2022-05-07 - Radio 
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I'm still on my quest to understand a bit more about analog 
electronics, and while reading, i have become quite intrigued with 
inductors and capacitors, and all the things they can do when you 
put them together. 

However, learning from theory only is no fun, and the best way to 
learn is from practice. So to have some practice building, i've 
taken up the challenge of building a crystal radio.

As a base for information i've taken the book "Making a transistor 
radio" by G.C. Dobbs (724 from the Ladybird series) from 1972. 
While this book was intended for kids, i've found it great to read. 
The content is great and educative and the illustrations are 
excellent. 

From a local radio electronics shop, i've purchased a polyvaricon 
capacitor (2x 266p), a germanium diode (OA91), a Crystal (piezo) 
ear piece and a roll of 0.3mm enameled wire. With these parts i've 
made some attempts to create a functional radio roughly as follows:

     Y
     │       OA91
     ├─────┬─|>|────┐
     │     │        │
     C     │ vari   o
coil C     = cap      Earpiece
     c     │        o
     |     │        │
     |─────┴────────┘
    === Ground

The first attempt was only partially succesful. With the 
266pf capacitor i consided that i needed a inductor of around 380uH 
to get a resonant frequency of around 500kHz. And on a ferrite rod 
i still had it would take around 60 windings to get such an
inductance value (i thought). 

After wiring all the parts up, connecting a wire antenna and earth, 
i couldn't really hear much going on. And after fiddling around 
with things, the primary issue was that the crystal earpiece was 
faulty and seemed to have a loose connection.

After some fiddling with the earpiece, it was somewhat working 
(if i held still and didn't move). However, not much could be 
heard. After some more figuring i started to hear static noise of 
some sort. But this seemed mostly a signal picked up from the 
building/surroundings (transformators and such).

While fiddling with things, i disconnected the variable capacitor, 
and all of a sudden i started hearing very faint voices (in a very 
foreign language). 

But with the capacitor reconnected, nothing could be picked up. As 
i don't have any equipment to test resonant frequency of the tank 
circuit, or even ways to test inductance or capacitance of the 
polyvaricon capacitor, i was very much in the dark. The faulty 
earpiece constantly failing also made things more complex.
 
So the next day, i considered the coil might have had too many 
windings combined with the ferrite core, and that i had been 
picking voices from some longwave stations.  However as i had no 
idea, i created another coil using an old plastic caulk tube (dia 
approx 4cm) and some breadboard wire.

Again, i wound 60 windings (but had to make an unintentional tap, 
after 40 windings as i ran out of wire. 

Now with this coil all of a sudden i could receive some local 
stations at the MW frequency band that i expected. Still the faulty 
crystal piece was very annoying, failing every 5 seconds. 
Furthermore the audio level was so low that words were barely 
distinguishable

So today i've made an audio amplifier on a breadboard using a BC547 
and a LM386, so that i could at least use my normal earphones.

At first i couldn't get the amplifier to work (only silence in 
combination with the crystal receiver), so i set off to test the 
amplifier by itself. Unfortunately i don't have any sensible 
equipment to measure anything. I wanted to feed a signal that i 
knew which was working (from TV or PC), but i didn't have a 
suitable plug (or cable) to do that. 

So i used a 555 timer, to create an audible square-ish wave signal, 
and after some fiddling with the breadboard (ugh breadboards) the 
amplifier came to live. 

The crystal receiver above was connected to the amplifier and all 
of a sudden i could hear crystal clear radio... Albeit multiple 
stations at once. :)

However, it was very satisfying to finally hear something 
distinguisable on a actual headset.  This way i could really follow 
conversations on the radio.

Furthermore, the unintended tap at 40 windings proofed to be useful 
as well, as with the first 40 windings bridged by a croc-clip, i 
could tune in some completely different band, which gives me all 
kinds of foreign stations... The reception changes all the time 
(based on atmospheric circumstances?) but i've heard, Romanian, 
French, Portugese and American stations i think.

Now this last bit i find really cool!  And while the radio is still 
pretty shitty (and cannot tune properly to a single station 
properly yet) the audio amplifier really makes listening to the 
thing much more fun and easy.

So now the next steps are to figure out how to get the 
tuning/selectivity to work, and to build the audio amplifier onto a 
PCB.  Furthermore, i think i should consider getting some proper 
measurement equipment (Functioning multimeter, scope, maybe a 
function generator)...

Anyway, it's good to have something working!