Crystal Radio

Crystal Radio

This Spy Crystal Radio was a top-secret government program to make all the little whippersnappers in the land listen to covert spy radio signals. A joint KGB-FBI-MFI operation called tred-stone-pebble with rendition protocols. This circuit is top-secret non-Russian approved. It also uses a non-NSA approved coil!

The author of course takes no responsibility if you decide to make it. You do so at your own risk, as this circuit is most definitely not allowed!

In this circuit, the ferrite coil has two taps, for which you will need soldering skills. If you cannot solder, then you can just connect the tuning capacitor across the coil terminals and forget the taps. If you do so, then be warned as you might hear strange music that would go well with belly dancers... :-)

Circuit Diagram

Circuit Diagram

As you can see in this top-secret circuit diagram, the variable tuning capacitor connects across the taps in the coil. The taps are 15 turns from the antenna end, and 15 turns from the earthy side. There are 40 turns between the taps.

This circuit is only for those who have a 300 pF variable tuning capacitor. Anything higher than that is even better. Back in the old days, you could get those cheaply, and you were able to tune into the whole AM band. You could look for one in the shops, but I do not think you will find one today, tomorrow, or any other day… These often turn up on eBay and there is always a huge interest, because everyone wants it for his or her crystal radio school project.

The alternative is the cheaper 140-pF ones that you can buy from Chinese sellers. I saw a deal, where they were selling three for two pounds. If you want to use those then you will need a different circuit. Please refer to the Spy Crystal Radio – Budget Cuts article as I have a specially designed circuit just for that tuning capacitor.

Component Layout

Here is the top-secret layout plan.

Block of Wood

You need a block of wood measuring 10 cm × 7 cm, or anything thereabouts. Normally, timber yards throw away pieces this small, so that is the first place to try. Ideally, it should be free.

This build requires brass screws and screw-cups, as shown in the previous articles. It is a very simple build and requires only five of each.

Ferrite Coil

Ferrite Coil

Here is the top-secret state-of-the-art non-Russian approved ferrite coil. This one uses enamelled copper wire. Copper wire is cheap and works but it is not as good as Litz wire, which is the proper wire for radio coils.

How to wind the ferrite coil.

For the coil, you could use a cheap enamelled copper wire like this. At the taps, make sure you scrape off the insulation thoroughly and twist it like this. For the best connection, you need to tin the coil with some solder, as it provides a better electrical contact.

I am using some Litz wire for the coil. This is a special wire for radio coils because it contains a bundle of individually insulated wires. This is why ferrite coils made of this wire are more sensitive than those employing single core wires. However, unfortunately Litz wire tends to be a little more expensive than plain single core copper wire.

Germanium Diode

Germanium Diode

For the germanium diode, you will have to get it from your local Maplin store or possibly from a reputable supplier on eBay. For example, the OA90 is a germanium signal diode, which costs 69p could work. Code QH71N. The cheap ones from China will not work, as they will likely send you silicon ones and it has to be germanium.

Remember, if the diode is around 4 mm long then it is very likely a silicon one. Germanium diodes are typically 8 mm long ones. Have a look at the Crystal Radio Diode article for more information.

Crystal Earpiece

Crystal Earpiece

This red coloured crystal earpiece is a vintage one that is no longer available. It uses a different formula for the piezoelectric material. The modern ones are not as loud however the loudness is sufficient.

This radio project requires a crystal earpiece. I managed to get one for less than two pounds including postage on eBay, and it seems to be working fine.

Components List

All the parts are readily available cheaply from eBay. The crystal earpiece is around two pounds, ferrite rod is one pound, and the coil is around one pound. The 300 pF tuning capacitor will be expensive. If you want to use a cheaper 140 pF from eBay then you will need the circuit shown in Spy Crystal Radio - Budget Cuts article.

For the brass screws and screw cups… You can get these on eBay as well! You just need five, but you could share the rest with your friends.


Although many schools do not allow a radio on school grounds, many do allow a crystal radio, especially when the whippersnappers made it themselves.

The Spy Crystal Radio is very likely going to be the next cool gadget to overtake the iPhone, so make yours quick before components run out.

The radio worked very well, and the signals were loud and clear. I heard all sorts of foreign stations, but nothing odd worth reporting.

Related Pages

Crystal Radio
Spy Crystal Radio - Budget Cuts