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.
Crystal Photocell Circuits
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
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.
This article 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.
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. line.
, 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 resistor, R3
and the relay are also connected across the line through the switch
. 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. contacts. C2
shunts the relay
coil to prevent chattering. The light, bell, or other device to be operated
plugs into receptacle SO1
. 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.
The starter anode of the 5823 thyratron obtains its voltage
from the voltage divider made up of R1
, the crystal cell,
. 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 alarm bell.
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.
As 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.
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
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 of R2
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
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 capacitor,
, depends upon the amount of light available. For .1 footcandle,
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. R7
must be shunted by
, 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.
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 into SO1
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 this point.
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
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