Fiber optic control apparatus

Electro-optical apparatus for transmitting Boolean switching signals over substantial distances includes a transmitter having a light-emitting diode operable to apply light pulses at a line frequency rate (or double a line frequency rate) to an optic fiber. A remote receiver includes a photosensor for receiving light pulses from the optic fiber, means for amplifying signals from the photosensor, an opto-isolator comprising a light-emitter and a photo-thyristor operated by said light-emitter, an electronic switch connected to energize a load device, and means responsive to said photo-thyristor for controlling the electronic switch.

This invention relates to electro-optical apparatus for transmitting 
switching control signals over substantial distances, and more 
particularly, to apparatus for transmitting Boolean or "on-off" control 
signals along an optical fiber. 
The advent of inexpensive microprocessors and other electronic components 
has made it possible and desirable to provide much more automatic control, 
and much more complex automatic control, to many industrial plants and 
processes, and to various other systems, such as multi-story office 
buildings. The addition of desired types of automatic control frequently 
requires many added control circuits to route signals between sensors, 
controlled devices, such as motors and valves, and processor or like 
equipment. While sufficient numbers of signal circuits (such as twisted 
pairs) needed to properly interconnect such devices can be provided in new 
installations in a straightforward fashion, it would be prohibitively 
expensive to add such signal circuits in many existing installations. 
While it has been common practice for many years to provide some empty 
spare conduits in many installations to allow for future system expansion, 
many systems were installed with no idea that such extensive addition of 
control circuits would ever become desirable, so that their spare conduits 
are insufficient in size to accommodate a desired number of added signal 
circuits. In addition, spare conduits installed many years ago often do 
not extend to locations where added sensors, controlled elements or the 
like must be located. The installation of added conduits would be very 
expensive in some existing installations, and nearly impossible in others, 
due to space limitations, for example. One object of the present invention 
is to provide control signal transmission apparatus which will allow 
economical substantial expansion of automatic control of a system in which 
the electrical conduits and/or wiring ducts are already full, or nearly 
full. 
Wholly aside from the aspect of system expansion, it has become 
increasingly desirable to locate numerous sensors and controlled elements 
in hazardous (e.g. combustible) environments. It has been the practice to 
house such devices in expensive "explosion-proof" housings, and to route 
the wiring for such devices through conduits using expensive 
"explosion-proof" fittings. Another object of the invention is to provide 
improved control signal transmission apparatus which does not require, or 
requires a minimum amount of, explosion-proof apparatus when used in a 
hazardous environment. 
In accordance with one concept of the invention, an "on-off" control signal 
is routed from a transmitter to a receiver over an optical fiber in lieu 
of a wired electrical circuit. While wire circuits tend to present 
sparking and fire hazards and they often must be contained within metal 
conduits or ducts, optical fibers can be run outside such conduits or 
ducts without creating any sparking or fire hazard. 
It has been known that analog voice signals or control information can be 
transmitted over an optical fiber link. For example, a voltage-controlled 
oscillator responsive to an analog input voltage may operate a light 
emitter to produce light pulses at a repetition rate or frequency 
deviation commensurate with an analog value for transmission of such 
pulses along an optical fiber, with a frequency sensitive circuit used at 
the receiving end of the cable to re-constitute the analog voltage. Such 
circuits tend to be complex and expensive, and tailor-made for a given 
application, with automatic gain control ordinarily being required. It 
also has been known that digital data pulses can be transmitted at very 
high rates over fiber optic circuits, either in the form of serially-coded 
pulse trains; or, when plural separate fibers are provided, in parallel 
digital code form. Such apparatus also tends to be complex and expensive, 
frequently requiring pulse re-shaping circuits at the receiving end. One 
important object of the present invention is to provide "on-off" or 
switching control signal transmission apparatus which is very economical 
to construct and install, and which is reliable in service. Another object 
of the invention is to provide economical signal transmission apparatus 
which has great versatility, so that a given form of the apparatus may be 
used in numerous different applications. 
In accordance with one aspect of some forms of the invention, great economy 
is achieved by energizing a light emitter substantially directly from an 
alternating line voltage, to provide light pulses at the line frequency or 
double the line frequency, for example, without the need for any 
oscillator or added frequency generation device. It is generally deemed 
undesirable to transmit low-level control signal pulses over wired 
circuits at the line frequency or a low multiple thereof, since nearby 
magnetic or electric fields can introduce serious noise through inductive 
or capacitive coupling, but such fields cannot inject noise into an 
optical fiber, and hence transmission of pulses at the line frequency or 
double the line frequency proves to be completely practical. 
Other objects of the invention will in part be obvious and will in part 
appear hereinafter. 
The invention accordingly comprises the features of construction, 
combination of elements, and arrangement of parts, which will be 
exemplified in the constructions hereinafter set forth, and the scope of 
the invention will be indicated in the claims.

In FIG. 1 a switch S is closed to apply an alternating voltage from a 
conventional line source (e.g. 117 volts, 60 hertz) having terminals L1 
and L2 to a transmitter shown within dashed lines at XMTR. The transmitter 
comprises a pulsating DC power supply section having a series fuse F, a 
voltage-dropping impedance formed by capacitor C.sub.A and resistor 
R.sub.A in parallel, a full-wave bridge rectifier formed by diodes D1 to 
D4, and a zener diode D5. The power supply section provides a pulsating DC 
voltage between terminals 1 and 2. The transmitter is shown as including a 
load which includes light-emitting diode LD1 in series with a resistor RB, 
and light-emitting diode LD2 in series with resistor RC. 
Switch S may comprise a manually-controlled switch, but in many 
applications it will comprise a relay contact, an electronic (solid state) 
switch or some form of condition responsive switch, such as a pair of 
thermostat contacts, or a pressure-operated switch or the like. The two 
light-emitting diodes may comprise coventional LEDs which emit visible 
light, or if desired, LD1 may comprise a non-visible light emitter, such 
as an infra red emitting diode. LD2 and resistor RC may be omitted in 
various embodiments of the invention, as will be explained below. 
Assume switch S is closed. During the half cycle of the line voltage when 
line terminal L1 is positive with respect to line terminal L2, current 
flows through fuse F, the voltage-dropping impedance, diode D1, zener 
diode D5 (and the load in parallel therewith), and diode D4 to line 
terminal L2. During the half cycle when terminal L2 is positive with 
respect to terminal L1, current flows through diode D2, zener diode D5 
(and the load in parallel therewith), diode D3 and the voltage-dropping 
impedance to terminal L1. Zener diode D5 limits the voltage between 
terminals 1 and 2 to a peak value of about 5.1 volts, and that voltage 
varies as shown in FIG. 1a, comprising essentially a square wave voltage 
having brief notches occurring at a repetition rate of 120 per second 
(with a 60 hertz source). 
Capacitor C.sub.A, which typically might have a value of 4.0 microfarads, 
functions to limit the current applied to the bridge rectifier, thereby to 
control the current through zener diode D5 and the LEDs. Resistor RA, 
which typically may have a value of 100 kilo-ohms, insures that capacitor 
C.sub.A will discharge when switch S is open, so that a residual charge on 
capacitor C.sub.A cannot cause excessive current flow through zener D5 and 
the LEDs if the line voltage is high at the instant switch S closes. 
The pulsating voltage between terminals 1 and 2 applied across the 
light-emitting diodes LD1 and LD2 and their associated current-limiting 
resistors R2 and R3 causes each light-emitting diode to emit pulses of 
light at twice the line frequency, with the intensity of light from these 
diodes varying with time in substantial conformance with the voltage 
variation shown in FIG. 1a. It may be noted that the pulsating DC voltage 
between terminals 1 and 2 is substantially unfiltered, and hence the LEDs 
are extinguished with no appreciable delay when switch S is opened. 
Further, the LEDs are illuminated with no appreciable delay when switch S 
is closed. In addition, avoiding substantial filtering in the transmitter 
allows it to be constructed physically very small. A small capacitor CB, 
which typically might comprise an 0.01 microfarad capacitor, theoretically 
could be said to provide some small amount of filtering, but its actual 
purpose is to merely smooth out small amounts of noise when the waveform 
(FIG. 1a) is near zero. 
Light-emitting diode LD1 is located adjacent the end of a fiber optic cable 
FC to inject light pulses into the optical fiber O therein. The fiber O of 
cable FC may be fitted closely adjacent the light-emitting face of LD1 if 
the LD1 itself has no optical fiber attached thereto. Light-emitting 
diodes having short lengths of optical fiber attached to their faces are 
commercially-available, however, and if such a device is used, fiber O of 
the cable FC may be coupled to the end of the fiber extending from the 
light-emitting diode. Light-emitting diode LD2 is mounted in the 
transmitter to project light outside the housing of the transmitter to 
indicate that light pulses are being applied to cable FC. While the 
provision of LD2 is useful for testing and maintenance purposes, it and 
its associated resistor R3 manifestly may be omitted in some applications. 
In one satisfactory embodiment of the invention cable FC comprised a silica 
fiber of 0.008 inch (0.2 mm.) diameter core covered with a protective 
covering having an outside diameter of 0.095 inch (2.4 mm.). Control 
signals were satisfactorily transmitted over lengths of such cable as 
great as 1200 feet (365.76 m.), and it will be apparent that signal 
transmission over distances up to that length is sufficient for a wide 
variety of control purposes. In FIG. 1 cable FC is shown leading to a 
receiver shown within dashed lines at RCVR. In some applications of the 
invention, however, light switches will be connected in the cable, as will 
be further explained below. 
The receiver RCVR is shown connected to an alternating voltage source 
having terminals L3 and L4. In many applications of the invention the same 
source will be used at both transmitter and receiver, so that terminals L1 
and L2 will correspond with L3 and L4. However, it is by no means 
necessary that the same source be available for use at both the 
transmitter and the receiver, and if two different sources are used, the 
phase angle between them is of no concern, nor need their voltages be the 
same. 
During half-cycles when terminal L3 is positive with respect to terminal 
L4, current flows through fuse F1, capacitor C1, diode D12, zener diode 
D15 and load circuits in parallel therewith, through diode D14 to source 
terminal L4. During the other half cycle of the L3-L4 source line voltage, 
while source terminal L4 is positive with respect to terminal L3, current 
flows through diode D13, zener diode D15 and load circuits in parallel 
therewith, diode D11, capacitor C1 and fuse F1 to terminal L3. Thus diodes 
D11 to D14 comprise a full-wave rectifier. Zener diode D15 limits the 
voltage between terminals 20 and 21 to about 12 volts. Capacitor C2 is 
connected across diode D15 to provide substantial filtering. The 12-volt 
DC supply so formed furnishes power for transistors Q1,Q2, for operational 
amplifiers U1,U2 and U3, and for a light-emitting diode contained in 
opto-isolator OI. The DC voltage at terminal 20 is applied via resistor R6 
to terminal 19, and capacitor C3 is connected between terminals 19 and 21, 
further filtering the DC supply voltage which powers the operational 
amplifiers and phototransistor Q1. The power supply connections to U2 and 
U3 are not shown, it being assumed that they are mounted on the same chip 
as amplifier U1. 
Assuming that the transmitter XMTR is injecting light pulses into cable FC, 
pulses of light at the receiver end of the cable are applied to 
phototransistor Q1. In the absence of light pulses being applied to it, Q1 
remains cutoff, and terminal 22 remains at the level of the negative 
(zero) terminal 21 of the 12-volt DC supply. As a pulse of light causes Q1 
to conduct, the voltage at terminal 22 rises relative to that of terminal 
21 in proportion to the conduction through Q1. The voltage developed 
across resistor R1 is directly coupled to the non-inverting input line of 
operational amplifier U1, which is connected as a non-inverting amplifier. 
Resistor R2 is an offset bias resistor, and resistor R3 is a feedback 
resistor, these resistors establishing the gain of amplifier U1. In a 
typical embodiment of the invention the voltage swing at terminal 22 is of 
the order of 0.5 to 20 millivolts, an U1 is arranged to provide a gain of 
10, providing an output voltage swing at terminal 23 from zero to a 
voltage of the order of 5 to 200 millivolts, depending, of course, on the 
amount of light applied to Q1. 
The voltage swing at terminal 23 is connected directly to the non-inverting 
input terminal of operational amplifier U2, which is also connected as a 
non-inverting amplifier, and provided with a gain of 450 in a typical 
application, providing an output voltage swing from zero to approximately 
2.25 to 11.0 volts at terminal 24 in a typical application. Amplifier U2 
is arranged to saturate at about 11 volts output. The signal swing at 
output terminal 24 of amplifier U2 is direct coupled to the non-inverting 
input terminal of operational amplifier U3, which is provided with a gain 
of 450 in a typical application, providing an output voltage swing from 
zero to approximately 11 volts at terminal 25 in a typical application. 
Amplifier U3 saturates as its output voltage closely approaches the supply 
voltage. 
The voltage at terminal 25 is applied via diode D16 to charge capacitor C4 
through resistor R14. When light pulses are applied to Q1, a voltage of 
3.5 volts at terminal 25 will charge capacitor C4 up to that voltage less 
the forward drop (e.g. 0.7 volt) of diode D16, or to a voltage of 2.8 
volts. In the case where very strong light pulses are received by Q1, an 
output voltage of 12 volts at terminal 25 will charge capacitor C4 to 
approximately 11.3 volts, and the voltage at terminal 26 approaches the 
output voltage of amplifier U3. During the brief instants once every 
1/120th of a second between successive light pulses the voltage at 
terminal 25 will return to zero, cutting off diode D16, but leaving C4 
charged. Whenever the voltage across capacitor C4 exceeds a predetermined 
level of the order of 0.7 volt, base-emitter current flow in transistor Q2 
will cause that transistor to conduct, providing current flow through 
resistor R10 and the light-emitting DO diode of opto-isolator OI. The 
predetermined level of voltage in capacitor C4 at which Q2 is turned on is 
reached when the voltage at terminal 26 equals the Q2 base-emitter 
junction voltage (e.g. 0.7 volts) times (R15+R16)/R16. In a typical 
embodiment of the invention the values of R14,R15,R16 and C4 are 220 
kilo-ohms, 5.6 kilo-ohms, 4.6 kilo-ohms, and 2.2 microfarads. 
When the application of light pulses to phototransistor Q1 is terminated, 
so that voltage 25 drops to near zero and diode D16 is cutoff, capacitor 
C4 discharges through R14 and R15 and through R16 and the base-emitter 
circuit of Q2, with a time constant determined largely by the values of 
R14 and R15 and capacitor C4. Using the values given above the time 
constant is of the order of 20 milliseconds. It will be seen that the time 
required for Q2 to turn off after the cessation of light pulses depends 
upon the level to which C4 has charged and hence upon the intensity of the 
light pulses received at the receiver, but in any event the turn-off time 
is no more than about 25 milliseconds, which is quite fast enough for most 
applications. 
If Q2 turns on when the voltage at terminal 26 reaches 1.0 volts, it will 
turn on when the voltage at terminal 25 reaches 1.7 volts. Thus it is 
important that drift or offset in amplifiers U1 to U3 be sufficiently 
small to insure that the voltage at terminal 25 never exceeds 0.7 volt 
when no light is applied to Q1. This is assured by use of low-drift 
operational amplifiers (e.g. Type LM 2902), particularly for amplifier U1, 
and properly matched offset resistances. Zener diode D21 also decreases 
the effect which offset voltage in amplifier U2 will cause at amplifier 
U3. 
An output device typified by the coil CR of a relay is shown connected in 
series with a bridge DS between source terminals L.sub.3 and L.sub.4. 
Bridge DS comprises diodes D17 to D20. Bridge DS together with transistor 
Q3 comprise a known form of AC static switch. The collector-emitter 
circuit of transistor Q3 is connected between diagonally-opposite 
terminals 27 and 28 of the bridge. If transistor Q3 is cut off, current 
flow cannot occur in either direction through load device CR, because 
diode D19 is poled oppositely to diode D17, and diode D20 is poled 
oppositely to diode D18. If transistor Q3 is turned on, however, current 
will flow from L3 via D17,Q3,D20 and CR to L4 during one half-cycle, and 
current will flow in the opposite direction from L4 via CR, D19,Q3 and D18 
to L3 during the other half-cycle. It will be seen that terminal 27 goes 
positive with respect to terminal 28 during both half-cycles, and that the 
photothyristor PT supplies equal base currents to Q3 during both half 
cycles. In the absence of current through the light emitting diode DO of 
opto isolator OI, the photothyristor PT of the opto isolator is cut off, 
no base current is applied to Q3, and hence no current flows through load 
device CR. When light pulses at the receiver turn on Q2 and cause the 
light-emitter of the opto isolator to emit light, that turns on the 
photothyristor, supplying base current to turn on Q3, and hence supplying 
current to load device CR. Opto isolator OI may comprise, for example, a 
Type No. 4N40 device commercially-available from General Electric Company, 
Syracuse, N.Y. 
It may be noted that each of the diodes of bridge DS and transistor Q3 must 
be capable of carrying the full current supplied to the load device, which 
may be several amperes in various applications, but that the 
photothyristor and other portions of the receiver need only carry small 
currents. It is also necessary that transistor Q3 be capable of 
withstanding the peak value of line voltage when it is cut off. The load 
device CR may be resistive, or quite inductive as well as resistive, or 
even capacitive. The use of transistor Q3 in lieu of a thyristor insures 
load current flow during substantially all of both half-cycles even if the 
load is inductive. If the load is appreciably inductive, one form or 
another of many known types of snubber circuits should be used to limit 
overvoltage from the inductive field collapse when the load device is 
de-energized, and in FIG. 1 a pair of back-to-back zener diodes at SN are 
intended to represent one form of snubber circuit. Varistors may be used 
instead. 
The load device CR in FIG. 1 represents a device intended to be supplied 
with alternating current, and if device CR comprises a conventional 
AC-operated relay or contactor coil, it may include a conventional shading 
turn or winding to minimize chatter. In some applications it is desired or 
required that relays or other load devices be supplied with direct 
current. It is a further feature of the invention that the receiver may be 
very readily modified to perform that function, by minor changes in its 
output circuitry. Indeed, one may construct the receiver so that either 
type of output circuitry may be plugged into the receiver. A modified form 
of output circuit shown in FIG. 1b provides half-wave rectified DC to a 
load LD. By comparison of FIG. 1b with FIG. 1 it can be seen that the 
device of FIG. 1 may be modified to the form of FIG. 1b by merely removing 
diodes D17,D18 and D19 and placing a jumper wire 30 between the terminals 
to which D17 was connected. 
FIGS. 2a to 2d illustrate a variety of modifications which may be made in 
the transmitter. In FIG. 2a half-wave rectification is provided by diode 
D1a to provide 60 (for a 60 hertz line) pulses per second from the 
light-emitting diodes. The voltage across those diodes is not clipped as 
was done by zener diode D5 in FIG. 1, so diodes LD1 and LD2 in FIG. 2a 
provide light pulses of sinusoidally varying intensity. FIG. 2a also shows 
that the light-emitting diodes may be connected in series rather than in 
parallel and that they do not need individual current limiting resistors 
in series with them. Further, dropping of the line voltage to limit 
current through the light-emitting diodes can be done using resistance 
(RA1 or RA1 and RA2 in parallel) in lieu of using a capacitor. Resistance 
RA1 is used alone for 230-volt operation, while resistor RA2 is connected 
in parallel with resistor RA1 for 115-volt operation, by placing a jumper 
between terminals 31 and 32. FIG. 2b further illustrates that current 
limiting, if done by means of resistors, can follow rather than precede 
rectification. 
In the transmitter of FIG. 2c half-wave rectification is used to illuminate 
diodes LD1 and LD2 during half-cycles when line L1 is positive with 
respect to line L2. Capacitor CA1 and resistor RA5 both limit current 
through the LEDs. When source terminal L1 is positive and switch S is 
closed, current flows from L1 through switch S, capacitor CA1 and diode 
D1b, then branching into two paths across terminals 33 and 34, one path 
being through zener diode D5a and the other being through resistor RA5, 
light-emitting diodes LD1 and LD2. When source terminal L2 is positive, 
current flows through diode D3b, capacitor CA1 and switch S to terminal 
L1. Diode D3b allows capacitor CA1 to act as an AC impedance, and diode 
D5a limits the peak voltage across the RA5,LD1,LD2 network. Resistor RA5 
controls the current flow through LD1 and LD2. This transmitter circuit 
furnishes pulses at a 60 hertz rate from a 60 hertz source, using a 
capacitive reactance for current limiting, minimizing heat dissipation and 
not requiring the use of a transformer. 
The transmitter circuit of FIG. 2d is arranged to furnish pulses at a 120 
hertz rate from a 60 hertz source using capacitive reactance to minimize 
heat loss. When source terminal L1 is positive and switch S is closed 
current flows from L1, diode D1d, branching through three paths between 
terminals 35,36, and thence from terminal 36 to source terminal L2. When 
source terminal L2 is positive current flows from terminal L2 through 
diode D3d to terminal 35, through the same three paths to terminal 36, and 
thence through diode D4b, and capacitor CA2 to source terminal L1. 
It will be apparent that a receiver constructed according to FIG. 1, or any 
of the various forms of transmitters shown, can be readily made to fit 
inside a standard 4.times.4.times.1.5 inch outlet box, for example. 
It sometimes is desirable that switching control signals emanate from 
devices located in dangerously-combustible atmospheres, or from devices 
which have no source of electrical power near them, and an added feature 
of the invention allows such operation. In FIG. 3 a light source LS, such 
as a simple incandescent lamp, located in an area A.sub.1 which does not 
have a combustible atmosphere, applies steady (or pulsating) illumination 
to one end of an optic fiber cable FC1. Cable FC1 leads into an area A2 
having a combustible atmosphere, to a non-electrical switching device 35. 
A second optic fiber cable FC2 extends from device 35 outside area A.sub.2 
to a third area A.sub.3 (or, if desired, back to area A.sub.1) which has a 
combustible atmosphere, to a receiver RCVR, and a device it is desired to 
selectively control. As shown in FIG. 3a, switching device 35 comprises a 
transducer 36 which moves a shutter 37 in between the ends of the optical 
fibers in cables FC1 and FC2, either allowing or occluding passage of 
light from cable FC1 to cable FC2. 
It should be recognized that while exemplary values of various components 
and voltage levels have been stated above in order to afford a clear 
understanding of the invention, that various changes in such values and 
levels will become apparent to those skilled in the art as a result of 
this disclosure. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained, and 
since certain changes may be made in the above constructions without 
departing from the scope of the invention, it is intended that all matter 
contained in the above description or shown in the accompanying drawings 
shall be interpreted as illustrative and not in a limiting sense.