The objective: Do you have a transistor radio at home? No? A Transistor radio is great fun; you can listen to music or news almost anywhere. Do you know what a transistor radio is?
A transistor radio (or transistor) is a small, often handheld, radio receiver. You can buy a transistor radio these days for as little as Rs. 50. The transistor radios sold in the market require an external power source -- electricity from the mains or from a dry penlight cell.
You can make a transistor radio that does not require any form of electrical source to operate. All you need is a few electrical/electronic components. These should be easy to get provided you have some electronic repair shop or a shop that deals in electronic components.
The components that you will need and how to get them are described below.
You will need several meters of something called enameled wire. Enameled wire is copper wire, coated with dark colored enamel paint. It's thickness is measured in ‘gauge’. What you want is something that is not as thin as human hair, but not too thick either. It is going to be about 22 or 24 gauge wires. If you buy it, it will probably come on a roll and be a couple of hundred feet and I have no idea what it might cost these days. Barring buying it, you can obtain magnet wire from the inside of a transformer. Just about any kind will do. The things that you use to power telephones, tape recorders, scanners and such. These are usually called "AC Adaptors". Problem is, you don’t want to tear one up, if it works. A few people have broken things like that at garage sales and so on. Look around.
You will have to CAREFULLY disassemble the plastic case to get the actual transformer out of the inside. You may perhaps use a hammer to crack the case and pull it apart and remove the heavy metal thing inside. That is the transformer. If you can see a reddish colored wire going around and around the inside of the heavy metal part, then you have hit pay dirt! Unwind all the wire and put it around a wooden dowel or something similar, you will need most of it. Once you collect the wire, you will need a coil form upon which to wind the wire.
The tube that is easily obtainable from right around most homes is a toilet paper cardboard tube, or a shuttlecock box. Alternatively, one can use a 500 ml. PET bottle used for dispensing treated drinking water/soft drinks or the reel that you use to wind the string for flying a kite. When you begin to wind the wire, you will need to lay it from one end and tape it or some other way secure it to the tube. Carefully laying the wire about ½ " from one side, begin winding the wire neatly, and not overlapping anything. When you are finished, you should have a nice, neat coil that goes around and around the tube, leaving about ½ inch of space on either end of the tube. The coil, if you measure will be 3.5 inches wide along the tube. You will need to make sure the wire is tight around the tube so that it does not move at all.
Attach a copper strip to a plywood/cardboard base plate using screws, nut and washer. If you cannot find a copper strip, don't bother. If you understand its purpose you can always innovate a different setup to fulfill its purpose.
Now mount your paper roll to the cardboard base plate using two pushpins (drawing-pins used to fix paper on a soft board) through either end. Locate the paper roll so that the copper strip will gently rub across the top of the coil.
Once it is, tightly secure it at both ends and make sure you leave a few inches of wire at either end. The accompanying figure shows you what it should look like.
Next we need an earphone. What we really require for this to work is something called a "High Impedance" earphone. It is perhaps the most difficult part to acquire easily. In the past the most easy source used to be kabariwala who deals in old electronic products (like those located near Jumma Masjid in Delhi). The headphone of an old black telephone serves the purpose best. So if you can find a old discarded telephone instrument somewhat like the one depicted in the picture, you can be fairly sure that you can make this radio. You will the need to screw off the earpiece to get out the earphone device inside. Otherwise you may have to experiment with the ceramic earphones like the one depicted in the picture on the top available these days for most music systems/radios/tape recorders.
The other item that you may find hard a bit difficult to acquire is a germanium diode. The most common is perhaps 1N34 diode. Its looks are depicted in picture beside. It should not cost more then Rs. 5 in any market in India. If you cannot find a shop that sells this diode you may request a relative or friend residing in Delhi or Mumbai. It is available in many shops at Lajpatrai Market or Lamington road. But, just in case you cannot find IN34, try with any other germanium diode, chances are that it will work.
Add 3 more push-pins to the cardboard baseplate as shown in the figure above. Connect the germanium diode beween the screw holding down the copper strip and one of the nearby pushpins. You'll make a better connection if you loop the ends of the diode around the screw and the pushpin. The side of the diode with the black band should point away from the copper strip.
Loop the loose wires from the ends of the tuning coil a couple of times around each of the remaining pushpins. Using some sand paper, gently remove the enamel insulation from the last inch or so of the wires coming from the tuning coil. Also gently sand (by rubbing in the same direction) the enamel wire along the path on the surface of the coil where the copper strip rubs.
All that is left now is to hook your radio up to an earphone, antenna, and grounding system (such as a cold water pipe) and you should hear music! By sliding the copper strip along the tuning coil, you should be able to hear several weak, but distinct, radio stations. For good electrical contact, you may need to press the copper strip firmly against the coil with your finger while listening to the radio. If you do not succeed in the first attempt and want some more information, follow any of the following links. You will surely emerge wiser