[AdrianoKF]

Not a ham operator, but I'd assume that a transmitter like this will emit a significant amount of noise across the RF spectrum (also, make sure to check your local laws before transmitting in potentially reserved frequency bands, e.g. with the FCC [0], or the Bundesnetzagentur in Germany [1]).

Other than that, looks like a fun little project!

[0]: https://www.fcc.gov/engineering-technology/policy-and-rules-... [1]: https://www.bundesnetzagentur.de/SharedDocs/Downloads/DE/Sac...

[wrycoder]

I am a ham.

This is a nice, very educational project that a couple of kids could use to communicate between their adjacent bedrooms at night.

It probably provides less rf than a typical noisy LED lightbulb power supply.

The instructions show how to set the frequency between any local AM stations.

If you are worried about neighborhood rfi, listen for it on your AM car radio in the drive and cut back on the length of the one meter antenna until you can’t hear it.

Also, notice how loud the radio frequency interference (RFI) actually is between stations, due to power lines, passing vehicles, LED and fluorescent lighting, many other sources due to modern technology, and lightning and other atmospheric effects. [0]

If someone completes this project, they are more than ready to get a copy of the ARRL General class license manual and go for their ticket. Communicate worldwide with Morse (hams usually call it CW, for “continuous wave”)![1]

[0] https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...

[1] https://en.m.wikipedia.org/wiki/QRP_operation

[AdrianoKF]

Thanks for providing some context! Reading your response makes me realize that my initial assumption was probably way too pessimistic - and that the project serves a nice educational purpose.

On top, sparking interest in tech and ham in kids is always a good thing, a reminder about the possible side effects can always come at a later time :)

[jah]

You're absolutely correct about RF noise. In the US, the FCC limits the power of all spurious emissions to be at least 43dB below the power of the fundamental frequency. Typically transmitters use a low pass filter (e.g. Buttersworth or Chebyshev) to reduce harmonic content.

Some micro transmitters (at the very low mW level) omit the LPF entirely. Looking at the datasheet for the ATtiny85, the maximum output current is 40mA and maximum output voltage is 5.5v which means this transmitter could theoretically produce an output of 220mW. PWM will reduce this obviously. The harmonics should be below this value, but they should probably add a filter. I've transmitting across the Atlantic Ocean at those power levels using a sub-optimal antenna and WSPR (https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)) encoding.

[wrycoder]

If you cut back the antenna length so you can’t hear the fundamental 100 ft away, the harmonics shouldn’t be an issue. But, I’d have to build it to be sure. I might do that!

Here’s some info on suitable low pass filters for $5. You’d have to modify the 600 meter kit for 555 kHz, after reading up on the design at the linked technical pages.

https://qrp-labs.com/lpfkit.html

[mlyle]

The thing is, ... anyone who's playing with microcontrollers and longish wires is making similar RF noise.

Efficiency of, and coupling to, this antenna is going to be garbage, even for harmonics. And the breadboard itself provides a fair bit of attenuation of the already-small 10MHz-and-up content.

> the maximum output current is 40mA and maximum output voltage is 5.5v

Looking at the maximum power you're supposed to draw from a pin isn't useful. The output transistors can provide much more: it's just not good for the part.

It'd be better to consider the radiation resistance of the wire and the AC voltage driving it.