to enlarge

The Power is ON

The ideal A.C. frequence is 50 Hz.


From the DIY A.C. Spectrum:

The fundamental Frequency: 48.45 Hz

3rd Harmonic: a little bit larger than 150 Hz

The frequencies of the Harmonics do not exactly match the theory, 48.45 x 3 should be 145.35 Hz. Why ???