Spy 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

As you can see in this top-secret circuit diagram, the variable tuning capacitor is connected 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 Crystal Radio -- Budget Cuts 140-pF article as I have a specially designed circuit just for that tuning capacitor.

Here is the top-secret layout plan.

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 also 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

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.

Remember to wind the coil on a paper former so that you can slide the coil along the ferrite rod. For the coil, you could use a cheap enamelled copper wire like this. At the taps, make sure you scrape off the insulation at the ends of the coil wires 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

For the germanium diode, you will have to get it from a reputable supplier online or eBay. The OA90 germanium signal diode works very well and is well tried and tested. The cheap ones from China usually do 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. Have a look at this Crystal Radio Diode article for more information.

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 it 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

A small block of wood - off-cut or free-scrap
No. 6 Brass Screw Cups & No. 6 Screws (1/2-inch)
Germanium Crystal Radio Diode
Copper coil wire or Litz Crystal Radio Coil
Crystal Earpiece
3 m antenna wire
3 m earth wire
300 pF or 500 pF Tuning Capacitor
Ferrite Rod between 80 mm to 100 mm long and 10 mm diameter

All the parts are readily available cheaply from eBay. The crystal earpiece is around two pounds, the ferrite rod is approximately 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 Crystal Radio -- Budget Cuts 140 pF 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.

Summary

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.