Follow our instructions as we show how to make a saucepan into a radio - but first, see how
Mike Leahy: There, can you reach? [laughter]
Kate Humble: The boys have just one more chance to get the radio working and crack longitude. And high winds aren't going to stop them. They want a bigger, longer aerial.
Jonathan Hare: What the aerial supplies is perhaps hundreds of stations, all at once. We need something to filter out most of them and just let one through. And these two devices act as a filter and you can actually tune them, so you can tune them through the radio spectrum and then this tiny feeble signal then goes into this beautiful crystal. This is called a detector, and it strips off from the radio wave the audio, or the music, or the presenter or whatever the programme is. And that tiny little voltage then comes down here, into the headphone, and the headphone converts it into sound which you can hear in your ear. And that's how it works.
Do It Yourself
Radio waves fill the space around us every second of the day. They emanate from local radio stations, mobile phones, short-wave radio transmitters around the world, and some even from satellites 36000 km out in space.
Radio is fundamentally about electricity and magnetism. Every time an electric current is turned on or off it creates electricity and magnetism in the form of radio waves. That’s why you often hear a click on the radio when a light switch is turned on or off. The radio waves radiate from the switch rather like the ripples formed when a stone is dropped into a pond. Radio waves, like visible light, X-rays and microwaves, are all forms of electromagnetic radiation travelling at the speed of light – an incredible 300 000 000 metres per second!
A radio transmitter is simply a device that continuously produces radio waves. It uses an electric circuit that causes a current to oscillate. By varying the strength of this oscillation according to, say, the loudness of music in a radio programme, you can encode information about the music onto the radio waves. These encoded waves are then transmitted from an antenna and picked up by a receiver that decodes the signal so that you can hear the music again.
How a radio works
The simplest radio is a crystal set which makes use of the radio waves’ electrical energy and so doesn’t need a battery. It only works if you have a very large antenna and if the radio signals are strong. A crystal set radio is composed of five basic parts: an antenna, coil, tuning capacitor, crystal detector and earpiece (or headphone).
Any piece of wire (or indeed metal) can act as an antenna. As the radio waves pass across the antenna they create tiny voltages in the wire. In general, the longer the wire the larger the voltage produced. This voltage is actually a mixture of voltages from all the radio waves that are passing through the wire, perhaps from a hundred or more different stations!
The antenna is connected to a coil and a device called a tuning capacitor. These act as a filter to select which of the radio-wave voltages should be picked out. As the name suggests, the coil is made up of many turns of wire. The capacitor is essentially two metal plates separated by a small gap. It’s designed so that the extent to which the plates overlap can be varied. In this way, you can control the filtering – in other words, tune the radio. The voltages that make it through the coil and capacitor pass to the crystal detector, a device that converts them back into the original signal (as a tiny, varying voltage) which the earpiece converts into sound!
The crystal detector is used to strip off the required programme from the radio signal. In the castaway radio, the detector we used was a small crystal of the naturally occurring mineral galena (lead sulfide).
The saucepan radio was designed to pick up short-wave signals so that we could get the time from the BBC World Service. After hours of experimenting, we managed to pick up a few short-wave signals, but none of these was a BBC station. Eventually, we picked up a programme in English on Swiss Radio International, as well as programmes in Italian, German, French and Arabic.
What you need
- wood for base (30cm x 30cm)
- wooden broom handle (30cm)
- enamelled copper wire (22 swg)*
- half a saucepan
- metal plate (similar in size to the half saucepan)
- 8 nuts and bolts and washers, or wire connectors
- galena crystal**
- crystal earpiece*
- insulated copper wire (100m)*
- heavy-duty plastic sheet (20cm x 20cm)
* Obtainable from electronics stores
** Obtainable from any shop that sells minerals and crystals
Making your radio
Construct the radio on a flat, wooden base, measuring about 30cm x 30cm, to which is fixed a coil of about 80 to 100 turns of approx. 22 swg uninsulated wire, wound around and along a 30cm length of broom handle (2-3cm diameter). It’s important that the turns on the coil don’t touch. Attach the wires at each end of the coil to two connectors: A (antenna) and E (earth).
The Tuning Capacitor
Our capacitor was constructed from half a saucepan and a semi-circular metal plate (make sure all edges are filed smooth). The metal plate should be just a little bigger than the base of the saucepan, with a 2–3cm diameter semi-circular hole cut out from the middle of its straight edge. Screw this plate at its corners to the wooden base.
Place a piece of heavy-duty plastic sheet over the plate and tack or tape it down to stop it slipping around.
The saucepan should be fixed above the plastic-covered metal plate by a bolt. Use washers to ensure that the spacing is as small as possible but not so tight that it can’t move. Pass the bolt through the semi-circular hole in the bottom plate to avoid making electrical contact. Use the saucepan’s handle to tune the radio by varying the overlap between the metal plate and the saucepan. Run a flexible wire from connector E to the saucepan, and from connector A to one of the fixing screws on the bottom plate.
The Detector and Earpiece
The device that holds, and makes the electrical connection to, the galena crystal consists of a small home-made spring, a bolt and a set of washers and nuts. The crystal should be positioned so that one of its faces can make contact with a fine, coiled wire attached to connector A. Move the free end of this wire around on the crystal’s surface to get the best signal. This is very much trial and error, and requires a lot of patience.
galena crystal and its holder
The earpiece should be wired in to the crystal holder and to connector E.
Use about 50 to 100m of wire for the antenna, which should be raised as high as possible and insulated from any metal supports, using plastic tubing. A good earth connection between connector E and something like a metal pipe driven into the ground greatly improves the crystal set’s response.
How to set up and use the radio
- After checking that all the connections are as clean as possible, touch the fine coiled wire on the face of the crystal and listen in using the earpiece. You should hear a click every time this wire is connected or disconnected. If you hear nothing, try changing the contact position on the crystal. You’ll need to be patient. You’ll also need to listen very carefully!
- Next, slowly turn the saucepan handle and listen for a signal. If after a while you hear nothing, try using half the number of turns on the coil. To do this, undo the connection between the end of the coil and connector A, and remake a connection between the centre of the coil and connector A. If that doesn’t work, halve the number of coil turns again. When you do find a station, mark the position of the handle on the board.
- The capacitor is probably the most variable component. The gap between the plate and the saucepan needs to be very small, but not so small that they touch. The plate and saucepan need to be large enough to get a decent overlap (capacitance), but not so large that there’s little variability in capacitance, and so won’t tune very well.
- Reception varies throughout the day. Short-wave signals tend to fade in and out, sometimes so quickly that it sounds as if something has suddenly become disconnected. If you wait a while, any signal will usually return.
The BBC and the Open University are not responsible for the content of external websites
Darrington P., A Guide to Broadcasting Stations, 20th edn., Newnes
Poole I.D., An Introduction to Amateur Radio, B. Babani Press
Penfold R.A., Simple Short-wave Receiver Construction
Supported Learning in Physics Project, Physics Phone Home, Heinemann/The Open University
Barrow J. D., The Artful Universe, Oxford University Press
A quite remarkable book that will change the way you view the world. Extremely accessible.
Burton et al., Chemical Storylines, G. Heinemann Educational Publishers
Part of the Salters Advanced Chemistry course, which explores the frontiers of research and the applications of contemporary chemistry. For A level and other science courses aimed at 16 to 19-year olds.
Fraser A. and Gilchrist I., Starting Science (Book 1), Oxford University Press
Part of an integrated science course for the National Curriculum Key Stage 3 and Scottish Environmental Studies (science) for S1 and S2.
Northedge A. et al., The Sciences Good Study Guide, The Open University
Indispensable for students of science, technology, mathematics and engineering. Packed with practical exercises and activities, all aimed at making studying more enjoyable and rewarding. Lots of hints and tips for those returning to study.
Selinger B., Chemistry in the Marketplace, 5th edn., Harcourt Brace
An excellent and informative reference source for all kinds of real-life applications of chemistry. Explores the world of chemistry that surrounds us in our daily lives, explained in terms that everyone can understand. ‘Makes chemistry come alive.’
PS547 Chemistry for Science Teachers course materials, The Open University, 1992
A course designed for use by science teachers from a wide variety of backgrounds, with varying experience of teaching science. A familiarity with some basic science (perhaps physics or biology) is assumed, but little understanding of chemistry is required. The mathematical understanding needed for the course is not great.