
Cost Control Circuits,"
a section of Seminar
Applications
booklet No. 671.9, published by
General Electric's
Semiconductor
Products
Department
(Electronics
Park, Syracuse,
New York 13200).
Referring to Fig. 3, we find that the de-
sign
features an npn photo- Darlington
de-
vice (Q2)
controlled by a conventional npn
bipolar
transistor, Ql.
Resistors 121 and R2
form a
voltage divider which establishes
a net positive
charge across CI when the
circuit is in its off (nonconducting)
state.
Both R3 and R4 serve as base
current -
limiting resistors.
The
load is
a conventional
incandescent lamp while a 12
-volt
d.c.
source
furnishes
circuit power.
In
operation, a momentary high- resistance
contact (such
as
that
caused
by
a person's
touch) across the
control terminals applies
base bias current
to
Ql
through resistor
R3, permitting QI
to conduct and apply
Cl's charge to Q2's base
through R4.
Thus,
Q2 is switched to
a conducting state, light-
ing lamp
Il.
Q2 is held "on" by
light cou-
pled from It to its photo-
sensitive surface.
Under
these conditions, most
of the supply
voltage
is dropped
across 11
so
that
very
little
voltage
is applied
to voltage-
divider
R1
-R2. As
a
result,
CI's
charge is
near
zero and
its
d.c. potential is less
than
that
between Q2's base
and El's negative
termi-
nal due to
internal
current
flow. If,
at this
time, another momentary contact
is
made
across
the
control terminals, Ql again
con-
ducts, and Cl, discharged,
acts as an
effec-
tive short across
Q2's
base
and emitter
termi-
nals. This switches Q2 back
to a
noncon-
ducting (or high- resistance)
state and
turns
off
II.
The circuit cycles between its
on
and
off
states
each time the
control terminals
are
touched. If
a
steady contact is
made,
the
circuit
recycles
automatically,
flashing
the
lamp at a
rate
determined primarily
by
the bias circuit's R -C time constant
in
con-
junction with II's thermal lag.
With neither layout nor lead
dress
criti-
cal,
the circuit may be assembled
bread-
board
fashion
for
experimental
study
or on
a
small
chassis or board.
Regardless
of
con-
struction
technique,
however,
care
must
be
taken
to provide adequate light
coupling
between 11 and Q2's photo- sensitive
elec-
trode (the
curved surface).
The
completed
unit may
be mounted in a
wooden,
plastic,
or metal
cabinet,
depending
on
application
needs.
Industry
Items. A new
series of
low
-cost
complementary
silicon power
transistors
has
been
introduced by Motorola
Semi-
conductor
Products, Inc.
(P.O. Box
955,
Phoenix, Arizona
85001). Identified
as
types
2N5190
through 2N5195,
the
six new
units
April, 1969
The
remarkable
difference
about
Pearce -Simpson's
Guardian
23 and
Guardian
23 -B:
they work.
What's the
most important
thing
about
a CB
base -station
and mobile -unit
combination?
When some guy
says something,
the guy at
the other
end should
be able to hear
it.
In
other words,
"10
-4"
should sound
like
"10
-4"
- not "snap,
crackle and
pop."
So the great
thing about the
Pearce
Guardian
23 and
Guardian 23 -B
(in fact all
Pearce
-Simpson's
radios)
is that the transmitters
transmit and
the receivers
receive.
Don't
laugh.
Not every
CB manufacturer
can
make
that statement.
If
you want to
get
technical,
here's the
explanation.
Pearce -Simpson's
exclusive
HetroSync circuitry
sharpens
transmitted signals.
High
level saturation
limiting
provides
automatic
speech clipping.
And the Superhet
receiver
hears
signals
that ordinary
sets distort or
fade.
Guardian
23 comes
with palm microphone,
mounting cradle,
AC
and
DC power cords.
Guardian
23 -B comes
complete
with
built
-in, all
transistor,
solid state pre -amplifier
that
lets you
stay a comfortable
distance from your mike
and
still broadcast
loud and clear.
And
Pearce -Simpson's
new
SuperMod
desk mike
is
available
as an option.
Guardian 23 and Guardian
23 -B-
Pearce- Simpson's
super
twins.
They
work.
For more details on all
our New
CB Radios
write
Pearce
-Simpson,
Inc., Dept. PE -469,
P.O. Box 800,
Biscayne Annex,
Miami, Fla. 33152
Pearce-
Simpson
Division of
GLAmINNG
Corp.
CIRCLE NO. 38
ON READER
SERVICE PAGE
97
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