Electronics > Beginners
Crystal Radio Breadboard - Zero Sound
chris_leyson:
I remember as a kid sitting for hours reading an old science book on my dads bookshelf, Science for the Citizen by Lancelot Hogben. "Dad, I want to build a crystal radio" so next weekend of we go to L.H.Evans in Cardiff market and buy all of the required parts. A 350pF film tuning cap, a Denco coil, an OA47 and a crystal earpiece. Wow that was fun and it worked reasonably well with a 50' long wire. I didn't have any luck building a single valve super regen but 50 years later I might have another stab at it just for fun. L.H.Evans is still there in Cardiff market and A simple crystal radio was the "spark" that got me into electronics.
StillTrying:
Anyway to check that the variable cap connections actually vary 20pF to 330pF.
With a DVM in place of the earphone you should see a few mV DC on a strong station at least.
With a scope you could ping the tuned circuit to see if it rings at ~700kHz.
metrologist:
--- Quote from: StillTrying on December 20, 2018, 10:06:32 pm ---With a scope you could ping the tuned circuit to see if it rings at ~700kHz.
--- End quote ---
How do you ping?
t1d:
The diode is a 1N34A, from a proper Crystal Radio supplier. I did proof it.
The v cap is also from a proper Crystal Radio supplier. I did proof it.
To ping the signal... Replace the antenna with a Function Generator and listen to the radio, for tone?
Thanks
vk6zgo:
--- Quote from: t1d on December 21, 2018, 01:52:41 am ---The diode is a 1N34A, from a proper Crystal Radio supplier. I did proof it.
The v cap is also from a proper Crystal Radio supplier. I did proof it.
To ping the signal... Replace the antenna with a Function Generator and listen to the radio, for tone?
Thanks
--- End quote ---
Do you have a function generator?
If so, you can couple its output very loosely to the Crystal radio & tune across the range of frequencies you expect to be receiving.
Most function generators go up to 1MHz, so, with the gen set to sinewaves, as you tune across the LC tuning circuits expected resonant frequency, you should hear the background hiss suddenly drop, as the unmodulated carrier from the FG passes the actual frequency of resonance.
As there is no modulation, you won't hear any tone.
To actually hear a tone, try a square wave or sawtooth at around 1kHz.
Such waveforms are rich in harmonics, which the radio will see as a MF carrier pulsed at 1kHz----near enough to AM!!
Again---- put the tiny little RF chokes back in the parts bin, & wind a proper coil.
If you look at enough websites*, you should get a good idea of the number of turns on a given sized form.
As to not always needing an antenna, this is "sort of" correct.
If you have an earth connection to a metal water pipe, the radio will "use the earth as an antenna".
Another old time trick was to clip a short antenna lead to the metal of an old style dial phone.
This would capacitive lay couple to the phone wires.
Millions of kids over many decades made Crystal sets that "worked".
Making really good ones is a specialist hobby, but you might be best to stick with basics first.
* PS----take a look at this:-
http://www.techlib.com/electronics/crystal.html
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