Recycling an old Smartphone to easily remote VLF probes
by Massimiliano Recchia, IK0VVE 

What an old Smartphone, a geophone in a cellar and a SpectrumLab session can have in common? We see it in this article where an old phone is used as a wireless connection to transfer the geophone signal to a PC, placed at home on upper floors.

All this started when I decided to move my home-made geophone from my lab room to my cellar at about 3 meters under the ground.

It was a better position for this device because of absence of electro-magnetic noise. Only a simple 220VAC socket and an incandescent lamp are located in the cellar, no other electric or electronic device, surrounded by reinforced concrete and innocent camping tent, skis, bikes, etcÖ

So, just for this electric ďlonelinessĒ it was difficult for me to have a connection with the PC located 3 floors up. The only way was to use my house wireless net with a repeater on the first floor, but how to connect the geophone amplifier to SpectrumLab? Should I to transform analog signal into data signal? Then back to analog-digital signal. Iím too lazy for these.

An old Samsung smartphone was into my lab powered off from a while, so I thought: why donít exploit the embedded smartphone technology for my goals?
Iíve downloaded and tested some applications for the Smartphone to stream audio by its Wi-Fi connection creating a simple audio server.

The easiest and lightest application that I supposed to work well is free and its name is Wireless Mic, available on Google Play Store.

If the APP is not available on Google Play Store, it can be downloaded directly from the developer's website:

It is very easy to use and configure, moreover latency is very low, it didnít overload the old smartphone CPU, in general it doesnít require many resources, this absolutely avoid audio flow interruption.

As seen in the principle scheme, I connected the output from the geophone amplifier (but itís possible to use any VLF probe high impedance level) to the headphones-microphone input of the mobile phone, a capacitor separates eventual DC continuity. After some regulations for audio strength I tried to receive the stream by my LAN.

I used a 4 rings 3.5 mm jack to RCA female to connect the probe to the Smartphone. The third ring is the signal input, the forth is the ground. In my cabling the white RCA is the input.

Once the application is installed you only need to turn on the Wi-Fi connection from the Smartphone and connect it TO your local Wi-Fi net.

Then you have to:

   1)  Start the application and
   2)  Press the Start button to start the Audio server.
   3)  Youíll see that the IP and the port number of the connection will appear on the screen.

To get and listen the stream from the PC Iíve used the freeware VLC media player software, it can simply manage a network flow just inserting server IP address and port.

Start VLC Media Player, go to Audio Menu and then Audio Device, choose your output VAC (Virtual Audio Cable) device. You have to configure an audio VAC to send the audio to SpectrumLab.

Unfortunately, SpectrumLab at the moment canít receive directly a compressed audio stream from Wireless Mic App.

Click on Media menu > go to ďOpen MediaĒ

Insert the NET URL complete of http:// and its port number.

Press ďStartĒ to connect to Smartphone server and send audio stream via VAC.

At first start the server will ask for USER and PASSWORD, write admin and admin for each field.

Just started the player I was enough sure that connection was ok, of course audio was non audible because of the low audio frequency, this was the big question, would this system transport a so low frequency audio signal? To see if all was right, Iíve started SpectrumLab configuring its input as the VAC that Iíve used with VLC media player output.

To my surprise all was working perfectly! Great! Giving some weak touch close to the probe I saw the signals on the low frequencies of the spectrum. Another test was made injecting a sub audio signal to the smartphone.


Iíve tested with success this system also for VLF reception for about a couple of months before to move it all into the cellar.

Iíve tried this system with other smartphones and Iíve find that not all models can well manage the audio, some phones use to cancel what they consider an audio noise, like for embedded microphone also from headphone-microphone plug it works so, you should get out a too digitized or too weak sound. Some models can manage this some not. The old Samsung made its dirty job well.

An example of the needed material with another cell phone brand

Finally an example of an h24 monitoring: an hourly multistrip geophone signal

As power supply you can choose for a big 12V gel battery and a 5 V stabilizer to keep in charge the phone battery or a wall power supply, this second solution doesnít give the safety from AC noise but you can build your own low noise one.

The range of this system depends on the Wi-Fi coverage. Usually itís 5/10 meters or more inside the house, more than 20 outside. You can extend it using apposite Wi-Fi extender so to easily overcome the 25/30 meters and one or more floors.

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