[Scott] Made A Single-chip Hellschreiber On Earth

[Scott Harden] is drilling teeth by day and designing radios that send secret messages by night. He’s set his sights on the Hellschreiber protocol which was used by the Germans in World War II along with their Enigma encryption system. The protocol is a viable alternative for transmitting and receiving code in environments with too much background noise for other communication systems.

His goal was to develop his own transmitter using just one microcontroller. He picked an ATmega48 and coupled it with a 40 MHz crystal oscillator. [Scott] mentions that there is no other hardware necessary, but static messages stored in an array so you’d need some other hardware to push your own characters through via the chip’s UART or otherwise. The AVR sends messages by converting the data into audio using PWM. That signal is fed into the crystal oscillator, which produces an amplitude modulated signal (AM) that can then be transmitted.

Check out his video after the break for a demonstration. He’s decoding the transmitted data using a free program called Ham Radio Deluxe.


38 thoughts on “[Scott] Made A Single-chip Hellschreiber On Earth

  1. Not like I’ve seen much (only a few examples online), but that’s the cleanest Hellschreiber output I’ve ever seen. Obviously it’s because there’s nothing long-range involved, but my first reaction to the picture was “Oh wow, it’s not all tilted!”

  2. I really like the idea of doing the frequency domain encoding with PWM. This got me thinking of trying to find a SPI EEPROM that will clock out its data as a continuous bitstream with minimal or no supervision.

  3. I wonder what will FCC say about this…

    OK, given it is very low power you may not have a problem. But what’s the point when you can transmit for a longer distance, which will get you into FCC’s radar (no pun intended)

      1. I don’t think it was considered encryption then. all you have to do is use the incoming pulses to draw dots… and as long as you’re anywhere between say 0.75x and 1.5x on your time sync you basically get the dots to print, using vertical scan lines…
        It’s a very very simple protocol. Hell is basically analog and has no sync.

  4. Thanks for all the feedback guys! I’ll note that I do have a FCC amateur radio license (extra class) which allows me to operate digital modes (such as Hellschreiber) using hundreds of watts of output power into a massive antenna if I desired to, as long as I stayed in the frequency ranges allotted to such activities – commonly around 3.5 MHz, 7.0 MHz, 14.0 MHz, and many more listed here: http://www.arrl.org/files/file/Hambands_color.pdf

    The comments about low pass filtering are absolutely correct. Note that I’m simply powering a clock oscillator and producing square waves – that’s my only generation of radio signals. You have similar oscillators in your computers, TV, and various other devices. If you opened your case, read their frequency, and tuned to it on a radio, you’d hear its tone too. The output power is probably in the nanowatt to microwatt range, and if I were to walk 20 feet away it would probably be silent. In that state, it’s useless as a radio transmitter. It could become a great transmitter if you added a few amplifier stages. Do not add an amplifier to such a device unless (a) you are legally able to transmit on that frequency, and (b) you apply low pass filters to turn the odd-harmonic-rich square waves into pure sine waves. With that being said, this could easily be taken to 10 or 100 watts of output power, put on a long wire antenna, and be copied around the world!

  5. Just a nit-pick — since when is Hellschreiber a “protocol?” It’s a line-encoding at best.

    “Protocol” implies a bi-directional, mutual agreement between at least two endpoints which must be agreed upon before communications can begin and/or continue. Hellschreiber has no two-way conversation semantics; you just transmit and hope the receiver can see it.

    1. I dont know where the talk about 40Mhz is comming. On his page all the pictures feature a 3582Khz TTL can oscillator. And that’s the thing getting modulated. I’ll grab on or two similar one’s from his sourse just to experiment on.
      And one could always build a AM/CW transmitter out of it for 80m, or with some bandpass filters 40m and maybe 20m, the squarewave is rich in harmonics.

    2. Yeah, I don’t know where Hack-A-Day got 40 MHz from… I just let it slide. I absolutely am using a 3.579545 MHz can crystal oscillator to generate square waves, and those are in the 80m amateur radio band (which I’m licensed to use). There are similar cans for a plethora of frequencies – I especially enjoy the ones around 28 and 29 MHz (just ordered a 10 on ebay this morning for about $1 ea) because they’re in the 10m amateur radio band, and handheld receivers are common for this frequency, and quarter-wavelength vertical antennas are small enough to be mounted on top of a car (similar to 27 MHz CB antennas). 28.704 MHz is the frequency of the crystals I got to go on my next high altitude balloon transmitter. With a small amplifier and a lowpass filter to keep it legal, it will make a wonderful telemetry transmitter!

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