Simple Crystal Set Radio Receiver
Written by Bryce Ringwood   

 

Introduction

Crystal set (xtal set) radios were developed at the turn of last century and were used to receive early broadcast transmissions until valve (or vacuum tube) radios ousted them from about 1920 onwards. Nevertheless, crystal set radios have always remained popular with home constructors, who have tried to achieve more and more performance with many fantastic designs.

A crystal set has three parts to it:

  • A tuned circuit,

  • A crystal detector

  • Headphones – using an amplifier is considered to be “cheating”.

The tuned circuit is connected to an “Aerial”, consisting of a long piece of wire, and an “Earth”lead connected to mother earth. The tuned circuit consists of a coil of wire with a capacitor connected across it. The coil of wire posesses a physical property called inductance, represented in formulae by the letter L. The inductance can store a magnetic field. The capacitor is no more than two metal plates separated by an insulating material, such as plastic. The capacitance of this arrangement is represented in formulae by the letter C.

Don't worry about this too much for now it will all become clear in time.

The important thing is that in such a tuned circuit, energy is continually swapped between the magnetic field of the inductance and the electric field of the capacitor. When the aerial receives small electric voltages from a radio station and passes them through the tuned circuit to Earth, at the same frequency as the swapping is going on, then you are tuned to the station. In fact, the tuned circuit effectively amplifies these small electric signals.

Alas, these signals are flowing in both directions, so if you attach sensitive headphones to the tuned circuit – you will hear nothing.

The crystal detector goes between the tuned circuit and the headphones. It simply makes the small currents flow in one direction, so that the radio station can be heard. In the early days, people made their own crystal detectors out of a piece of galena (a naturally occurring mineral) and a piece of fine wire (appropriately called a “cat's whisker”) with a sharp point resting gently on the crystal. Although this is quite easy to do yourself – it is much easier to purchase a device called a germanium diode, which will do the same job.


 

Circuit Diagram

The circuit diagram for this simple set is as follows:

Circuit Diagram of Basic Crystal Set

But Wait! - where's the famous capacitor ? Well, its there, but you can't see it. It turns out that between the aerial conection and the ground, there is a natural capacitance of around 250 pico Farads. (Capacitance is measured in Farads – if you have ever seen a 1 Farad capacitor, you will understand why we mostly deal in microFarads – one millionth of a Farad).
 

To tune our radio, we will simply vary the value of the inductance by sliding a contact over the turns of our coil.

Things you will need
 

  • A piece of PVC pipe 50mm in diameter, about 80 mm long

  • A metal coathanger to make the slider mechanism

  • A germanium diode (A silicon diode won't work) Try 1N34A from RSE or NTE109 from Mantech. 

  • The innards from an old telephone earpiece OR a crystal earpiece – the latter you will have to pay for.

  • A piece of Supawood about 150mm square by 15 mm thick

  • A packet containing at least 3 M5X20 Countersunk screws

  • A packet of M5 Hex Nuts – at least 6

  • A packet of M3x25 containing at least 6

  • A packet of M3 Nuts – at least 10

  • A packet of M5 lock washers

  • 11 Metres of enamelled wire, around 28 gauge, carefully unwound from an old transformer – be sure there are no kinks or loops!

  • About 100mm PVC covered wire (Buy a reel and share it).

  • Lots of patience

Tools you will need

  • Electricians screwdriver for M3 screws (It will also do for the M5s) be sure you get the point that works with the screws you bought.

  • Good quality small hacksaw and blades. Best to use a Stanley, not Chinese.

  • Good quality electricians pliers – pointy nose. Some good S African ones, Mitco(?) - not Chinese, unless you know of some strong ones.

  • Chinese electric drill (Ryobi, Black & Decker, Skil) OR good quality hand drill. Be sure the chuck will hold a 1mm drill bit securely. You could also use a drill press.

  • A Wire Stripper (MITCO) – DON'T USE YOUR TEETH!!!

  • 3.2 mm Drill bits – jobbers drills

  • 5.1 mm Drill bit – jobbers

  • Pack 1 mm drill bits.

  • Countersink bit (or large drill bit).

  • See if you can borrow a Record vise

  • Fine marking pen

  • 200 Grit Emery Paper

  • Quick drying clear varnish

  • Masking tape

You don't need

  • A soldering iron - some time you mightwant to learn to solder.

Making a crystal set coil former

1. Cutting the PVC Pipe

The coil is wound on the pipe

2. The coil is Wound on the Pipe

Partly Wired Simple Crystal Set

3. The Base is drilled ready

The Complete Crystal Set

4. The Finished Radio


 

Build the set

  • Cut the PVC pipe to about 80mm long. Wrap a piece of masking tape round the pipe as a guide.

  • Drill 2 1 mm Holes about 25 mm in from each end (in line looks neatest).

  • Drill 2 5.1mm Holes 10 mm from each end – these must be in line, because they will be used to mount the coil.

  • Wind the coil Thread about 200mm wire through one of the holes, then start to wind the coil neatly with the turns touching. You need about 70 turns, one or two more or less doesn't matter. Finish by threading the wire through the other 1 mm hole. Keep the wire taught by wrapping it round a pencil to stop the coil unravelling.

  • Spray the coil with quick drying clear laquer. Let it dry

  • Spray it again. Let it dry completely.

  • Drill two 5.1 mm holes in the Supawood, about 50 mm in from one edge, spaced the same as the 5.1 mm holes in the coil.

  • Mount the coil on the supawood using two M5 screws. Best to deeply countersink the Supawood underneath. Pass the screws through the wood, then tighten using a nut. Then mount the coil so that its spaced away from the supawood base.

  • Drill a 5.1 mm Hole in the centre of the supawood base about 30mm from the front of the coil.

  • Drill two 3.2 mm holes about 25 mm apart 15 mm from this

  • Drill a further 4 3.2 mm Holes about 10mm in from the front edge, in-line (See Illustration 3)

  • Cut 155 mm off the straight part of the coathanger

  • Using the electrician's pliers, bend one end in a tight loop to fit round an M5 machine screw. (At this point you will understand why good quality pliers are needed!)

  • Make a right angle bend 25 mm from the loop at right angles to it.

  • Make another bend 40mm beyond that. You will now have a Z shaped piece of wire.

  • Wire the set using the PVC covered wire. Strip about 20mm insulation and wrap the wire round the machine screws protruding from the base. Finish by placing a washer over the wrapped wires and tightening with an M3 nut. The large M5 nut will be used for the coathanger slider. The two M3 screws in the centre of the base are used for the germanium diode. You will need a short piece of wire from the M5 screw to the nearest M3 screw. The non-coloured end of the diode is fastened to this screw. The coloured end goes to the nearest M3 screw.

  • I used the two screws at the centre of the edge set of 4 as the earth. You will need to connect the nearest free end of the coil to the nearest of the two middle screws. Connect the two middle screws together with a short piece of PVC covered wire. Baring the enamel wire of the coil is difficult to do without breking the wire. Use the fine emery paper to do this. (If you set doesn't work – be sure to check this connection.

  • Now connect the wire from the screw holding the red end of the diode to the edge screw at the extreme right.

  • Finally, connect the aerial connection – the screw at the extreme left to the screw holding the non coloured end of the diode.

  • Now the tricky bit. You need to gently sand across the top of the coil to provide a track for the slider (coathanger straight bit) to contact. You can use masking tape to assist in getting an even track. The turns shouldn't short, yet the copper wire must shine through the insulation.

  • Now fit the coathanger slider between two lock washers. Tighten the central nut so the coathanger slider makes firm but smooth contact.

  • Wrap some masking tape round the free end of the bent coathanger wire. The set should look something like this;

  • Connect the earphone as in the illustration to the two right hand screw terminals.

  • Connect the aerial and the earth as shown.

  • By moving the slider, you should be able to tune in to local medium wave stations.

Aerial and Earth

  • Aerial needs to be about 20 metres of wire between two insulators mounted as high as possible. Downlead goes to the aerial terminal

  • Earth needs to be a water pipe, or copper stake buried in the ground and watered. Connect earth terminal to this.

If the set doesn't work

  • Check the wiring and connections

  • Use a crystal earpiece

  • Maybe there are no stations within range – wait for night time. You might receive short-wave stations at the extreme (minimum turns) end of the tuning range

  • Check you didn't break the diode

  •  
 
Joomla template by a4joomla