Crystal Sets

s a child, my father got me into building my first crystal set.  I don't fully recall the details, but I seem to recall it was a kitset and I was in Nelson when I built it - If I remember I'll ask him.  What I do remember is building a few of them, including one that fitted inside a tic-tac container and another that had no tuning cap, just an icecream stick with a screw through the end that scratched through to vary the coil windings in use.

Anyway, I've recently starting teaching my daughter how to make them, and how they work - and I thought it might be useful to someone to show what steps we went through (and are still going through), which sites were helpful, and what parts to use.

When I first started looking around for a kitset I was disappointed to find there was little or nothing available here in New Zealand... although Jaycar carry a kit (shown to the right) - but its not exactly educational - nor is it built in the simple spirit of crystal radios (ie: using a minimal amount of parts scrounged from a box of used bits and pieces and screwed to an old slab of wood).  It also has a very narrow tuning range due to only using a small trimmer capacitor rather than a full tuning capacitor.  It did tune in a station, however, and the look of amazement and excitement on my daughters face when she could actually hear it was enough to get me thinking, googling and researching.

I don't recall all the sites I came across while looking for more information, but some of the most useful are listed at the end.  What I discovered was that spider coils were the new thing, where coils wound on a ferrite rod (similar to the Jaycar kitset one) were in-vogue when I was young.  Spider coils - Higher 'Q' than a cylindrical coil due to less interwinding capacitance.  Don't sweat the technobabble - but just know that spider coils work better, and are easier to wind - especially for small hands.

So, I made a former up by drawing it in TurboCAD, printing it, gluing it to some corrugated cardboard then cutting it out with a sharp hobby knife.  I used a small jewellers screwdriver to make some holes to feed the wire in (24 gauge enamelled copper wire), and the tuning cap.  I then soldered everything together in place.  

Note: The tuning cap has two gangs - a 60pF and a 160pF one.  By soldering the two outside wires together and using that as one connection, and the center wire as the other we get 220pF... not quite enough, but it works.

The actual winding details for the coil I calculated with Dan Petersens calculator (Version 5.7 is here - its all imperial, but it does the job).  What I worked out was that around 56 turns would be good with the bog-standard 60-160pF variable capacitor that you can get at Jaycar.  It should, by my calculation, cover the entire AM band.

The circuit I settled on is about the most basic possible - and the performance was pretty good - and coverage was excellent.  With a little tweaking of the two trimmers on the back of the tuning cap (sometimes called a polyvaricon) I could tune both Radio Live (570ish at one end and Coast AM (1590ish) at the other... a pretty good spread for such a basic design.  Whats even more impressive is how little aerial it needed (although we are in the city, and Coast in particular is transmitted very close to us).

The 47k resistor is there for the crystal headphones.  If the resistor isn't there then you probably won't hear anything.  If you have old-school vintage headphones then you can remove the resistor.  Here are better pics of the front and back.

FM Crystal Set - Experimental

This is one I built as an experiment to see if it would work... no joy yet, but I'm still trying.  Its based on this one:


FM Crystal Set - version 2

I built this version after a little advice on The Radio Board... but yet again, no joy.  Still tinkering tho.  (Oh, there is a typo - I used a 10pF cap over the coil, not 33pF).


Links: - Dave Schmarders site - An absolute crystal set craftsman - The Birmingham Alabama Crystal Radio Group - lots of sets to look at - Plenty of hard-to-find parts - Incredibly clever guy, lots of good theory - The Radio Board - Crystal Set Forums - air-cored L calc - advanced air-cored L calc - Bandspreading 


UPDATE: More info here