Chapter V Appendix
5.1. Making PCB’s
5.2. Computer - Aided Radio Receiver Control
5.3. Receivers with NE612 IC
5.3.1. Synchrodyne AM Receiver
5.3.2. AM Receiver with Synchro - Detector
5.3.3. Input Circuits for the Receivers with NE612
5.4. Universal Audio Receiver
5.5. Additional Circuits
5.5.1. Fine Tuning
5.5.2. Electronic Tuning
5.5.3. Signal Suppressing of Local Radio Transmitter(s)
5.5.4. Dual Tuning
5.5.5. Separation of Stages - Preventing the Oscillation
5.6. The Boxes
5.7. Bimboard, Protoboard
5.8. Universal PCB Plates
5.9. A Modern Oldtimer
5.1. Making PCB’s
Design and manufacturing of the PCB’s has been explained in detail in the
second issue of P.E. Here, we’ll consider how to make a PCB whose drawing is
already done. As an example, we’ll take a drawing of the board of the
receiver from pic.3.19, which measures 45 mm x 30 mm.
a. The PCB is being made of pertinax or vitroplast, i.e. a thin plate (about
1.5 mm) made of isolation material, which has a lean layer of copper put on
one side. From the plate you buy in some electronic components’ shop, a 45
mm x 30 mm piece should be cut. In amateur conditions, this means
refracting. First, points A and B are marked on the non-copper side of the
plate, acc. to pic.5.1-b. A ruler is placed over them and a groove is made
by pressing with a screwdriver or a bodkin along it. Its depth should be
about 0.5 mm (on picture it is shown in dashed line). When this is done, the
plate is placed on the edge of the table, with copper facing downwards. With
one hand the plate is pressed firmly to the table, and with the other, the
piece that has to be refracted. And - it cracks just along the groove.
On the refracted piece, a new groove, measuring 30 mm from the edge, is
made, and the procedure is repeated. In this way we finally have our 45 mm x
30 mm plate.
b. All of the copper has to be clean and shiny, since only in this case the
etching and, later, soldering is performed quick and easy and well. If it
seems to you that the copper you have just bought is clean enough, you’re
probably wrong. The plate must have spent some time in the shop, and the
copper surface is certainly more-less corroded. The cleaning is most
efficiently done with some abrasive powder (VIM or similar) which is
otherwise used for cleaning of the cookers, bathrooms etc, but also the
sodium bicarbonate, laundry detergent and even plain salt can well serve the
purpose. Take a piece of cloth, wipe it with water, extract the water well
and muss it to be ball-shaped. Dip it then in the powder, and scrub the
copper until it “shines like the shiny sun”. After that rinse the plate, and
pay attention not to touch the copper with your fingers, since that will
make it dirty again.
c. Put the plate, facing the copper up, beneath the sheet that contains the
PCB layout, right under this drawing. In our example, that would be the one
on the pic.5.2-a. With the pike of a bodkin the holes are made through the
centers of all the contacts, and in the centers of two bigger holes that are
placed sidewise, taking care not to move the plate. The bodkin has to be
pressed firmly, in order to obtain good prods on the copper. When this is
finished, the plate should look as on pic.5.2-b. i.e. it has to contain as
much prods as there are contacts, plus two. If the drawing contains many
contacts, the plate can be easily dislocated, and the procedure is to be
done all over again. It is better practice then to make a copy of the
picture, cut it out, and attach it to the plate with two pieces of scotch
tape.
d. Drawing the contacts and lines on the plate is done with the
acid-resistant marker paintstick. It can be recognized by its characteristic
“alcohol smell”, and is being sold in bookstores as a marker for “writing on
glass”. You can test it: write in the store (it will be later afterwards)
something on the glass, piece of plastic and similar, wait for a couple of
seconds, then try to wipe it out with your fingertip. If the paint remains -
the marker is OK. Nevertheless, this test isn’t 100% certain, it is much
better to buy the marker in the electronic components store (you have to
accent to the salesman that you need a marker for drawing lines on PCB’s).
With the tip of the marker draw a circle around every prod (except those two
that are for bigger holes), measuring 2-3 mm in diameter. Move the marker
slowly, in order to leave a thick layer of paint on the plate. Take care to
leave a small copper isle around every hole. Then,

you should, carefully and slowly, draw all the lines, by looking at the
pic.5.2-a. They do not need to have the same shape as on the picture,
especially they don’t have to be that “chamfered”. Line thickness should be
about 1 mm, but that either is not obligatory, they can be somewhat thinner
or a lot thicker (where applicable). The important thing is not to connect
the nearby lines or contacts during the drawing, i.e. not to make junctions
that do not exist on the drawing. If that happens anyhow, remove the paint
surplus with a razor or a small, sharp screwdriver. Pic.5.2-c shows the
beginning of drawing, several contacts and 3 lines are drawn. The drawing is
finished when you have a pic.5.2-a on the copper foil.
e. Next step is etching, i.e. removing the copper that is not covered with
marker paint. For this purpose, a mixture of hydrochloric acid (HCl),
hydrogen peroxide (H2O2) and water (H2O). Pure hydrochloric acid is not
used, but its 35% solution, that is being sold as a household cleaning
agent. Hydrogen peroxide is being sold in drug stores and cosmetic stores.
It is being sold as 30% solution, or even more diluted, 8-12%.
Hydrochloric acid and hydrogen peroxide are very aggressive media,
especially for the eyes and skin, therefore care should be exercised when
working with them. It would be the best for you to work with them in the
bathroom, or some other place close to the running water supply. If some of
these liquids spills on your skin, metal tool or clothing, wash them down
with water immediately.

The etching mixture is being made directly before the etching, and is
CERTAINLY being disposed of, right after the process. The plate is put at
the bottom of a plastic, glass or porcelain dish, with copper facing
upwards, and the acid is poured, in quantity enough to fully cover the plate
(pic.5.2-d). Hydrogen peroxide is then added, being poured from the
container directly over the plate. The amount of peroxide depends on its
concentration, as well as on the concentration of the acid. So, put some
peroxide, raise a little left end of the dish, then the right one, to allow
a liquids to mix, and observe the plate. The mixture is transparent, and if
the copper starts changing the colour after a dozen seconds - the etching
has begun. During this process, the bubbles are formed in the mixture, in
the amount somewhat more than in a glass of mineral water. If too little
bubbles are present, add some more peroxide. Be careful, however, not to
exaggerate, since if you happen to have too much bubbles, the mixture is
going to heat up and the marker paint can be destroyed. From time to time,
you should raise one end of a plate with a pointed wooden or plastic stick,
in order to remove the old liquid from its surface, and allow for fresh
mixture to take its place.
Etching is finished when there is no more uncovered copper on the plate.
Raise one plate end with the stick, wait for the liquid to decant, take a
plate with a laundry clip and wash it thoroughly in a jet of running water.
You can then remove the paint by scrubbing, as previously described, with a
wet cloth dipped in some powder. The copper contacts and lines will emerge
on the plate.
f. If you were careful enough to leave a uncovered isle of copper in the
centre of every contact, after etching this will be a small cavity, in the
centre of the contact. Through these cavities, that will guide your drill, a
1 mm holes should be drilled (it is better if the holes are 0.8 mm in
diameter, but such drills are harder to find, and a lot easier to break).
Two holes for the fixing screws are usually about 3 mm in diameter. While
drilling, a piece of thicker plywood or some flat hard-wood plank (beech,
oak) should be put beneath the plate, and not a piece of polystyrene or
something similar. Do not press the drill too hard, since the tool will be
plucking tiny pieces of plastic on the other side of the plate.
5.2. Computer-Aided Radio Receiver Control
In Book 7/8 of Practical ELECTRONICS methods for simple control of
various electrical devices by computer were discussed. Practical realization
of various interface circuits and sensors was described, by which the
computer is being connected with the outside world, so that it can turn
on/off the heater, light, fan, TV set or some other electrical device at the
desired moment, based on data comprising temperature, light intensity,
humidity etc. In this chapter we shall present, in short terms, one of the
projects from the aforementioned book, that deals with simple computer-aided
turning on/off of the radio receiver at the desired moment.
The radio is connected with the computer via parallel port, the one where
the printer is also being plugged. It is a 25-pin female connector, called
Sub D-25, which is given on Pic.5.3.
With appropriate programme, logical ones (voltage +3.6 V) and zeroes (0 V)
can be sent to the outputs marked as DI-1, DI-2,...DI-8, that are located on
the legs marked with numbers 2 to 9. Electrical devices that are being
controlled are connected to these outputs over the interface circuit that is
given on Pic.5.4. Two connected devices are shown on the picture, their
maximum number is 8.
Pic.5.5-a shows the electronic diagram of an extremely simple interface
circuit, which can serve to connect to computer the radio receiver, that can
then be switched on or off at certain time, with adequate programme. The
low-power transistor BC547 can be used for the consumers that use the
current from the battery that is not greater than 100 mA. In case you have
bigger power consumers, some stronger transistor or two transistors in
Darlington junction can be used instead of BC547. The transistor bas is
connected to the pin No.9 of the mail Sub D-25 connector over the R1
resistor, while the emitter and minus pole of the battery are connected to
the pin No.25, i.e. to the computer ground. As long as there’s a logical
zero on the DI-8 output, the base voltage is zero and the transistor is
locked and no current runs through it, therefore also through the consumer.
When a logical one emerges on the DI-8, the transistor goes to the
saturation regime, the voltage between collector and emitter becomes very
small (practically zero) and the transistor behaves as if the collector and
emitter are short-circuited. In that way almost the entire battery voltage
is available on the receiver power supply input.
The PCB layout is shown on Pic.5.5-b: The component side is in the upper
part, and the soldering side in lower part of the picture.
Pic.5.5-c shows how a small transistor radio receiver, powered by a 9 V
battery, is connected to the parallel port, over the interface from
pic.5.5-a. The plate is connected with clamps via the cables A and B, and
with the battery over C and D.



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