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Crystal radio help anyone?

(If you want the blow-by-blow of my questions, skip the following gigantic piece of text and proceed to the 2nd list after the first)  

I was reconsidering making a crystal radio the past week and so I set out to research the topic on the 2nd most popular information database on the internet which comes out of the 1st most popular one. According to wikipedia, all you need is one diode in order to split the AC signal coming in via the antenna in half, therefore it uses half-wave rectification (it's not quoted; it's what I understand from it). Could full-wave rectification be also used? Like connecting the antenna to a bridge rectifier (aka. diode bridge, aka. 4 diodes stuck together)? I know that half-wave rectification yields pulsed DC, but I also don't understand how. By that I mean what happens to the other half? Does the other half of the wave connect directly to the output? One last thing, I know the largest metallic object I can ever use as an antenna is basically just my roof, but then I don't like coming up to the roof and looking for some exposed metal to connect to. Any thoughts on what huge, metallic items I can use in my household? I've already thought up a list of what I could use for an antenna:
-Window screens
-Extremely long piece of wire (I have an 8 meter long CAT5 cable going from the 1st to the 2nd floor)
-My front gate
-Sheet metal laying outside my front door (not the gate)

Here is the summary of all my questions:
-Is this wikipedia article true? (see at the list "semiconductor" area)
-Can full-wave rectification be used on such radios?
-In the case of half-wave rectification, where does the other half of the signal go?
-What can I use as an antenna (excluding my roof)?

Aside from the asked questions:
-Is there any other way to tune to a particular frequency?
-I'm asking opinions to people who know what they're doing (no offense to beehard44, this is more of a poll that anything else).


sort by: active | newest | oldest
- Good starter article, lots to glean from it if you read it ALL.
- No, cuts both halves of the signal off to produce a slightly pulsating DC. Picture #6 in the WIKI article shows how it works.
- Blocked by the "diode" action of the rectifier, ie, only lets it come through one way... on the positive half. The negative half is just stopped.
- Random length antennae are usually used in these instances, preferrably 100' (30+ meters) or more. That and a GOOD earth ground, mostly found on a cold water pipe (metal one that comes into the house) or a ground rod (found near the electric meter outside).

Tuning can be done (usually) 2 ways.
1. Different taps on the coil - a small loop is brought out of the coil and the insulation is scraped off to allow connection or the coil is wound all the way from start to finish and a "strip" of insulation perpendicular to the coil is removed and a slider moves up the coil on that strip.
2. A variable capacitor can be added to the coil to make a tuneable tank circuit. These are usually in the 5-325 pF range, IIRC.
The outputs from either is then fed to the detector. This makes your tuneable radio.

If you cannot find high impedance earphones (~2000 ohms), use a LM386 to drive the headphones / speaker you have. There are plenty of 'ibles to teach you how.

Now, you can make a tuneable crystal radio from 2 paper towel tubes, tin foil, magnet wire, a 1N914 diode and a high impedance earphone. I don't remember all the specifics, but If I can find my old copy of Poptronics, I can fill you in on them if you'd like.

Qa
By cut both halves of the signal off you mean run it through a full-wave rectifier so that I'll have a negative and a positive to connect the earphones to or what? By "I know that half-wave rectification yields pulsed DC, but I also don't understand how. By that I mean what happens to the other half? Does the other half of the wave connect directly to the output?" I mean you just connect one diode to the negative/positive side then just stick the remaining side in there?
The detector of an AM radio is a 1/2 wave rectifier. A bridge rectifier is full wave. Technically, you could use only 1/2 of the bridge (1 input and just the positive [+] output) and it should work. Thing is, the diodes used for power rectification make for poor dectectors. If you want to use a diode, the 1N914 has been used for years with great success. Forget the bridge rectifier. Put it out of your mind for this project.

If you want to understand more, just Google full wave bridge rectifier and look for explanations. There are many out there. Then Google How does a diode work as an AM detector? and look for related explanations. Perhaps you can understand more from one of them better than from me.

Good luck,
Qa
Or you could make a bridge with four 1N914 diodes.
maximzodal3 years ago
As a kid I used just a high impedance surplus WWII ear phone and a germanium diode! The ground was a gas pipe in the bedroom, the antenna was the screen on my window. No coils as all. It only picked up KVOO in Tulsa where I lived. It was loud enough I had to put it under the pillow in order to sleep.
rickharris6 years ago
Also needs a very high impedance ear phone - most people these days use a crystal ear piece. NOT er buds from you ipod.
I had success using small earbuds with an impedance matching transformer and a step up transformer found in a old liner power supply. (those large black wall adaptors).
nutsandbolts_64 (author)  rickharris6 years ago
That would be hard to source.... thanks anyway.
Dr.Bill5 years ago
This this wikipedia article is true.

Full wave rectification in crystal radio is also called 'Push Pull' . It uses both sides of the sine wave to make a louder signal to the earphones. [see answer below this one]
Dr.Bill5 years ago
People sometimes use a half wave rectifier to make sound in the earphone using the positive side of the sine wave.
People sometimes use two separate half wave rectifiers to make sound from both the positive side and the negitive side of the sine wave. This kind of crystal set is called a 'Push Pull' crystal radio. It makes sound in the earphone that is louder than the half wave set.

I have built a crystal radio that had a signal strength meter on it. I even added an led that actually lit up and modulated to the announcer's voice !

Push Pull Crystal Radio is the way to go. If you want more study materials try www.MidnightScience.com , they can even show you how to make headphones from Piezo Buzzers !
Re-design6 years ago
A full wave rectifier would create DC with a slight static.

Random metal objects don't usually make good antennas since they are usually grounded somewhat and aren't usually the right length. Length matters more than mass in this case.

8 meters is not a long antenna for a crystal set. You can string 100'-150' of wire just zig-zagged on your ceiling and that would be better than your other options.

Do some reading as suggested.
nutsandbolts_64 (author)  Re-design6 years ago
30 meters of wire... If length is more important, does it matter if the wire used for the receiving antenna were to be thin or does it have to be acceptably thick?
You can use thin wire as long as it's strong enough to support itself. I've unwound old transformers and used the thicker wire.
Here's a really great site for crystal radios.
rickharris6 years ago
http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html

How to and unless you live in a very remote place the materials should be available to you.

Some people use a coin and a razor blade instead of a diode.
Get yourself a copy of this book


Come back when you've read it.
rickharris6 years ago
Half wave rectify or the two audio signals cancel out
Not sure what you asking about the wiki - it looks reasonable on a quick scan
Get a germanium diode if possible.
No
To ground
A long wire - longer the better and a GOOD GROUND.

a variable capacitor as in the lowest diagram in the wiki
Fun to make - instructive - hard to get working.

there is a lot of crystal stuff on theWWW look around.