Lingo of the Schematic
is a very basic introduction to schematic reading from the August
1955 edition of Popular Electronics. As someone who has been exposed
to schematics and mechanical drawings for 40 years or more, reading
them is second nature. However, to the newcomer to electronics,
it can be a bit cryptic. It is the equivalent of handing someone
who has never read music a sheet from Beethoven's 5th and asking
him to make sense of it. Of course there is a lot more to schematics
than presented here, but you have to begin somewhere.
August 1955 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history
of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights
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Lingo of the Schematic
By George Berry
Strictly for the beginner; the how and why of radio or
electronic schematics plus pictorial vs. wiring
If you enjoy building and experimenting with electronics, but
still feel a bit shaky about wiring up circuits directly from the
schematic diagram, take heart. Pictorial diagrams are all right
to start with, but they are not as easy to follow as they look.
But schematic diagrams are actually much easier to understand than
they would seem to be. Most of them are a lot simpler than, say,
a road map, and easier to figure out than a diagram of an end-run
play in football.
Let's assume you already recognize the standard symbols for the
common parts tube, capacitor, resistor, and coil ... battery,
headphones, antenna, and ground. Once you have some idea of what
these different gadgets do, it's almost impossible to mistake what
the symbols mean. They were designed that way.
Fig. 1 (above). A wiring schematic contains the identical
information conveyed in the pictorial schematic in Fig. 2 (below).
Beginners tend to favor the use of the pictorial; but once familiar
with the essentials behind the true wiring schematic, it becomes
obvious that the latter will always be best.
of the schematic diagram is simply to show what is connected to
what, in the simplest, most direct manner possible. You can take
a schematic and compile from it a list of what connections to make.
Then you can follow that list with a soldering iron and pliers and
the circuit will be all wired up. Experienced people, including
engineers, usually do this mentally while they are working.
Let's take an example, say, the simplest possible one-tube radio
receiver. This set will work, incidentally, but don't build it except
for practice. A very few more parts would make a vastly better set,
but that would spoil the neat simplicity of our example. The schematic
is shown in Fig. 1. Now let's proceed through the mental processes
that go with wiring it up.
on a board those parts which ought to be screwed down. That means
the tube socket and the tuning capacitor. Let's use a big board
and set the batteries on it too, so that it can be carried around.
There are few sounds as discouraging as the "clunk" of a heavy B
battery plunging to the floor, accompanied by the ripping noise
of taut wires pulling radio parts out by the roots.
The coil can be screwed down, too, if it has a bracket. Some screw
terminals or Fahnestock clips for the antenna, ground, and headphones
need to be fastened on as well. Now we are ready to wire. Starting
at the left, we see that the antenna terminal, one end of the coil,
and one side of the tuning capacitor C2 should all be connected
together. Do so. Next, and rather naturally, the ground terminal,
the other end of the coil, and the other side of the tuning capacitor
C2 all go together. The frame side of C2 is always the ground side.
Now note that the little mica capacitor C1 and the resistor R1 are
connected directly across each other. Let's solder them together,
and then see where the combination goes ... one end goes to pin
No.5 on the tube socket while the other end goes to the tuning capacitor
- that side which is connected to the antenna.
do the other socket terminals go? Well, No.3 goes directly to one
side of the phones. While we are at it, let's take care of the other
side of the phones. A long wire goes to the positive side of the
B battery; make it red for "+". Now there are only two connections
left on the socket. Pin No.2 goes to the negative side of the A
battery, so let's solder a wire there. It goes to a couple of other
places, too. In fact, it looks as if pin No.2, the "A minus," the
"B minus," and ground are all connected together. Does it matter
just what wires go where, so long as these four places are all connected
together? Well, it matters at v.h.f., but not down here in the AM
broadcast band. Let's do it in some reasonably direct way. A wire
from socket pin #2 should go to the frame (ground) side of the tuning
capacitor. It can be easily routed around via the terminal on L1.
Color should be black for ground, negative, and such things, if
we are particular. As the B battery will be sitting alongside the
A battery, let's make the wire about six inches longer than necessary.
Skin the insulation six inches back from the end and connect that
to "A minus"; then skin the end and connect that to "B minus." It
is simpler that way.
Now we ought to be finished ... except
for pin No. 7. That goes to the plus side of the A battery, and
nowhere else. Better put a clip on the battery end of the wire so
we can turn off the unit.
Clip a wire from a water pipe
or bedspring or something onto the "ground" terminal, drape ten
feet of old magnet wire from a defunct auto speaker field over the
window frame for an antenna, connect the phones, and presto... Grandpa
Jones, Peggy Lee, news bulletins, and used-car commercials.
Why "A" and "B" Batteries
"A" and "B" for batteries, for the benefit of those youngsters who
were not playing with radio in the 1920's, goes back to the time
when all home radio sets ran on batteries. Since "filament" and
"plate supply" and such expressions did not ring bells in the minds
of the general public, the battery people came up with some nice
simple names having one letter each. When the family radio started
to get laryngitis, you opened up the top of the cabinet (they all
had piano hinges) and looked at the filaments. If they looked dim,
you hauled the 6-volt storage battery off the shelf under the table
and took it out for a recharge. If this didn't fix it, you had to
run down to the corner radio store and exchange eight bucks for
a pair of 45-volt heavy-duty B batteries.
But suppose we
end the history lesson and get back to diagrams. Figure 2 is a pictorial
diagram of the same circuit as Fig. 1. Now, honestly, do you still
think it's simpler than the schematic?
Actually, one big
trouble with pictorials is that the man drawing the diagram has
less choice as to where he locates the parts. This makes for more
of a tangle in the wiring. Not too bad for simple circuits like
this one, it gets rapidly worse with two-and three-tube circuits,
and the multiplicity of crossovers makes the wires hard to follow
without a flock of colored.
Wiring Schematic Preferred
A schematic, if it is drawn right, will have very few crossovers
of leads. Then, there are standard conventions that help, which
are followed by almost everyone: (1) stages are laid out in approximately
a straight line, with the signal proceeding from left to right;
(2) auxiliary circuits go below the tube they affect, e.g., in a
superhet the oscillator is drawn below the mixer; (3) power supply
circuits are drawn below, near the bottom of the sheet; (4) ground,
filament, a.v.c., and such low-voltage wiring is drawn along below
the tubes, while high-potential or "B plus" wiring may be drawn
along above the tubes. It is hardly possible to follow these conventions
in a pictorial. Each pictorial is, hence, a one-shot affair so far
as learning goes. But every schematic understood, preferably redrawn,
and perhaps built, is a step toward a better understanding of electronics.