spectrum graphics - OpenLibra

Anthony Marlow from Cheshire has sent
in a useful little device for helping with
SAVEing programs
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If you're fed up with starting to
SAVE a program on cassette
and only then realising that
you've forgotten to disconnect
the earphone plug, or with exerting
a considerable force in pulling
out and pushing back in the
power supply plug to provide a
system reset, then this simple,
inexpensive hardware project
may be what you're looking for.
None of the components used is
expensive or hard to get hold of,
and even when put in a plastic
case will only cost about £3.50,
This is much less than some
commercially available designs
which just comprise an on/off
switch for £5.00. You don't
have to be an electronics boffin
to build it, although you will
need to have access to a finetipped
soldering iron.
Feedback
The need to remove the earphone
plug while saving a program
stems from the fact that
most domestic portable
cassette recorders feed the
signal that is being recorded to
84
C2
100U OUTPUT
• i f
01
8C108
d^TO TO SPECTRUM
f ARPMONE
SOCKET
the earphone socket. If the plug
is not removed, then this signal
is returned to the Spectrum and
interferes with the signal being
sent from the Spectrum to the
cassette recorder. So a vicious
circle is created which prevents
the correct signal being recorded.
Obviously, one could install
a switch in the earphone line but
the result is no more convenient
than having to remove and
reinsert the earphone plug.
However, a number of cassette
recorders also possess a 5 pin
D.I.N, input/output socket,
which is normally used for recording
between two cassette
recorders. A similar feedback
problem could also exist between
two cassette recorders
and for this reason the output
from the DIN socket is normally
switched off inside the cassette
recorder when put into RECORD
mode. Thus, if use could be
made of this output rather than
the earphone socket, our vicious
circle will be broken
automatically without us having
to unplug or switch anything,
unlike many commercial
devices. On most recorders,
however, the DIN socket output
is not sufficiently powerful to
drive the Spectrum input when
loading a program. To overcome
this problem the simple transistor
amplifier stage (Figure 1 )
was designed and built. This
amplifies the signal by nearly
five times, so that the output
signal is capable of driving the in
put circuitry of the Spectrum.
Note, however, that on some
machines the DIN socket output
is independent of the volume
and tone controls.
How it Works
To fully understand how the cir
cuit works would require a
detailed knowledge of semiconductor
theory. However, a brief
description of what a transistor
does will give some insight into
how the circuit works. Basically,
a transistor (TR1 in our case)
consists of two pieces of an impure
semiconductor material
(usually silicon) with a thin slice
of the same semiconductor (but
with a different impurity) in between
them. The two outer
pieces are called the "emitter"
and "collector" and the thin
slice in the middle is called the
"base". These are labelled as e,
c and b respectively on the
diagram (Figure 2a).
One of the properties of a
transistor is that a small variation
in voltage applied between
the base and emitter (the signal)
can cause a large variation in the
current, I, which flows from the
collector to the emitter. Thus, if
a resistor is placed in the collector
lead (R3) then by Ohm's Law
(V IR) the voltage at the collector
will be proportional to this
changing current, the precise
value being determined by the
value of R3. By choosing ap
propriate component values in
the circuit, any desired
amplification may be achieved.
Before the transistor can do
anything, however, certain
static d.c. conditions have to be
set up. R1 and R2 form a poten
tial divider which sets the static
base voltage. Because the
values of R1 and R2 are high, the
best voltage can still be altered
by a small amount when a signal
is applied to it via capacitor C1 .
The static base voltage in turn
sets the static emitter voltage.
Given this, resistors R3 and R4
set the necessary standing cur
rent which flows between the
emitter and 0V. The gain, or
amplification, of the amplifier is
set by the ratio R3 /R4 .
Capacitors C1 and C2 allow the
signals to pass through but pre
vent the standing voltages on
the transistor getting through
and upsetting the d.c. conditions
in the recorder or Spec
trum. The circuit takes its power
from the Spectrum power supp
ly. Since this involves cutting
the wire from the power supply,
a switch was added at the same
time so that power could be
switched off and on to effect a
system reset.
Construction
The prototype was built on a
small piece of 0.1" matrix
Veroboard, available from most
electronic component suppliers.
The actual size required is 1 6
holes by 18 holes with the copper
strips running along the
shorter dimension. This can be
cut from a larger piece. The
component layout and connec
tions are shown in (Figure 2a).
Begin by fitting the resistors on
the plain side of the board. Pass
the leads through the appropriate
holes and solder them
to the copper strips on the other
side. Use only fine multicored
solder and make sure (he solder
does not bridge the tracks Trim
the leads close to the soldered
joint. Next, fit the capacitor and
transistor in the same way, tak
ing great care that they are inserted
the right way round (the
capacitors will be marked as be
ing positive or negative at one
end and the tag on the transistor
case is adjacent to the emitter).
Now take a length of stereo
screened cable (about half a
metre) and carefully cut one
wire about eight inches from one
end, leaving the other wire in
tact. Strip the outer insulation
off both cut ends and separate
the screen wires from the central
core, then remove about a
ZX COMPUTING DECEMBER JANUARY 1985