Crystal Photocell Circuits
January 1957 Radio & Television News Article

January 1957 Radio & Television News

[Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Electronics 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. This 1957 article in Radio & Television News magazine is a prime example. 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 photocell.

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.

The basic circuit, shown in Fig. 1A, is sensitive enough to operate at one-tenth of a foot-candle of light. A protective resistor, R₁, the crystal photocell, the Clailrex CL-2, and the variable load resistor, R₂, are connected in series across the 117-volt a.c. line. C₁, 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 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. 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, 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 alarm bell.

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.

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.

The unit is put into operation by turning switch S₁ to "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 of R₂ 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.

The circuit shown in Fig. 1D is useful when high speed operation is required at very low illumination levels. The value of cathode capacitor, C₄, 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. R₇ 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 foot-candles 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 will sound.

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 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 April 5, 2021
(updated from original post on 6/21/2013)