Sending Morse codes over GSM network - Alternatives?

Hi Ravi,

My name is Saikat and I’m writing you from Singapore

I came across your very useful article (link here) in Embedded World when looking for information for my son’s science project. Some of the terms were difficult, so I wondered if you would be kind enough to spare a minute to help me?

Project description: To set up a morse-code channel using only 2 standard cellphones without using the cellular network or wi-fi

Theory: A mobile app will do the following:

  1. Get cellphone # 1 to send a Morse signal via the speaker, but on ultrasonic or RF frequency across a distance of 5-10 kilometres
  2. Get cellphone #2 to receive the Morse signal via its microphone and translate it into English alphabets on the screen

What I need your advice on:

  1. Whether an average cellphone battery (3.5 V, 7.98 Wh, 2100mAh) and earpiece is capable of transmitting an ultrasonic or RF signal powerful enough to reach across 5-10 km
  2. Whether the average cellphone speaker and microphone can detect such a signal
  3. If not, is there any signal on any frequency which the average cellphone can transmit that would cross 5-10 kilometres?

Please accept my thanks for your help, and for taking the time to go through my mail. I look forward to your reply.

With thanks and regards,
Saikat Ghosh

Hi Saikat,
RF frequency cannot be transmitted on the Audio channel. Its bandwidth is limited to Human voice, from 20Hz to maximum 8KHz.

So, you can transfer audio tones. You can generate tones and decode yourself by building a hardware or you can use builtin functionality called DTMF. Its the same principle, except you can transfer tones for numbers 0 -9 and * and #.
Check this video for demo , I am using SIM800 module for this.

Hi Ravi,

Would DTMF tones carry over 5-10 km using an average cellphone speaker ? Won’t the attenuation/loss over distance will make it too feeble to be detected by the 2nd cellphone.

OR -

Is there any RF frequency that the cellphone antenna (not speaker) can transmit which the 2nd cellphone can detect 5-10 km away ?

Thanks and regards,

Hi Saikat,
You can reach anywhere in the world. Its just speech on a call.

Check the demo here, You can control devices from anywhere in the world

Hi Ravi,

Perhaps I should have repeated my first posting for clarity.

There is NO cellular or wi-fi network available.

Imagine both the phones are in the middle of a desert, separated by 5-10 km.

How would a DTMF tone from one phone reach the 2nd phone ?

Alternately - is there any other kind of signal which could travel the 5-10 km WITHOUT a cellular or Wi-fi network ? For example, RF or any other.


Ok, now its clear,
You have to use LoRaWAN for this. For getting such range.