hobbyists are always anxious to hear the announcement of a new device
that is forecast to revolutionize the tech world. In the late 1950s
something as relatively tame as a crystal photocell satisfied that
urge. Today it takes something like a negative refractive index
metamaterial to invoke the same sense of awe and wonder. Those were
simpler times, but then again even today's beginners in the world
of electronics circuit designing and building have to start somewhere,
and these types of circuits are as good as any place.
By Allan M. Ferres
The development of this tiny, light-sensitive
cell makes possible an interesting variety of compact control units.
The photoelectric cell is a most interesting subject for
the electronic experimenter. When connected to relays, the number
of uses to which it can be put is limited only by the ingenuity
of the experimenter. It can be used for such diverse applications
as announcing a patient in a doctor's waiting room, turning on house
lights and advertising signs at dusk, automatically opening and
closing doors, warning householders of intruders, preventing shoplifting,
etc. The list seems almost endless, but may now be even further
extended with the development of the crystal photocell.
Overall view of the control unit and a closeup view 01 the
Clairex CL-2 crystal photocell as wired into the Cinch-Jones
plug and as the component looks unmounted.
This tiny cell has several advantages over the high vacuum and gas-filled
tubes usually employed in photoelectric relay circuits. Its characteristics
are such that light-controlled relays may now be used in applications
which are impractical with the conventional photoelectric tubes.
The crystal photocell is a very small device about the size
of a lead pencil eraser, 1/4" in diameter and 1/2" long. The light
sensitive element is a pure cadmium sulphide crystal which responds
to light over the entire visible spectrum. The crystal is a semiconductor,
its resistance decreasing with an increase in light intensity. Its
electrical characteristics are such that it may be operated at a
considerable distance from the associated amplifier and relay. This
factor and its small size make it ideal where concealment of the
device is desirable, or where space is limited. The crystal is so
sensitive that operation is practical with normal room illumination
when used with a simple amplifier. This eliminates the necessity
for using special exciter lamps and optical equipment. In some applications,
the relay may be operated directly by the crystal itself. Its low
cost and mechanical ruggedness make the crystal photocell an ideal
device for light-controlled relay experiments.
describes a control unit using a simple, basic photocell amplifier
and relay circuit. Four other circuits are also discussed which
will be of interest to experimenters.
The basic circuit,
shown in Fig. 1A, is sensitive enough to operate at one-tenth of
a footcandle of light. A protective resistor, R1
crystal photocell, the Clailrex CL-2, and the variable load resistor,
, are connected in series across the 117-volt a.c.
, which shunts the load resistor, charges to
peak voltage on each cycle to provide a higher striking voltage
for the thyratron. The miniature thyratron, its current-limiting
, and the relay are also connected across
the line through the switch S1
. The relay is a plate-circuit
type having a coil resistance of 5000 to 8000 ohms and an operating
current of not more than 6 milliamperes and provided with s.p.d.t.
shunts the relay coil to prevent chattering.
The light, bell, or other device to be operated plugs into receptacle
. As a photocell relay is usually operated continuously,
no a.c. "on-off" switch is included, but, of course, one may be
added if desired, in series with the line cord.
anode of the 5823 thyratron obtains its voltage from the voltage
divider made up of R1
, the crystal cell, and R2
When the cell is dark, its resistance is high and the starter anode
voltage is too low to allow the thyratron to draw plate current.
When light strikes the cell, its resistance drops, increasing the
voltage across R2
, and the tube conducts. With the switch
on, the relay contacts are wired so that the line voltage is connected
to the output receptacle only during the interval of time when the
light on the cell is interrupted. When the switch is off, a momentary
interruption of the light will remove the plate voltage from the
thyratron and power will be furnished to the receptacle continuously,
even though the light to the cell is restored. This locking type
of operation is desirable when the device is used to sound an intruder
Fig. 1. Five practical circuits using the
Clairex CL-2 crystal photocell. (A) Basic circuit which will operate
at 1/10th footcandle. (B) Variation of basic circuit in which power
is furnished to the output receptacle when light falls on cell.
(C) Circuit for operating lighting sequences. (D) Circuit for high
speed operation at low illumination levels. and (E) A simple setup
to be used as "intrusion" alarm.
shown in the photographs, the necessary parts can be easily mounted
in a 2"x 3" X 5" case. No ventilation of the case is needed as the
power dissipated in the unit is less than one watt. The placement
of the parts is not at all critical, so any convenient arrangement
may be used. Socket terminals 2, 5, and 6 of the 5823 should not
be used as tie points as these pins are used for internal connections
in the tube. The crystal photocell is wired into a Cinch-Jones type
P-302-FHT plug. A matching receptacle is mounted on the case, so
that the cell can be either attached to the case or used at a distance
of 20 feet or so from it by means of an extension cord.
Ordinary lamp cord is adequate for this purpose, provided that its
insulation is good, as leakage between the conductors will reduce
the sensitivity and may cause erratic operation. The cell can be
shielded from stray illumination by a short length of cambric tubing.
Overall view of the photocell unit. It is built into a 2"
x 3" x 5" case. The placement of parts is non-critical as
there is no heating. CL-2 unit is at right.
The unit is put into operation by turning switch S1
"on," plugging the line cord into an a.c. outlet, and pointing the
photocell toward a source of light. R2
is adjusted so
that a steady blue glow appears in the thyratron and the relay pulls
in. Cutting off the light to the cell will cause the blue glow in
the tube to disappear and the relay will drop out. The adjustment
is not critical, except with very low levels of
illumination. Line voltage variations will have little effect on
the operation of the unit.
Fig. 1B is similar to the basic
unit except that power is furnished to the output receptacle when
light falls on the cell, instead of when the cell is dark. The switch
must be set to "on" for locking operation. The only additional part
required is R4
a 1-megohm, 1-watt resistor which is wired
to hold the tube in conduction when the relay pulls in. This circuit
might be used to open a gate or garage door when the car's headlights
illuminate the cell.
Fig. 1C is a good circuit to use to
turn on signs or lights at dusk, and to turn them off again at dawn.
The cell load resistor is divided into two parts, and an additional
set of relay contacts is used to change the value of the load when
the relay drops out. This modification of the circuit is desirable
to insure positive operation of the relay at the time of day when
the light is slowly fading down to the operating value. R5
must be adjusted first so that the relay pulls in, turning off the
artificial light at the desired amount of daylight, and then R6
is adjusted to turn on the light at dusk. The photocell must be
shielded from the artificial light, or the light will blink on and
off in a form of oscillation.
Fig.2 - Average characteristics of the Clairex Type CL-2
crystal photocell. See article.
Table 1. - Specifications on CL-2 photocell.
The circuit shown in Fig. 1D is useful when high speed operation
is required at very low illumination levels. The value of cathode
, depends upon the amount of light available.
For .1 footcandle, C4
should be 100 µfd. and for
1 foot-candle, 10 µfd. is adequate. 150-volt capacitors should
be used. If fast recovery from an overload of light is necessary.
must be shunted by capacitor C3
, its value
being determined experimentally, under actual operating conditions.
The relay pulls in when the cell is exposed to light and drops out
when the cell is dark. The wiring of the output receptacle is governed
by the type of operation required. V2
may be either a
6C4 or a12AT7 with the sections connected in parallel. Higher operating
speed is obtained with the 12AT7 The relay must be capable of fast
operation to take full advantage of this circuit. In some applications,
a counter may be used in place of the relay.
A simple circuit
which will sound a chime when someone enters a doorway is shown
in Fig. 1E. For the direct operation of the 1 milliampere relay,
a light intensity of 40 to 50 footcandles is required. This can
be conveniently obtained by placing a 25-watt lamp on the same side
of the doorway as the photocell and reflecting its light into the
cell with a magnifying mirror placed on the opposite door jamb.
A shaving mirror is suitable for this purpose. A small magnifying
lens should be mounted in front of the cell, focused to obtain maximum
relay current. When the light on the cell is interrupted by someone
entering the doorway, the relay will drop out and the chime, plugged
Many other circuits and
applications will occur to the experimenter as he works with light-sensitive
cells. The graph of Fig. 2 and Table 1 provide a useful guide to
the operating characteristics of the Clairex CL-2. When working
out other circuits, care should be taken not to exceed the 50-milliwatt
power rating of the cell as operation tends to become unstable above
The service technician might well be able to
add to his income by building and installing light-operated equipment
in stores, doctors' offices, machine shops, etc. Building the basic
unit described in this article, or one of the other circuits, is
a good way to get started toward designing commercially profitable
devices. In a more frivolous vein, they can be worked into some
amusing parlor tricks.
The Clairex type CL-2 photocell is
available from the Allied Radio Corporation, 100 North Western Avenue,
Chicago 80, Illinois; Sun Radio and Electronics Company, 650 Avenue
of the Americas, N. Y. 11, N. Y., and from the Clairex Corporation,
50 West 26th Street, New York 10, New York. The net price is $3.50.
The author wishes to thank Mr. Al Deuth of the Clairex Corporation
for his cooperation in providing data necessary to the preparation
of this article. Posted
June 21, 2013