Crystal Sets Revisited

 Well, my original crystal set post had some experimentation on it, and I have done a little more recently due to wanting my daughters Cub Group to build one as an experiment.  The biggest hurdle was getting the cost down to a resonable figure - otherwise the project was dead in the water.

In New Zealand, tuning capacitors (polyvaricon types) are around $5 each, as are headphones.  The rest (enamelled copper wire, diode, resistor, etc) adds another $5 approx, so all said and done, its a $15 project.  That might seem pretty cheap, but then to some of our families thats a huge amount - and I didn't want to exclude some cubs due to cost... so, it was time to start digging up some old tricks.  The first one I thought of was a variable ferrite slug within the coil, and a fixed capacitor (reducing the cost of capacitor to around 1/100th of the price).  Ferrite is not too bad, $5 will get enough to make three radios, so thats a significant saving.  The headphone is pretty much unreplaceable... so that cost stays.  The original coil was a spider coil as well, as seen in the original crystal set article... this one is a traditional coil, based on the size of the ferrite rod and so much smaller... thus using less wire... which means less cost.  Effectively I can put together the parts for one radio for under $10... and the feasability is now looking better.

So here is the experiment: make it small, but buildable by young hands - make it cheap, and most importantly: make it work.

The basic design is one I have built before, as a child, in a tic-tac container.  But this time (to save little teeth) I've used a matchbox... and here's the prototype:

And inside:

The coil around the outside of the box was an experiment to try and improve selectivity, although it made no difference at all (if anything it was slightly worse).  The coil is about 70ish turns of 22-gauge wire, a 1n914 (1n4148, OA91 or similar would be fine), and the ferrite slug is 60mm long (meaning I can get three from the 180mm rods I get locally).  The resistor is 47k just to add an electrical path to allow the piezo headphone to operate.  The circuit is as follows all apart from the variable capacitor - I have used a 220pf disc ceramic here:

And the results are great - Using the full length of rod (180mm) I had louder reception than any of my spider-coil sets - and even with just 60mm its equivalent to them.  I also tried a shorter 15mm slug with a wooden stick glued to its end, but the volume was terrible.  Winding the coil is tricky - once wound, whatever you wound it on is too tightly bound to let go.  I used some stiff paper wrapped around the ferrite rod first time around, some plastic cut from a soft drink bottle the second time and went back to the first option but using a penthat was slightly larger than the ferrite rod, and with a slightly tapered barrel so I could get it off.  Once you wind the coil it becomes very tight and impossible to remove without damaging it otherwise.

The band coverage of this set is very limited, and I did use a polyvaricon to find the best capacitance for a loud local station (Coast AM - top of the dial at nearly 1600kHz).  I might look at using two 100pf caps and a paperclip switch to improve the range - the cost addition will be minimal, but the fiddlyness goes up...

Also, I have used a socket for the earpiece here, but I won't bother in the final version... I'll just solder it straight in.

So there you have it.  Small, loud and cheap - nothing like me at all Cool

UPDATE:

My Daughter and my Neice made a mkII version of this the other night, just to prove they could be built by small fingers.  I made the tubes and taped the wire on, they wound them and I taped the other end.  Then they bent up the components and helped solder them (actually, I held the parts, and they soldered... I've never been so nervous! :)

They work well, look neater than the original, and are quite inexpensive to make - even at retail costs for parts.

  

 

Footnote: I've been experimenting with selectivity - an issue with the above radio.  This little experiment has two tuning circuits, and by tuning the second one with the aerial attached to it I can zero in on specific stations I couldn't pull out of the clutter before.  Unfortunately I can't keep it as its soldered together on and around my leg! :)

Footnote 2:  I was experimenting with different diodes to see what performance gains could be had and discovered something interesting that might improve the spider coil sets my daughter and I originally made.  Using an OA61 gave the best performance on loud and weak stations - and the best tone.  A BAT48 Schottky diode performed almost as well, just a slightly scratchier sound but very similar volume on loud and weak stations.  The worst performer was the 1N4148 / 1N914 diode that I used in the spider coil sets - well, apart from a standard green LED I tried which gave no output (no surprises there, they have a forward voltage drop of somewhere around 2V!) - most surprising was a standard silicon rectifier diode which worked ok (albeit quietly) on loud stations... I wasn't expecting anything from that either.

So, I'll replace on of those diodes with a BAT48 (I just used my last OA61) and update the original thread with the results comparing the two (nearly identical) sets...

And this is the basis for my loudest set ever.  I'm using the dual-gang tuning cap shown above in series with my incoming aerial, and using a 640pF (dual gang unit) in parallel with the sliding coil (lower one shown) to do all the tuning.  The diode rectifies the signal on the red coil and feeds the speakers directly.  Its so loud I can hear it at the other end of my house!  Here is the diagram:

LOUD Crystal Set

 

Loud Crystal Set - photo