Crystal Radio/AM Transmitter

Thread Starter

Lightfire

Joined Oct 5, 2010
690
http://scitoys.com/scitoys/scitoys/radio/am_transmitter.html

That's great.

Let's start with the crystal radio.

http://scitoys.com/scitoys/scitoys/radio/radio.html#crystal

Building a crystal radio is a good hobby and thing to do. Isn't it?

Now, I need some suggestions on how to make one and an improved one.

Let's move with the transmitter.

According to the link I posted on the very top,

Depending on the antenna, the transmitter can send voice and music across the room, or across the street.
That's so great.

Please give me some suggestions on this project.

Any help will be appreciated.

Thank you.
 

Wendy

Joined Mar 24, 2008
22,177
What parts do you have so far?

You will need a variable capacitor for tuning. A matching coil (this is important).

A diode. Germanium or schottky is best, but almost anything will work.

A length of wire, 20' to 100'. I used to put them between trees, but they always broke.

A sensitive earpiece. Piezo preferred.

A small block of wood to mount everything on.

Since this qualifies as a project I'm moving it to the projects thread.

Let me know what parts you need help with.
 

Thread Starter

Lightfire

Joined Oct 5, 2010
690
Variable capacitor, dont have.:(

Germanium diode, I can afford this one.

I have wires but not that long. :(

I have earpiece but not piezeo.

Yes, I have small block wood...
 

Thread Starter

Lightfire

Joined Oct 5, 2010
690
Um, I guess the broken old radio is still here in our house. let me check......

How 'bout the transmitter? Do you have any suggestions???
 

MrChips

Joined Oct 2, 2009
22,104
You can wind your own coil with enameled copper wire wrapped around a cardboard tube.
Make your own variable capacitor by using two cardboard tubes from spent rolls of toilet paper or plastic wrap. Find two tubes that slide into each other. Wrap metal foil over the tubes and slide one into the other. A tube of about 4 inches long will give a capacitance of about 450pF.
 

wayneh

Joined Sep 9, 2010
16,401
Lightfire, you might like to read about a project I recently completed using the technique you linked to. One challenge you will have is find the 1MHz (=1000kHz) crystal oscillator. It's old and not still made. I found some from a surplus, hobby parts supplier.

The other posters are reacting to your title. A "crystal radio" is one of the very most common hobby projects for beginners in electronics. I made one as a boy scout, like my big brother before me. I'm sure many of the regulars here have also made one. But what they all think of as a "crystal radio" is quite a lot different than the transmitter project you linked to.
 

Adjuster

Joined Dec 26, 2010
2,148
This is worth getting straightened out. "Crystal" may have (at least) two different meanings for a receiver or transmitter. To add to the mix, a "crystal" (diode) receiver may even be listened to using a "crystal" (piezoelectric) earphone. That kind of earphone is actually a distant relation of the other kind of crystal which might be used in a transmitter.

Enough confusion: what are we actually talking about? The first kind of crystal, used in radios since the very early days, is a form of rectifier diode used as a demodulator, or detector of radio transmissions. The earliest versions used certain natural mineral crystals. They required the user to attempt to find a workable point on the crystal with a movable wire point, not the most convenient arrangement. Later, more convenient fixed diodes were made.
http://en.wikipedia.org/wiki/Crystal_radio#Crystal_detector

The resonating crystal, most often made of quartz, was developed to define tuning frequencies, at first mainly for radio transmitters. The quartz crystal has the property of generating electric potential under stress (piezoelectricity). This effect also works "backwards" so applying a voltage to the crystal causes it to change shape slightly.

Taken together with an ability to "ring" at high frequencies like an extremely high pitched bell, these properties makeθ the crystal useful for regulating the frequency of electronic oscillators. Such crystals are also used to keep time in electronic clocks and watches, and to provide stable "clock" frequencies for things like computers.
http://en.wikipedia.org/wiki/Quartz_oscillator

Finally what about that "crystal" earphone? This also depends on piezoelectricity. The piezoelectric effect is used in reverse here, so that signal voltages cause vibrations which make sound in the ear. The crystal earphone has a high input impedance at low frequencies, and it does not pass DC. A parallel resistor (usually of tens of kΩ) is therefore needed for use with a simple diode radio, or any other circuit where a DC path is needed. http://en.wikipedia.org/wiki/Crystal_earpiece
 

Wendy

Joined Mar 24, 2008
22,177
Actually the resistor is not required for a classic crystal radio piezo earphone, which is what LF is building. It is contraindicated, since you want as high an impedance (resistance) as you can get to not load down the reciever. This is free energy, but the power levels are so low you need every help you can get.

If you want to help, think simplicity. This is where LF is at at the moment. The transmitter comes later.
 

Adjuster

Joined Dec 26, 2010
2,148
Sorry Bill, but I can't agree on that last point, at least not necessarily.

It all depends on how much DC leakage there is in the diode and the crysyal earpiece. If this is less than a minimum value, the output of the diode detector will be unable to follow the carrier amplitude as it falls from higher to lower amplitudes. This may lead to muffled, distorted signals, or even no audio at all if the DC blocking is particularly "good".

At the very least, it is worth trying say a 100kΩ pot in parallel with the earpiece to see if better results can be obtaiined. This is not just theory, as a child I had a crystal set which worked well with magnetic earphones, but poorly with a piezo earpiece. A teacher suggested connecting a 56kΩ resistor across it, which did indeed give far better results. Note that this order of resistance is still more than the few kΩ of normal magnetic 'phones.
 

wayneh

Joined Sep 9, 2010
16,401
This is where LF is at at the moment. The transmitter comes later.
His first post is asking about building the circuit you can find on the internet to build an AM transmitter using a 1MHz crystal oscillator.

This really has nothing to do with the traditional crystal radio hobby project, which we all agree would be a good one for him. But it's not what he started this thread with.

Lightfire, come back and let us know what you are thinking about.
 

Wendy

Joined Mar 24, 2008
22,177
I've been talking with this guy offline, a lot. :) I was the one who suggested it as a good project.

The DC value going to the piezo earpiece is going to be proportional to the reception of the AM signal, which isn't going to be much.
 

Adjuster

Joined Dec 26, 2010
2,148
My reason for insisting on the DC path is not just functional. It has to do with Lightfire (eventually) grasping the principle that an AM demodulator is essentially a rectifier, and thus requires a DC path in order to deliver output current.

It can be shown that in the absence of a DC path no useful demodulation occurs: this can be proven by putting a low-leakage capacitor in series with the diode. This is notwithstanding the fact that sufficient leakage paths may exist through components for the circuit to work without an exlicit resistor.

Whether or not the lad is yet at a level of understanding to profit from this is another matter, but I always want to be orthodox when dealing with beginners (clearly I often fail, but that's how it goes). My time for giving advice may not be long now, and I want to make it count.
 
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Thread Starter

Lightfire

Joined Oct 5, 2010
690
Is it possible to have a tuner, frequency changer, in the crystal radio? :D :D :D

I am planning to build the AM transmitter also, you know. :D :D But I have to work first in the crystal radio.

I have problem now in AM transmitter if ever I made it, is it possible that I put more mic in parallel?
 

wayneh

Joined Sep 9, 2010
16,401
Is it possible to have a tuner, frequency changer, in the crystal radio? :D :D :D
Yes, the traditional crystal radio can be tuned.
I have problem now in AM transmitter if ever I made it, is it possible that I put more mic in parallel?
Yes, but you'll need to explain exactly what you want at that time. You will be able to broadcast any audio signal. If you want to first mix together various audio sources (like two microphones), this mixing of the audio is done before feeding it to the transmitter. And only the mixed signal appears at the receiver - not the individual sources.
 

Audioguru

Joined Dec 20, 2007
11,249
Did you make it? It is so simple that it takes only a couple of minutes to make it.
Did it work? If your antenna is long enough then its very poor sensitivity might pick up only strong local stations.
Did it pickup all the local stations at the same time? Real AM radios have many tuned LC circuits so that you hear only one station.
 
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