For all lovers of the electronics, the box where their device is to be put is the famous “production weak link”. The finished boxes are either impossible to purchase, or they can be bought but their dimensions or shape is inappropriate, or they are too expensive, or... In cases like this one should be quick-witted enough to find some square-shaped box that is being used at household, or some packaging box or similar. That is how it’s done in the “whole white world”. Two years ago, in the famous electrotechnical magazine ETI TOP PROJECTS the article named “TIC TAC RADIO” was printed, where a receiver with ZN414 that is placed in the transparent plastic box of TIC TAC mints (In the abstract, it was written that making this device serves well as an excuse for buying candies, which is probably meant for the readers that are on a diet for aesthetic reasons).
However, the “finishing touch” is of great importance for everything. The majority of your friends will be more impressed by a lovely box where the receiver is placed, than the reproduction quality, type of modulation and other technical characteristics. And a nice, appropriate box cannot be bought, it is up to you to make it. It can be something as on pic.3.11 or similar. The idea can be also found in some catalogue of radio receivers’ manufacturers, or you can think of something of your own. As far as the author of these lines is concerned, he likes best the wooden boxes from the 20’s and 30’s of the previous century, from the times of the charleston, E. H. Armstrong and Al Capone. They looked something like those on the pic.5.17 and can serve you as an inspiration for your personal design.
The mid button is for the variable capacitor for station tuning, the right one is for the potentiometer for volume regulation. The button on the left can be a rotary switch for turning on/off (S). It can also be a tone regulation button, and for the reaction-type receivers it can be a button of the potentiometer that regulates the magnitude of the reaction. In the last two cases, the on/off switch (S) is located on the regulation potentiometer. The outside antenna and ground hubs are located at the rear panel of the box. The wires connecting the hubs with the PCB should be isolated, flexible and long enough to be able to open the panel and put it at upright position.
If the receiver is power supplied from the outside net, a green LED should also be added, as the power indicator. The good place for it is just above the variable capacitor’s button, instead of the triangle-shaped marker.
Pic.5.18 shows the parts for the first box from pic.5.17. For the front and rear side two pieces of 5 cm thick plywood, measuring 22 cm x 15 cm are needed; for the side panels, two pieces of 10 cm thick plywood, 15 cm x 9 cm, and for the bottom side - one piece of 10 cm thick plywood, measuring 13 cm x 9 cm. The best way to cut these parts is to be done by the carpenter on the special machine, since only then will they be of strictly rectangular shape, and bottom and side panels will have exactly the same width, which is very important during assembling. On the front side, the circle and the arc are drawn with the aid of the sector, and the cutting is done with the carving saw. The part that is cut from the back panel will serve as a closure. When it is cut it isn’t necessary to treat it with emery, since it will fit nicely in the hole on the rear panel even if it isn’t cut evenly. On the inner side of the rear panel two plywood lattices measuring about 2 cm x 13 cm should be nailed. Four wood screws will be screwed in them later (the holes are shown as four dots), which will serve to tighten the closure. Connecting of the pieces is done with the wood glue and small nails. Before you start hammering, it is very useful to drill a few holes for the nails in the front and rear panel with 1 mm drill. The nails are partially hammered into the panels, the edges are then covered with glue, and the nailing can then be done. When all this is finished, the box should look as the drawing at the right end art of the pic.5.18 *vertical stripes over the loudspeaker opening are not shown. They can be omitted, and you can nail in a few thin lattices, when the box is finished, as shown on the last drawing on the pic.5.17). The semicircle part is made of 5 mm x 5 mm lattices, or similar, which are put side by side on the upper edges of the front and rear panels, that are covered with glue (the picture shows only one of these lattices). When the last one is fitted, the space between them is filled with “putty” that is made by mixing the fine wooden chips with the wood glue, with the aid of a steel plate. After that, the lattices are tightened to the panels’ edges by two pieces of strong scotch tape, which are shown in dashed lines, and everything is left to dry well. When drying is, after about 10 hours, finished, all the edges and lattice parts that protrude are well flattened with emery. All the remaining holes are filled with the fast-drying putty, and everything is abraded once again, and the putty is applied again, and abraded again, etc., until the upper part is semicircle-shaped, all the sides smooth and the edges correct.
* Perhaps some of the readers will seem that there’s a lot of exaggeration in previous lines, and even too much pedantry. There’s a Latin proverb, that says: AGE QUOD AGIS - Do the things you do, which, in our case, can be interpreted as: You should either make the box properly or not making it at all.
* This box is relatively small, it is predicted for the loudspeaker that is about 12 cm wide. If you have bigger loudspeaker, and it will certainly play both louder and better, you should make a bigger box. The dimensions calculation is done by dividing the diameter of the bigger opening, that will suit bigger loudspeaker, in centimetres, by 11, and all the measures on pic.5.18 are multiplied with the number attained. E.g. if the diameter for the new, bigger hole is 15 cm, new dimensions are obtained by multiplying the old ones by 1.36.
The readers that have carefully studied all the radio receiver projects that are described here, have possibly noted that the author referred to experimenting either with values of some components or with entire circuits, and all that was in order to practically find the optimal solution. When small changes are discussed, such as finding the optimum value for some resistor, that can be done on the previously made PCB. In case of bigger changes that of course is not convenient, and sometimes it is almost impossible. For all kind of electrical diagrams check-outs as well as various experimenting with all electronic devices, including radios, it is the best to use a special experimental board, which can be purchased under various trade names: protoboard, bimboard, matador, steckplatine, steckboard etc. All of them have in common that component connecting is done without soldering, by simply inserting the legs into the small holes on the plate.
As an example, pic.5.19 shows a full-scale experimental board that can be purchased in one of the Belgrade electronic shops. It has 630 vertically aligned holes, connected internally in 126 groups of 5 holes each, and another 100 holes placed in two topmost lines, connected in two horizontal groups by 50 holes each. The connections between the holes are inside the board and cannot be seen, they are shown on picture in dashed lines. The two topmost parts are used fo bring the supply voltage, and the battery or the adaptor is connected to them. One of them, most often the one that has minus pole connected to it (in all the devices described herein), also serves as the device Ground. The holes contain miniature metal hubs that are elastic, so when a leg is popped in, a reliable mechanical and electrical junction is accomplished. The distance between the adjacent holes is 2.54 mm (1/10 inch), which allows for connecting the vast majority of electronic components, which are being produced with inter-leg distance that is equal to a hole number multiplied with 2.54 mm (in the producers and sellers’ catalogues the 2.54 mm distance is marked as R, which stands for raster, and the components that have their legs horizontally and vertically distanced to 2.54 mm multiplied by some whole number are said to have their legs in raster).
The necessary electrical junctions between the hole groups are accomplished with connecting wires that can be bought at Conrad, but are more often self-made from plastic-isolated 0.5 mm or 0.6 mm copper wire. These pieces vary in their length and can be bent as the biggest piece in the lower left part of the pic.3.19, although it is better and nicer to use regular pieces, shaped as the cyrillic letter P.
Pic.5.19 also shows an example of practical usage of one such board. The radio-receiver from pic.3.15 is made on it. As can be seen, the coil ends are stuck into the holes whose coordinates are j,37; j,39; i,45; i,47, the diode in holes i,39 and i,45, the pin No.1 of the IC in e,54, etc. With the connecting wires the legs No.1 and 3 are connected, the ones that are connected with the potentiometer slider, and legs No.2 and 4 are connected to the ground by means of 4 connecting wires (the minus pole of the battery), etc.
It is now clear that experimenting is done in a very simple manner. E.g. if you are interested how does a capacitance of C2 affect the tone colour in the headphones, all you should do is remove it and insert a capacitor of greater or smaller capacitance, etc.
* It is useful for the connecting wires to be made with isolations of various colours, so that red ones could be used e.g. for connecting with the + battery pole, the black ones with Gnd, the yellow ones for the signal etc.
Practical realization of simple radio receivers, as well as other simple electronic devices, can be done in many ways, as it was discussed in PE No.2. One of those is shown also in this number. That is construction of the detector receiver from pic.3.11, where some of the components are mounted onto the box walls (the variable capacitor, coil and the hubs), while other (the diode and two block-capacitors) are placed between them. With some skills, and by the aid of few smaller nails nailed from the inside of the front panel, a more complex device could be made, say, that from pic.3.12. But this solution would start looking as “the dead cockroach technique”, which will be discussed in the “Funniest Electronics”. The real solution is the PCB that can be made from the drawings that are given, or those you will draw yourself, together with the instructions given in chapter 5.1.
There’s another option for practical realization. It is a universal PCB, that can be bought in the electronic components’ stores. There are more sorts of these PCB’s, and all of them have in common that the holes on them are drilled on the distance of 1/10 inch (R=2.54 mm).
One of the universal PCB’s is shown on pic.5.27. It consists from a huge number of round copper isles, with hole in the middle. The components are being soldered first (resistors, diodes, IC’s, capacitors...), and then the component pins are connected by pieces of isolated copper wire, on the soldering side.
As an example, pic.5.28 contains the photograph of the receiver from pic.3.21-a that is made with the universal PCB from pic.5.27. It can be placed into a box as on pic.3.21-c, except the box should be bigger, in order for the loudspeaker to.
The receiver on pic.5.22 is designed for the readers that wish to make a semi-conductor model of a complete direct radio receiver that was being produced many years ago, with electronic tubes. It had a total of 3 tubes, one of the contained the HF pentode (utilized in the HF amplifier) and the diode (used in detector), the other one had a triode (pre-amplifier) and powerful pentode (power amplifier), whilst the third one contained the duo-diode (the rectifier).
