Hi there - I thought it was time to contribute another "Instructable", so here it is.

I know we've done the crystal radio thing here once before, (see the Nic Nac Tic Tac crystal radio) but this one takes the cake. This project will require the student to develop some hand skills in order to wind the largish coil and will help to develop an understanding of basic radio and electronics principles. If you built the Nic Nac Tic Tac crystal radio from last year, then you can recycle most of the parts from that project and upgrade to the Oatbox radio at minimal cost.

History

Back in the 1920's many people couldn't afford to buy a valve/tube radio as they were very expensive, so they scrounged enough parts and obtained the round 'box' made by Quaker Oats in order to build themselves a simple crystal radio receiver, and thus the name "Oatbox" radio.

The radio consists of 48 turns of insulated wire wound around the outside of the 4" diameter tubular box. Coil taps are inserted every 6 turns. Small electronic parts are then mounted in the lid of the 'Oatbox' and then the coil connections are wired into the right spots underneath the lid. Attaching an antenna wire, a ground wire and a set of headphones or a more modern crystal earphone, completes the radio set in its most basic form. The addition of an audio amplifier later on will enhance the performance of the Oatbox considerably

Selecting different coil taps for the diode and antenna leads (inserted during the coil winding exercise,) is the most basic form of tuning, and is effected by moving the antenna and detector clip leads, up and down the taps until satisfactory reception of one or more local AM radio stations occurs. Extra parts such as a metal "air variable" tuning capacitor or the smaller plastic version Polyvaricon can be added in later on, once the basic crystal radio is properly functioning. Using small value ceramic capacitors will also enable you to 'tune' the Oatbox, but more on that later.

This project is ideal for a class of young primary/grade school students, as well as home-schoolers too. Materials are cheap and readily available, and if you can't buy them at a local electronics store, you can buy all of them online, and even get mum or dad to scrounge them from old electronic circuit boards, if they have the skills.

Young children will need the help of an adult such as a parent or teacher, and it is recommended that close supervision be provided for any grade/primary school children, especially when it comes to using basic tools on the Oatbox lid and container itself. Safety glasses are a must for those who use sharp tools of any kind.

Step 1: Gathering the Parts...

The parts you will need are:

1. A Germanium diode, typically a 1N34A, as they seem to be still in good supply. Try and get a package deal and buy yourself at least five or so. Diodes look like a small glass bulb with a wire coming out either end. MTM Scientific in the USA sells a pack of 10 diodes for crystal radio set experimenters. Other alternatives are the OA90 and 91, the 1N60/P, a BAT46 or 48. Diodes such as the AA112 or the Russian D9 or D18 are also worthwhile hunting up online.

Beware of fake diodes, unmarked with any type numbers, which come from suspect suppliers. There are also some suppliers who will show photos of good diodes in online websites, but the supplier substitutes other poor quality ones and sends them to you, and not the good ones.

2. A 47k resistor. This will look a bit like the diode, but will be made of different material and will have several coloured bands to indicate its resistance value. For a 47k resistor that will be: yellow/purple/orange.

3. You'll also need one or more fixed ceramic capacitors. These components, in conjunction with the coil will help you to tune the Oatbox. You'll need to obtain one 68pF and 100pF and 120 pF ceramic caps. The "pF" stands for 'pico Farads' and is a tiny unit of measurement for small capacitors. These capacitors usually come in the form of small diameter orange coloured discs, with two leads poking out the bottom.

4. A crystal earphone - usually consists of a beige coloured shell attached to a clear plastic insert that goes into your ear. Some will come with a plug on the end and others will come with bare leads. If you can't find one of these, we can show you how to convert another electronic component to become a crystal earphone instead - and they work really well with this and other basic AM radio receivers too.

5. Wire and heaps of it! This will be the coil winding wire and you will need to decide just which wire you will use. I'm recommending cat 5 data wire. This comes in either a blue or grey outer sheath. It is used for sending computer data from one PC to another. Each protective sheath contains four pairs of wire, twisted together and if you can, get the multi stranded wire in preference to the single conductor wire. It's quite often tossed out into a skip during office upgrades so ask nicely and see if you can scrounge 10 metres or so as an offcut. Alternatively it can be found in small bundles at recycle centres or brand new at electronics stores, although it can be expensive to buy new. Untangling it all can be fun too... but more on that later.

6. Of course, you'll need an Oatbox container and the larger size (4" to 5" in diameter) is recommended. You'll need one with a plastic or cardboard lid as you will need to insert components and wires into the lid and join them together in order to make the Oatbox radio work. You cannot use a METAL container of any kind, because metal dampens any signals travelling through the wire windings of the coil.

Some food containers also come with a 'silvered' inside layer and this can also hamper signals inside the turns of the coil. If you can't get a simple cardboard Oatbox style container, then use a 90 mm diameter Postal Tube. They come with two plastic end caps and these are ideal for mounting the parts and wires of the radio in too. You'll have to cut the tube down to length too, but this may be done best with a band saw. Parents and teachers, if you're facing doing this for a class, approach a local wood shed and see if they'll cut the tubes properly for you, so that you get an even cut, and virtually no burred edges, so that the end caps are more or less a perfect fit.

You can see from the pictures above all of the basic bits and pieces you'll need to build the set.

Buying Information

Also, be aware that some suppliers package items in packs of two or more. For example, Jaycar Electronics packages resistors in packs of 8 and ceramic capacitors come in packs of two, so just be careful if you are ordering your parts online. If buying for a school or science club, there are discounts for more than 10 or 25 components, depending on what they are. It can be a good idea to get together with friends and buy in bulk and thus save on bulk discounts and even postage/shipping costs.

Tools and Other Odd Items

Simple tools such as wire cutters, pliers, and a sharp tool to make holes in the Oatbox lid and tube of about 3 mm in diameter should be about right. Hook up wire and some crocodile clips will also be needed so that you can connect to the diode and antenna taps you'll need later on to tune the Oatbox.

Tried and True Design

You can see from the second picture above, that this kind of design has been around for a long time. Slider mechanisms are good fun to use, but I've opted to use a number of coil taps instead. It is a bit difficult for the beginner, learning to wind a coil with taps, but once you've mastered the art of coil winding, you can make as many Oatbox radios as you want!

Step 2: Building the Parts Into the Lid...

Let's start off construction by identifying the components and making the right holes in the lid so that you can insert the wire ends and the three connecting wires between the lid, the coil and the Ground connection. You can see the lid and all components and wires that are inserted into it, or are wired to it. Next, there's a close up of the electronic components themselves. The wire link, then the 47k resistor, followed by the three recommended ceramic capacitors (68, 100 and 120 pF values) and finally the germanium diode. Then we can see an example close up of how all the components and wires fit into the lid. Following that is another picture of the wiring underneath the lid. Note that the white and orange wires are the two ends of the tuning coil and are connected to the lid at the right places.

Take the lid and mark a hole in the centre. This will enable you to keep a sense of balance when you make the holes for the rest of the wiring. You will need to make three holes on one side for the resistor, wire link and the BLACK Ground wire connections. You will also need to make five holes on the other side for the other end of the resistor, both ends of the diode and the RED wire, as well as an extra hole for the YELLOW antenna clip lead. Note that the earphone connecting wires go across the resistor end leads, underneath the lid. The hole for this wire should be made just below the body of the resistor, just near the edge of the lid.

Once you've poked all of the holes in the lid, begin mounting the components in turn and insert the short Link wire last. Then begin to gently twist the wire ends together, as shown, underneath the lid. When you've finished this part of the project, lay the completed lid to one side and then prepare to wind the Oatbox radio's tuning coil.

Step 3: Winding the Oatbox Radio Coal

Winding a large coil around the Oatbox tube may prove difficult for children with small hands, so some adult help (parents or teachers) will be necessary to complete this project.

Take the Oatbox tube and make two small holes 1" (25mm) from the top. Make them about 1/2" (12mm) apart. Take one end of the coil winding wire and thread it through one hole, inside the tube and back out through the other hole.

Leave about 5" (120mm) of wire for connecting up to the parts on the lid later on. Then begin to wind 5 turns around the tube, 'close wound' - side by side. At the end of the 6th turn, hold one part of the wire close to the coil with one hand, and mark where the middle of the two holes are at the top. Twist the wire so that it looks like a small loop about an inch (25mm) long. This will be your very first coil tap. Take care not to 'drop the turns' of the coil while making these taps along that side of the coil. Use small pieces of cello tape to hold the previous 5 turns in place while you make each tap on the 6th turn, until you reach tap number 7. This is the last coil tap (6 x 7 = 42) so when you get to turn number 48, make 2 more holes in the Oatbox tube as you did before, and secure the wire end in there, ready for connecting up. Leave about 5" (120mm) for this purpose.

Then you can proceed to bare the copper wire under the plastic insulation one by one until they are all exposed. If you take your wire cutters and mark out the centre of each looped tap, cut the wire at that spot and then strip the wire on both sides of the cut until the plastic insulation has been removed. Then twist the two wire ends together ad thus form each tap. Continue this process until all the copper wire taps are exposed. If you're handy with a soldering iron, warm it up and solder the tap so the twist in each tap is secure.