Talk over a long distance on a speaker wire

Here is one is I dreamed up but decided not to use. Use a DTFM encoder to send a comms over speaker wire by using an audio amp to boost the signal for line loss to a decoder

Do you have a question?

Just a thought

Moved to bar sport as it seems the only appropriate place.

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’Talking’ over dtmf seems to be a pointless exercise.
Maybe explain what you’re really trying to achieve…

I was looking for a way to have security stations on my two acres talk over the distance securely that can not be jammed at the wifi freqs or listened in on. The system would control lights and discourage the pigs from getting in my garbage. i hope to have video of them running from mp3 files of dogs barking and such

That’s a better explanation.
DTMF may be useful, but since you’re looking for more capabilities, you may like to rephrase your post’s title and ask for a more specific solution.

LoRa or other low power radio networks might be better suited.

Even 3G/4G data might offer a good package at reasonable cost.

Btw, DTMF is a VERY low rate signalling method - no use for voice quality.
Read up on FSK for the basics of low rate data over low quality audio mediums.
You can put DTMF on a voice channel, but not thee opposite.

Not long ago I was asked at work for a simple digital monitoring system for a one off installation, using an existing UHF FM comms system for voice.
The owner wanted minimal intrusion into the existing system, it was to be used at night when voice comms was not needed.
Found this library, it uses;

SoftModem is a wired, low-cost and platform independent solution for communication between an Arduino and mobile phones. It uses the phone's audio jack and Bell 202 modem-like FSK encoding with up to 1225 bit/s.

It is designed for phone application, so I coded a PTT line and some pairing protocol to check connection.

I used the lowest baud rate;

#define SOFT_MODEM_BAUD_RATE (126)
#define SOFT_MODEM_LOW_FREQ (882)
#define SOFT_MODEM_HIGH_FREQ (1764)

Using speaker wire will work, that's how I started my DEVELOPMENT IN STAGES of the project.
I left the transceivers out of the comms loop first to prove hardware and library performance.

Tom.... :smiley: :+1: :coffee: :australia:
PS, many would have seen this picture, its the prototype that ended up spending a week in the field till the PCB was assembled and tested.

A solution that uses three off-the-shelf Arduino units. if speed is not a requirement this could be set up cheaply and not be on crowded radio freqs. an FM radio link requires no license and could be broadcast all over town, but anyone with an FM radio hears you. I need to talk to units all over my 2 acres. id and discourage pigs, rats, and other desert pests. there seems to be no such system. many here in Tucson would pay big bucks to protect expensive cars from rats. much more hate to pick up the trash the pigs spread. still, more would like a system to light and run different sequences on their driveways. on top of that people want a security system they can know how it works and expand it themselves. then new radio system work by magic and you must buy expansions from them. an Arduino system is more a hobby than anything else.

What region do you live in that an FM transmitter doesn’t require a licence ?

What frequency and power limitations are imposed ?

America, you can buy 5 watt FM "parking lot" transmitters on amazon.


Australia has UHF CB, it is what is called a Class License, you don't need any license as such.

The conditions allow the use of;

You can use the CBRS for:

  • any voice communications
  • telemetry and telecommand

Tom... :smiley: :+1: :coffee: :australia:
PS, But you must use an approved transceiver to do it.

I'm curious - why are they called "parking lot" transmitters?


From what I gather they are transmitters that work in the FM broadcast band at VERY limited power, for very local transmission.
For example a drive in theatre, you tune your car stereo to the "parking lot" transmitter and enjoy full stereo sound from the movie.
I'm not sure how legal they are, but with limited power they might make use of a loophole in the legistation.

Tom.. :smiley: :+1: :coffee: :australia:

5W doesn't seem to me to come under the category "VERY limited power"

In lots of places in the World you can openly buy stuff that is not legal to use.

Are those 5W FM transmitters legal to use ?

Me thinks some form of encrypted radio system would suit.

But making it 'can not be jammed' would not be so easy.

And if you wanted to make it really secure, maybe dont openly discuss the method in public.

If 'line of sight' is possible you could use IR lasers.

Servo-driven semaphore flags, telephoto lens and OpenCV

so you want to make hack proof, jam proof, bullet proof system to scare pigs away?