Choccy Block Crystal Set Radio Modifications, The Single Transistor CBCSRK-1T

As a kid, I had a hard time trying to make any published crystal sets work, and had about zero success with single transistor radios either. So here's one that does work, if you put all the bits in the right places. I should add that in crystal set terms, this is a Massive Cheat. When you've added a battery, it's no longer a crystal set.

On another page about a different design, I describe how that radio doesn't use any, "funny electronic tricks."  Well, we're not in six transistor land now, so we're going for all the tricks we can, to get the most loudness and sensitivity out of one additional transistor and a single AA battery, using a few additional components, and those already in the kit.

In the original CBCSRK-1 kit, there is a 12 section terminal strip.  Four of the terminals are unused.  That gives us an opportunity to add a transistor and a power supply.  We can "supercharge" our crystal set. The intention is to keep the additional power requirements to a minimum, in this case a single AA battery, or an alternative 1.5V power source.

Here's how to build the single transistor circuit. The electronic schematic diagram is shown below, but you can just follow along with the pictures to make up the parts. I've assumed that you've already made the original crystal set, or at least read through the instructions. You can click on the pictures to see enlarged versions.

The Electronic Schematic Diagram For the Single Transistor Modification


 
PDF Copy Of The Single Transistor Terminal Strip Crystal Set Radio Modification

Assemble the Components and Wire Links

First, disconnect everything that you've built for the crystal set. Sorry, it's the best way. Then, build up the components as shown. One important point to note is that you need to turn L2, the audio transformer, the other way around so that the end with the two wires is connected to the earpiece.  The other important thing to note is that Q1, the BC549C transistor must be flat side up. I've used different coloured wires in the various pictures to make the photographs as clear as possible, but use any colour of wire provided in the kit.

The Components And Wire Links Connected Into The Terminal Strip
 



Connect in the Ferrite Rod Coils, Battery, Aerial, and Earth Wires.

Then it's time to connect up the ferrite coil L1, the battery holder J1, and the ground and aerial wires. The numbers in the photograph match the pin numbers on L1 in the schematic diagram. The two enameled copper wire windings are the same ones as used in the original crystal set.  Pins 4 and 3 are the regeneration winding shown in orange wire in the picture. This only needs to be positioned as shown to provide a small amount of signal feedback into the ferrite rod. It needs to be wound in the same rotational sense as the main coil 5 and 6.  In many situations with a decent aerial, you may not need it at all, and you could just connect the wire without taking it near L1. If you find that putting the regeneration winding near L1 makes the output quieter, you need to reverse the rotational direction of the winding. If the regeneration winding is too close to the rod, the radio will make a loud unpleasant squawking noise.

The Three L1 Coils Winding And Connection Detail
 



The Battery Holder, Aerial and Earth Wires Connected.

Zoomed out slightly to show the whole radio. The red and black wires for the battery holder have to be connected the right way round, and you have to fit the battery in the right way. The red wire is the positive (+1.5V) wire.




Replace The Tuning Knob And Test

After that, you can put the tuning control knob back on, and plug in a single AA alkaline battery. Do this without the earphone in your ear in case it's too loud. Start with the regeneration coil far away from the rod.  Connect up the crocodile clips to an aerial and earth connection of some kind. This does not need to be anything like as good as with the purist crystal set kit. Turning the tuning knob should bring in the main stations that you might hear on a normal AM radio. To try for maximum sensitivity, carefully edge the regen winding closer to L1. You can disconnect the battery after use, but the radio uses such a small amount of current that it should last for more than a year.

The Completed Single Transistor Radio Kit



Is it any good?  Yes.  If you've been unable to get CBCSRK-1 to work due to being in a valley, or a flat where you can't launch much aerial wire, then I hope that CBCSRK-1T can get you going. It will get both more stations where available, and greater loudness. In MAY-2025 in Southampton, I get good reception of BBC Five Live and Talk Sport, both from Brookman's Park in Hertfordshire. That was using a central heating radiator for the earth and a bit of wire leading up into the attic for the aerial. Radio Wales is still going strong on 882 kHz over in the West. If you're within 40 miles of Orford Ness, you should be able to hear Radio Caroline radiating on 648 kHz. As with the crystal set, moving the main coil further onto the rod will allow you to tune lower frequencies and vice-versa.  If you're in the USA where transmitter power output tends to be greater, and AM still has more popularity to cover the vast distances in some regions, you should have success too.

Depending on your aerial, you might find that the tuning is rather broad and that stations overlap. You can slide the aerial coil L1 pins 1 and 2 further away from the main coil to counter this. I found this when testing on a farm fence in Dartmoor where the upper wire and lower mesh of the fence were quite close together. Another way to fix this is to try fewer turns on the aerial coupling coil, anything down to 10 turns.

Some Radio Testing On Dartmoor



 
How it works, The Simple Version.

Simple looking circuits rarely have simple explanations. 

The aerial wire 2, to ground 1, induces radio signal into the ferrite rod system L1.

The high Q winding on L1 pins 5 and 6 resonates with VC1A to tune your station.

Transistor Q1 amplifies the radio signal from pin 6 of L1 and it appears at Q1 emitter.

The radio signal gets fed back a bit to the antenna coil L1, through regeneration coil connections 4 and 3.  This boosts the radio signal again via positive feedback.

The action of Q1 working only in the direction of the arrow acts like an active diode, so after the regen coil, you get the demodulated audio signal across C2 and L2 primary.

The audio signal gets fed back via C3 to be strengthened in current by transistor Q1 through L1 pins 5 and 6. L1 doesn't care about audio; It lets it straight through while still doing the business of radio tuning.  This provides reflex action or bootstrapping for the audio signal.

Three active processes are occurring at once. You might say that we're using one transistor three times.

More Modifications And Thoughts

I've shown this version of the kit bolted onto a piece of scrap hardboard about 3mm thick. I heartily recommend this, and though I don't provide the hardboard, I do provide the bolts. You might go for the old cassette case option shown in the original kit, use some stiff cardboard, or see what scrap bits are available for free outside the local hardware store.

The 0.2mA current consumption from the 1.5V AA battery is very low, so this is a great radio to run from a small solar battery or alternative power sources.

Solar Battery Power Operation

If you have a small solar battery that is specified to provide anything up to 3 Volts, you can use it to power the radio. In the pictures below, my miniature solar panel is rated at 3 Volts, 0.3 Watts, 100mA maximum. This assumes that you're in the Sahara desert at noon, but it provides more than enough power for this radio indoors in bright office conditions. The sky was quite overcast at the time that the picture was taken, and you can see the 2.894 V output from the miniature panel reading on the meter.  For an aerial and earth in these pictures, I'm connected to some chicken wire on the ground, and 3 metres of vertical wire attached to a washing line, which is more than adequate. You can sometimes find old solar powered garden lights where the plastic covering the photovoltaic cells has gone cloudy, remove the sun damaged plastic, and use that instead of buying something new.



Easy Peasy, Lemon Squeezey

...as Sooty used to say.  Or, "Lemon, Lemon, Difficult Lemon," as someone else said. Yes, you can use a lemon for a battery.  You will, however, need a piece of copper or a carbon rod, and a piece of zinc.  Copper foil can be found securing the output transistors to the chassis in old radios, or a small piece of copper pipe.  I'm not going to say where I got my piece of zinc or carbon rod from, because it's not a safe approach in modern times. (Let's just say, the 1960's Ladybird book, "Magnets, Bulbs, and Batteries," will not be found in your local library without a dire safety warning posted inside the front cover)  You can always get some online, or ask a friendly science teacher. The acid in the lemon provides an electrolyte. It's the difference in the chemical reactivity of the metals that is really providing the power, as the zinc plate slowly dissolves. Don't eat the lemon afterwards.

Carbon Rod and Zinc Plate Lemon Battery Operation at 0.6V



Copper Plate and Zinc Plate Lemon Battery Radio Operation At 0.8V




The Potato Programme 0.7V Operation

"Ah Helga, the foolish imperialist Western burgeoise use their fancy imported lemons, where we clever Soviets can use a mere potato!" 
'Yes, Boris. A potato is clearly a far superior option.'

Yes, you can use a potato. It's cheaper and you get less juice going everywhere. Don't eat the potato afterwards. I promise that this is the last of the, "fruit and veg" batteries.




Alessandro Volta's Voltaic Pile Operating at 0.75 Volts

For historic purposes, you might want to try the original battery invention from 1800. You could even try stacking more copper and zinc plates to get up to 1.5V or more. I used salt water, kitchen roll tissue for the separator, and just one copper and zinc plate.  Strictly speaking, that's a single cell. If you stacked two, that would be a battery of two cells. Alessandro Volta proved that you could generate electricity from metals, and disproved the then ongoing debate about whether it was generated from animal electricity with the whole, "twitching frog's legs by poking them with a fork on a metal plate," thing.



The alternative batteries are a bit of fun, and are great for science lessons about electro-chemistry. In practice, buy a AA battery for 20pee and use the battery holder provided. The "fruit and veg" batteries do, however, show how this active diode type of design works well down to ridiculously low voltages.

Aerials (Antennas) And Earth Connections

I can make this radio work in most locations by connecting the earth wire to a central heating radiator and just gripping the aerial wire with my hand. Can you make it work with no aerial and earth wire at all?  Sometimes yes, just about. At my house there are telephone wires radiating out like an umbrella from a central pole in the middle of the street. It forms a shield against MW radio signals. Solar panels on the house don't help the aerial wire in the attic work much, either. If you're in a better location and you want to try working with just the ferrite rod as the aerial, remove the aerial coupling coil L1 pins 1 and 2 from the ferrite rod completely. This worked for me in a location in Todmorden, Lancashire, which is a long way from the Brookman's Park transmitter. You could have greater success in the USA if you still have a high power AM broadcaster within 50 miles, or a small station nearby.

Longwave (LW) And Shortwave (SW) Operation

LW

Longwave radio broadcasting is on its last legs. For shame! To receive it on this radio you would need a main resonant coil that is 200 turns, an amount of wire that I don't provide in the kit, and surprisingly intuitively, a long aerial for long waves. BBC Radio 4 198 kHz still soldiers-on from Droitwich at 500 kW, while the remaining Economy 7 electricity meters in the UK are turned off from the phase modulated teleswitching signal which that transmitter provides, and are turned into smart meters.  If you live in Algeria, however, a French language service Chaine 3 from Tiapazia operates on a full height quarter wavelength monopole aerial on 252 kHz.  Such extraordinary transmitting effort can not go un-noted. I can receive it in Southampton, UK, during the daytime as a routine domestic experience with a cool cardboard loop resonant aerial antenna, though I have yet to make the single transistor modification work to hear it. 

SW

If you completely remove the main coil from the ferrite rod, it will resonate at SW frequencies. China are big on high power SW broadcasting at the moment, not least because the West have vacated some of their old frequencies. Problem. The ferrite rod is not only used to make the inductance change frequency, it coupled the signals together. So now, taking out the ferrite rod has made the reception frequency higher, but the aerial and earth coil is lost in space. Either wrap the aerial and earth wires connected together directly around the main coil, or connect them straight to the top of the resonant tank to hear various goings-on at 2MHz and above.

That's it for now. This is a new page, so please report any errors or confusion to me at the email address below.

How It Works For The Electronics Engineers.

The link above provides some more detail and oscilloscope pictures, mostly aimed at electronics engineers.

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Recent edit history

02-JUL-2025: nocache directive added