Looking for a way to create a radio link between a pair of antique telegraph keys

Question

I'm trying to design a way to link up two antique telegraph keys wirelessly.

This is for a friend to use with his granddaughter between the house and the playhouse, so we plan to use FRS to avoid licensing requirements.

My current thought is to use a corded mic/speaker/ptt jack on the side of a FRS handset, and build a circuit to 1) toggle the ptt, and 2) generate a tone into the mic input when the telegraph key is depressed.

The receive side would detect the tone and close a relay or transistor to activate the coils on the "tapper". The "tapper" can be activated with 6V 44mA.

I have disassembled a mic/speaker/ptt set from a baofeng, and struggled to understand the internal circuitry. Additionally the wires were unbelievably tiny and contained some kind of fiber. Soldering these looks difficult. And finding info I could understand on which jack contacts to manipulate to accomplish my goal was difficult. Also, I would prefer to use a cheap FRS set rather than a pair of baofengs.

Any suggestions on an easier way to accomplish this? Does a commercial product exist to do exactly this? I have searched, but may not have the terminology to search well.

I have also looked for an FRS set with a CW/Morse button, as that would eliminate the need to build a tone generator into my circuit, but this seems nearly non-existent.

I'm open to any blindingly obvious suggestions on available hardware or hints on how to accomplish this efficiently. It's been a few years since I worked with analog circuits!

Answer 1

I believe FRS is the wrong place to look. Once you crack open that radio to hook up a telegraph key the Part 95 FCC certification is lost. MURS is another Part 95 service and it allows telegraphy, but not hand sent Morse code from the looks of it. Other comments and answers mention 27 MHz radios but I'm not so sure that they are CB radios. There's a few channels in between CB channels for... something or other. If they are using a CB channel then it might be under Part 95 or it could be under Part 15.

I still have an old electronics kit somewhere that I could use to snap the blocks together to make a radio telegraph transmitter that could be received on an AM radio. These would be operating under Part 15. I'd suggest looking for a kit like this rather than try to do surgery on FRS radios.

So, there may be a MURS radio out there with the capability to hook up a telegraph key. Maybe a kit for an AM telegraph transmitter. Maybe there is an old 27 MHz radio you can find.

This is a place for ham radio questions so an answer that doesn't mention ham radio might be bad form. Getting a Technician license is not terribly difficult, and that opens doors to more options. Options that includes more kits that would be legal under that license.

My old electronics kit had schematics for the transmitter, at least I recall it did, so there are plans out there to find. If someone doesn't offer it for free on the internet or in some library then there must be a book to buy for not a whole lot. I could not find my kit right away, I moved some things around recently so it's usual spot isn't there any more.

I'd suggest looking for some kit or schematics to build a transmitter. If you are planning to build a tone generator then you are likely more than half way to a low power AM transmitter.

Answer 2

When I was a kid in the seventies, I got a set of walkie-talkies for Christmas one year that each had a Morse code button. I think I had to press the PTT switch before pressing the Morse code button. I had memorized the Morse letters, for instance "A is dot-dash", and so had a few friends, but none of us had actually been taught how to send clean code, or how to copy by ear, so the button was completely impractical; it was only cool for five minutes at most, and was mostly ignored after that. Of course the walkie-talkies were complete junk, and didn't work reliably from the front yard to the back yard, so they were really only useful for pretending to be GI Joe, rather than actually communicating with friends more than a few feet away. They used a CB channel; from time to time we'd hear someone with a real radio, who never seemed to hear us. Ah, those were the days ;) Modern FRS radios work so, so much better.

For someone with the right knowledge and skills, designing a circuit like you describe would not be so difficult. Rather than disassembling the radio or a microphone, I would put the circuit in an external enclosure with a cable and a plug that plugs into the radio's microphone and PTT jack(s). I'd also equip the circuit with a jack for the Morse key. An Altoids tin could serve as the enclosure, or you could buy something fancier, or gut some other device and use its enclosure.

The heart of the circuit would be the audio oscillator. If you search the web for code practice oscillator, you will find many examples of suitable circuit designs, or even kits that could possibly be adapted to do what you want. Probably the simplest way to do it would be to buy a code practice oscillator kit and build it, and just put that in front of the walkie-talkie, with no electrical connection between the oscillator and the walkie-talkie. Of course a proper circuit that plugs into the radio would have more panache, but you would need to solve many more little problems along the way.

The PTT would be a little more difficult. You could either train the child to push the PTT button, or build a circuit that would trigger it automatically and keep it open for a short time, perhaps one second, when the Morse code key is pressed. Try searching for one-shot multivibrator or monostable multivibrator for circuit examples.

I suspect that the circuit would be as much to get you back into electronics as it would be for your friend to play Morse code with his granddaughter, which is well and good. Just be aware that it takes several hours of determined study and practice to be able to send and receive Morse code even at 5 WPM, the lowest practical speed. (That I know from practical experience.) Of course one could always devise a simple code such as "three dashes means it's dinner time, four means it's time for bed". I'd guess that many children would prefer to skip the telegraph key entirely and just speak into the walkie-talkie, or send a text message through a mobile phone. I'm not a parent, but I'd guess that motivating a kid to learn Morse code is like trying to motivate a kid to learn to play piano: good luck.

Answer 3

Avoid modifying FRS radios

I think that you should avoid modifying FRS radios. It is illegal, unless you want to build your own, go to the FCC, and spend a possibly large sum testing the radio, to make sure it complies with their rules. it would not be worth it anyway, since the FCC does not, to the best of my knowledge allow Morse on FRS. I think, that if you know enough to build a Part 15 radio (which I do not), that could be a viable solution, as long as you comply with the rules, and build a good radio, that does not interfere with the FCC guidelines.

Otherwise, do one of the other things that people said.

Answer 4

I have also looked for an FRS set with a CW/Morse button, […] but this seems nearly non-existent.

The Family Radio Service has only limited provisions for digital data transmissions and it's likely that morse code audio transmissions (aka "MCW") wouldn't be considered that anyway. [Though this comment claims it would be fine. In that case, perhaps all you need is an FRS radio that works with an earpiece and then look up the pinout e.g. for your particular model.]

It might be fairly simple to hook up the telegraph keys in place of the PTT button and transmit plain CW which would come through as patterns of silence on the other radio. However, unless you rigged up some sort of automatic button presser, it still involve modifying the approved FRS radio though and turning it into an unapproved device. (But perhaps this is okay if the walkie-talkie already has a headset jack that includes a PTT pinout?!)

I'm open to any blindingly obvious suggestions on available hardware or hints on how to accomplish this efficiently. It's been a few years since I worked with analog circuits!

While in some ways it's going to be more complicated than modifying some FRS devices (which are officially not allowed to be modified), another route might be a very simple "remote control" style set of transmitters and receivers. They are sort of meant to transmit some sort of serial data, but if I understand correctly basically when the "Data in" pin goes high on the transmitter, the "Digital output" and/or "Linear output" pin(s) go high on the receiver. With two pairs you might be able to rig up a simple buzzer circuit on each end.

One step above there's also chips like the RFM69 that really are meant for data but in combination with an Arduino or Raspberry Pi Pico you probably send a stream of some known letter for "on" and have the microcontroller buzz while it's receiving that letter.

Answer 5

One way would be to use a 1 MHz crystal oscillator as the transmitter and a broadcast band AM pocket radio, in conjunction with a beat frequency oscillator (BFO), as the receiver.

Answer 6

I would be tempted to try using two Bluetooth LE capable Arduino-like devices (Adafruit Feather, ESP32, et.al.), and transmit the keying over the 2.4 GHz ISM BLE band. For possibly more range, there are also inexpensive 433, 868 or 915 MHz (depending on your region) transceiver breakout boards that can be used with Arduino's and Raspberry Pi's.

Then use a GPIO pin on the Arduino to buzz a piezo speaker on received key down notifications.