Two Morse Code AM Transmitters - Updated Jan 2014




Introduction: Two Morse Code AM Transmitters - Updated Jan 2014

About: I am an American teaching English at Shangluo University, Shaanxi. I like making machines that do interesting but fairly useless things - I call them Quixotic Machines.

Version 1: (photos 1, 2 and 3) using 555 chip
The version I am using now is the AM transmitter based on the 555 IC chip schematic in photo 3 with build instructions here:
I built this transmitter and placed it in a plastic container. ANT is about a meter of wire coiled up in the bottom of the container.
This transmitter works very well and transmits the signal all over the AM band. Just place the radio near the container, turn on transmitter and you are good to go. You get a very nice strong "beep beep" sound (sidetone) with this version. Number of components is still very low but you have to do some wiring and soldering. I built this one on a breadboard and then just transferred it to a circuit board and soldered the jumper wires. Not very elegant perhaps but I like the spaghetti wire look.

There is some question as to type of capacitors to use: I used simple round ceramic disk caps but the original link picture shows them using polarized caps so I think either works. Perhaps one works better than the other, will have to try an experiment on  that. Probably C1 should be polarized, C2 ceramic disk.

Version 2: (photos 4 and 5) using 1 mhz oscillator crystal
This was my original transmitter based on schematic in photo 5 and this instructable:

I had built this transmitter first and it is definitely the easiest with only a couple of components. It is also the most powerful transmitter but who needs power for just practicing keying? The thing I did not like about this one is that it only transmits a carrier wave with no modulation (no sound) so you only get a "buzz buzz" or "shh shh" sound by turning the carrier wave on and off - not very pleasing. 

Note: I also did a version 3 which was to combine versions 1 and 2 with an audio transformer to modulate the sound output from the 555 chip into the 1 mhz oscillator. It did broadcast a strong carrier and audio signal but the downside is that there was a terrible pop sound when I let up on the telegraph the key. That could be fixed with an RFI choke but was too much work as I just needed to transmit a clean signal to the AM radio which version 1 performs nicely.

I have learned how to send Morse Code pretty well now but listening at speed is still a challenge but am slowly getting there.

Photo 6: Picture of my 1/4 watt 40 meter Michigan Mighty Mite ham radio that I cannot legally use until I get a license. Just wanted to make one.
Photo 7: Picture of my portable practice CW keyer. See my instructable for details.

Update 1/18/201: I am learning more and more about radios as I go along. I found out that in "real" ham radios the transmitter doesn't send a sidetone, rather it is gotten from the transmitting oscilliator carrier by the receiver - called Beat Frequency Oscillator - so you have to have a special short wave radio receiver that has the capability of demodulating the signal from the carrier wave - usually.  I just built a QRP Michigan Mighty Mite transmitter, which I cannot use because I don't have a ham license - something I hope to do one day as you just have to pass a 35 question test for the entry level Technical License. But I am building a cw receiver for the 40 meter band so that I can get practice at listening to live cw code. It is very interesting learning the concepts that allow you to understand what is going inside the little "black box" radio. Learning about inductor coils and resistors and capacitors and transistors and how they function is fascinating.

Update 2/18/2013: I built the Michigan Mighty Mite Transmitter shown in the last picture. This is a real HAM transmitter that is not legal to use without a license. I hooked the antenna leads to a 50ohm resistor so that it wouldn' t transmit farther than my room and was able to hear the signal on my shortwave radio that allows me to hear CW or Morse Code. The receiver demodulates the signal from the transmitter carrier. You can find lots of details on how to build the MMM on the internet.

Update 4/13/2013: I have since built a couple more little transmitters and put them in tea tin boxes, built rf detector (fun to see what is producing rf signals and if my rc transmitters are working, wattmeter probe, a Ramsey HR-40 reciever (fun to experiment with but not the greatest receiver, and bought and use a Tecsun PL-660 SW receiver for listening to CW - fantastic little radio. Also built an antenna tuner which greatly increases receptivity. Have learned Morse Code but listening is a skill that is going to take a while. Practicing at 5/20 words per minute. No end to the stuff you can build and it is fascinating. Need to get a ham license but have to wait a year to get back to States.

Update Aug/2013: added photo of a bamboo key I bought. Pretty.



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    20 Discussions

    I realize that this is a little late, but it showed up on Pintrest this morning. The "AM" transmitter is nothing more than a square wave oscillator at an audible frequency producing a signal very rich in harmonics so it can be picked up on a regular AM broadcast radio and produces an audio tone at the fundamental frequency as the output. The strength of those harmonics falls off rapidly as the higher order harmonics are monitored so it would not have much range, which is a good thing since that setup is producing signals across the whole spectrum.

    The 1 MHz oscillator is a true continuous wave (CW) signal so it is, ideally, a sine wave at 1 MHZ, in the middle of the AM broadcast band. Since it is a sine wave, it is only a carrier with no modulation on it, thus no tone. In a real communication receiver, there would be a beat frequency oscillator, running near the IF frequency in a superhet, so that it would produce an audible tone when the oscillator was keyed. Alternatively, a classic regenerative receiver could be adjusted to oscillate and produce a beat note.

    When the 1 MHz oscillator was combined with the 555 audio oscillator, you produced a modulated CW signal so it could be heard as a tone in the AM receiver.

    Did a little more poking around and I think C1 should be polarized, C2 can be ceramic disk. The original project (link now fixed) looks like two polarized in the picture.

    Yes, good question and interesting because the schematic shows a polarized capacitor but I just used a non polarized ceramic disc capacitor. You might try a polarized to see if there is any difference. (or I will try because now Im interested.) Also, link to original project no longer works, will have to try and find it again.
    Thanks for checking this out.

    would this transmitter be effective over long distances using am frequencies? you mention that you have to place the radio close to the transmitter to hear it...?

    1 reply

    This particular radio (using the 555 chip) is not a long distance radio nor would you want to be as that would require a ham license (which is relatively easy to get by studying for and taking a short test). If you want a long distance radio for Morse Code then look at the Michigan Mighty Mite 40 meter radio (as pictured in my instructable and with construction plans found in many places on the internet). The MMM is just one of many simple AM CW radios that are easy to build but requiring a much larger antenna and of course a ham license.
    That said, the radio using the 1mhz oscillator carries much further than the radio using the 555 chip but as I said the signal is kind of ugly.
    Also, this information is only fairly accurate as I am not a licensed ham and have been building radios for self-education reasons.

    Hi. Here are components:
    R1 - 2.2k Ohms
    IC1 - 555
    C1 - 10 uF
    C2 - 220 uF
    + V - 9 V
    Antenna - 3 meters of copper wire No 25.

    I just coiled up the ant wire in the bottom of my container.

    Not sure what I used - I just asked the seller to give me something to work with the leds they sold me. Lots of programs on the internet that tell you which one to use based on led and voltage.

    Could range on this be improved with a oscillator with a higher hertz rating? What about increasing voltage? Or maybe just a larger antennae?

    3 replies

    If you want more distance go with a higher amperage output, not higher frequency or voltage. You also want the antenna to be proportional to the amperage. Example: (amps as a constant) a huge antenna would output a low intensity of am waves while a tiny antenna would output a high intensity of am waves. You probably want something in the middle o those 2 examples.

    I thought I replied to this but its not here so will again.
    A larger antenna will definitely increase range but then you may run into legal issues if you are not a licensed ham operator. A higher hertz will just change the frequency of broadcast and reception. Not sure about the voltage but I think you would not want any higher than 9 volts. These are just assumptions based on the comments I read in the above link.

    I remember the time when my friend built the computer controlled one. He keeps messaging me 'tune to the max on your radio' and I keep going 'it's already maxed out!'. I still don't get it, why didn't he use a regular 1mHz oscillator instead of a 30mHz crystal he found. It makes no sense for me to tune in on 30mHz when I can only reach 1.6mHz (or 1600kHz). Oh well, at least you got the LED right (I think it's supposed to be encircled, but it doesn't really matter to me). Oh wait, I'm staring at the radio detection finding schematic I've been looking for. Also, I believe that the tone is produced at the receiving end, but that's for the older radios back in the day.

    Ahh. Ok. Is there a equation for calculating range based on antenna size? I think you only run into legal issues if it is more than a mile, but then RC airplanes would be illegal because they transmit 3 or more....

    1 reply

    The comments in the above link: ( ) would probably be more helpful as I really have no expertise in these areas.