Worlds smallest RC airplane: 66 mm!:

Originally posted by sonhouse
https://www.youtube.com/watch?v=ayovPr_WjSM

This one is cool too.

YouTube

On the theme of "World's smallest", this video shows the construction and running of a miniature V12 engine:

YouTube

The above link seems not to work (at least for me, it's the first time I've tried to link a YouTube video) the URL is:

YouTube

Edit: It does work, but not from the preview thing.

I couldn't find a youtube video for this, but a few years ago Scientific American had a story about the worlds smallest radio. It was so tiny you had to look through a microscope to see it. The only components were one carbon nanotube connected to a power source and tiny amplifier. The speaker was probably visible... I'm working off memory here so I'm not entirely sure about some of the details. But I distinctly remember that the carbon nanotube served as both antenna and tuner, and it also prevented the carrier wave from entering.

Carrier waves are removed by passing the signal through a germanium diode. But the carbon nanotube was also able to do that, so no diode was needed. The nanotube could only tune in one frequency, and whatever frequency it could tune in depended on how the tube was bent.

Originally posted by lemon lime
I couldn't find a youtube video for this, but a few years ago Scientific American had a story about the worlds smallest radio. It was so tiny you had to look through a microscope to see it. The only components were one carbon nanotube connected to a power source and tiny amplifier. The speaker was probably visible... I'm working off memory here so I'm not ...[text shortened]... une in one frequency, and whatever frequency it could tune in depended on how the tube was bent.

So all you need for a tuner is an atomic force microscope then.....

Originally posted by lemon lime
I couldn't find a youtube video for this, but a few years ago Scientific American had a story about the worlds smallest radio. It was so tiny you had to look through a microscope to see it. The only components were one carbon nanotube connected to a power source and tiny amplifier. The speaker was probably visible... I'm working off memory here so I'm not ...[text shortened]... une in one frequency, and whatever frequency it could tune in depended on how the tube was bent.

For amplitude modulated signals the carrier wave is filtered out with a capacitor. The diode provides rectification, without it the loudspeaker will just see a high frequency AC signal which it can't respond to and not do anything. For FM it's more complex, but one way of getting the signal off the carrier wave is to use a tuning circuit as the voltage response is frequency dependent and more or less linear near the resonant frequency. I'd guess that the nanotube does its filtering this way.

Originally posted by DeepThought
For amplitude modulated signals the carrier wave is filtered out with a capacitor. The diode provides rectification, without it the loudspeaker will just see a high frequency AC signal which it can't respond to and not do anything. For FM it's more complex, but one way of getting the signal off the carrier wave is to use a tuning circuit as the voltage ...[text shortened]... ss linear near the resonant frequency. I'd guess that the nanotube does its filtering this way.

Does the same thing happen in a simple crystal radio set, does the crystal provide rectification without removing the carrier wave? If so then how is the carrier wave removed in a simple crystal radio receiver before reaching the speaker? Is it the tuner that does that?

I once built my own little version of a crystal radio set using parts from a small Am/Fm radio. It was simply a tuner and the headphone plug in, along with a germanium diode I'd gotten at Radio Shack. I used the diagram from a toy crystal radio set as a guide for putting it together inside a one square inch plastic box, with outside leads for attaching wires with alligator clips. I was able to pick up only one station with it, but it was an Fm station... I forgot to mention that one of the parts was a switch that could be set on Am or Fm. The switch was set to Am, but for some reason I was getting a station broadcasting on Fm.

Originally posted by lemon lime
Does the same thing happen in a simple crystal radio set, does the crystal provide rectification without removing the carrier wave? If so then how is the carrier wave removed in a simple crystal radio receiver before reaching the speaker? Is it the tuner that does that?

I once built my own little version of a crystal radio set using parts from a small ...[text shortened]... or Fm. The switch was set to Am, but for some reason I was getting a station broadcasting on Fm.

The diode does half-wave rectification - it cuts off the negative voltage. Handily there's a circuit diagram on wikipedia on the page [1], take a look at the diagram [2]. The diode D1 does half wave rectification and the capacitor C2 allows high frequency through but not the low frequency signal - it acts as a low pass filter. As it happens you can get away without the capacitor C2 as the speaker in the headphones can't respond to the high frequency of the carrier wave so it will tend to just follow the envelope.

I'm not an electronics expert so I'm guessing a bit here. The tuning circuit resonates at a selectable frequency. It's response close to the resonant frequency is approximately linear so V ~ f(t) - f_c where f(t) is the instantaneous frequency of the input and f_c < f_r is the frequency of the carrier wave and close to and slightly below the resonant frequency f_r of the tuning circuit. You had it set to AM so you'd managed to get the circuit tuned to a frequency just above that of the signal and the tuning circuit was providing this linear response that meant you got a usable signal out. I'm not surprised that it only worked for the one station - I think there was a bit of a balancing act going on

[1] https://en.wikipedia.org/wiki/Crystal_radio
[2] https://en.wikipedia.org/wiki/Crystal_radio#/media/File:Crystal_radio_with_impedance_matching.svg

Originally posted by DeepThought
The diode does half-wave rectification - it cuts off the negative voltage. Handily there's a circuit diagram on wikipedia on the page [1], take a look at the diagram [2]. The diode D1 does half wave rectification and the capacitor C2 allows high frequency through but not the low frequency signal - it acts as a low pass filter. As it happens you can ge ...[text shortened]... ttps://en.wikipedia.org/wiki/Crystal_radio#/media/File:Crystal_radio_with_impedance_matching.svg

I'm not exactly an electronics expert myself, basically all I was doing was following the same design templete of a toy crystal radio set. I later learned we were living very close to a transmission tower, so that's probably why I was able to tune in that one station.

If I used a tin can attached to a wire running to the metal stem of an umbrella, I might have been able to hear that same station... that's how close we were to that tower.

Originally posted by lemon lime
I'm not exactly an electronics expert myself, basically all I was doing was following the same design templete of a toy crystal radio set. I later learned we were living very close to a transmission tower, so that's probably why I was able to tune in that one station.

If I used a tin can attached to a wire running to the metal stem of an umbrella, I might have been able to hear that same station... that's how close we were to that tower.

Actually, the capacitor works in conjunction with the coil to respond to a certain frequency range, hopefully in the AM broadcast band, 550 to 1700 Khz. Crystal sets don't have a lot of "Q" and so are really wide banded so you can't expect to separate two stations 40 Khz apart. The diode DOES the demodulation because it cuts off the bottom or top half of the modulated CW wave, modulated with AM so the audio is riding on the carrier wave, which would be at something like 1000 Khz, for example.

So it is a sine wave but with much lower frequency audio on it and the diode chops off either the negative or the positive going waveform depending on which way you hook it up, either way works fine so you are left with some dc voltage with the audio riding on it.

Since it is pulsating DC, the thing you hear is the modulation.

You can further diddle with that signal by putting in a zener diode which does not let current through till a certain threshold voltage is reached and you could, if you knew the voltage that is reached before the modulation is reached you could cut down that to make less dc offset but it probably won't make much difference to the sound you hear in the earbuds.

It is amazing to me a signal can be sent through the air at all and be picked up well enough to hear with earphones from such a simple radio which uses no amplification.

There is a version of the crystal set radio that does what I said with the zener diode and then go to a simple one transistor audio amp but that is another story.

Originally posted by sonhouse
Actually, the capacitor works in conjunction with the coil to respond to a certain frequency range, hopefully in the AM broadcast band, 550 to 1700 Khz. Crystal sets don't have a lot of "Q" and so are really wide banded so you can't expect to separate two stations 40 Khz apart. The diode DOES the demodulation because it cuts off the bottom or top half of t ...[text shortened]... with the zener diode and then go to a simple one transistor audio amp but that is another story.

It's possible because mammalian ears are so sensitive. You can make a just-about-functional radio with some headphones, copper wire, a rusty razor blade, and a needle (the aerial will provide a bit of capacitance). We seem to have a slight difference of opinion on what constitutes demodulation - I'd regard the capacitor C2, or just the headphones, as doing the demodulation because after it's gone through the diode the carrier wave is still present.

Originally posted by DeepThought
It's possible because mammalian ears are so sensitive. You can make a just-about-functional radio with some headphones, copper wire, a rusty razor blade, and a needle (the aerial will provide a bit of capacitance). We seem to have a slight difference of opinion on what constitutes demodulation - I'd regard the capacitor C2, or just the headphones, as d ...[text shortened]... ng the demodulation because after it's gone through the diode the carrier wave is still present.

The key is the carrier wave, although technically still there, it is now pulsating DC and now can do some work using the modulation. The earphones could not vibrate very much at 1 megahertz but can at 1 kilohertz. The diode is the demodulator that allows that to happen. The capacitor is either in series with the coil or in parallel with it, either way it is an LC circuit which will have its frequency response curve somewhere, hopefully in the 1 megahertz band.

Take out the diode and no radio. The rusty razor was mentioned and that would work because it acts like a halfasssed diode. A good diode would be very low ohms in one direction but near infinite in the other. The rusty razor would sort of have that but a lot less difference between one direction of current flow vs the other. Say one side would be 100 ohms and the other side 10000 ohms, probably good enough to demod the carrier.

When you cut half the carrier wave away with the diode, the rest is an audio signal that might even be the same value as half the full carrier wave and would therefore be almost pure audio.

Here is a wiki about the diode used as demod:

https://en.wikipedia.org/wiki/Envelope_detector

Originally posted by sonhouse
The key is the carrier wave, although technically still there, it is now pulsating DC and now can do some work using the modulation. The earphones could not vibrate very much at 1 megahertz but can at 1 kilohertz. The diode is the demodulator that allows that to happen. The capacitor is either in series with the coil or in parallel with it, either way it is ...[text shortened]... Here is a wiki about the diode used as demod:

https://en.wikipedia.org/wiki/Envelope_detector

Right, I agree that a crystal set won't work without a diode, but you've missed what I'm saying slightly. There are two capacitors. One is in the tuning circuit, that is not the one I'm talking about. I'm thinking of a second one, in parallel with the earphones and after the diode whose job is to filter out the carrier wave. It's not absolutely necessary but stops the earphones trying to respond to the carrier wave. It's labelled C2 in the diagram in link [2] from my earlier post.

Originally posted by DeepThought
Right, I agree that a crystal set won't work without a diode, but you've missed what I'm saying slightly. There are two capacitors. One is in the tuning circuit, that is not the one I'm talking about. I'm thinking of a second one, in parallel with the earphones and after the diode whose job is to filter out the carrier wave. It's not absolutely neces ...[text shortened]... respond to the carrier wave. It's labelled C2 in the diagram in link [2] from my earlier post.

Yes, that would work. It's called a bypass cap. An earphone would never be able to respond to megahertz signals so it is really not needed. But the diode is what does the actual demod. Did you see my link?