CA1133101A - Vital optical coupler circuit arrangement for railroad signaling systems - Google Patents

Abstract

(Case No. 6997) ABSTRACT OF THE DISCLOSURE

A light emitting diode (LED) is coupled direct to the local source of alternating current track circuit energy through a transformer, with a band pass filter tuned to track circuit frequency included in the transformer secondary connections to the LED. The LED responds to each positive half cycle of the local signal and is focused to illuminate a photo resistive cell (PRC) to actuate similar periodic reductions in its resistance. The PRC is coupled to the electrified track section rails through a saturable transformer to receive track circuit energy from the same source through the section rails.
The LED and PRC function as a synchronous rectifier, when the local and track signals are of the same frequency and rela-tively in phase, to provide an averaged direct current signal component from the saturable transformer secondary to a biased direct current track relay, which picks up to indicate an unoccupied section and releases when the section is occupied or a broken rail occurs. The circuit arrangement rejects signals of propulsion power frequency and its harmonics and sharply attenuates the D. C. output if the inputs are out of phase beyond predetermined limits.

Description

(Case ~o. 6997) ~ ~ ~

VITAL OPTICAL COUP~ER CIRCUIT ARRA~GEME~T
FOR RAILROAD S IG~ Ç; SYSTEMS

BACKGRO~ND OF THE I~VENTIO~
My invention pertains to a vital optical coupler circuit arrangement for railroad signaling systems. More specificallyO
the invention relates to a vital optical coupler circuit arrangement usable as a track relay means in alternating current trac~ circuits on electrified railroads.
As a matter of economy and efficiency, electrified rail-xoads in the United States are planning to change to commercial 60 Hz propulsion power sources to replace the long used 25 Hz propulsion power generated by special apparatus at privately ~wned generating stations. Any new electrification will also be at the commercial power frequency. The signaling systems in such electrified railroad~ have for many years used lOQ Hz alternating current (A. C.) track circuits including centrifu-gal ~ype track relays which effectively are two winding, dual input, synchronous motor or detector devices designed ~o be immune to the 25 Hz propulsion current flo~ing in the rails.
- 20 One winding of the relay is energiz~d direct from the track circuit power source. The second winding recei~es energy through the section rails from the same track circuit source connected at the other and. The relay operates ~o indicat~ an unoccupied track section only when both windings are energized by track circuit frequency currents having a phase angle rela-tio~ship within predetermined limits~ It is impractical, in the process of changing propulsion frequency, to also change ~?

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out the existiny 100 ~z track circuits. However, the present centrifugal r~lays are not sufficiently immune to the 60 Hz frequency to assure no response by the relay to propulsion currents of the new frequency flowing in the rails. In addi-tion, centrifugal relays inherently require considerable and frequent preventative maintenance to assur~e proper and re~able ~ ~
operation. Therefore~ it is desirable, even required, to sub- ~1 stitute a passive network not responsive to 60 Hz currents which will permit continued use of the 100 ~Iz track circuits but with conventional, vital direct current (D.C.) track re-lays which will not respond to, that is, which are immunized from, propulsion currents whether of the 25 or 60 Hz frequency. ~;
OBJECTS A~D SUMMARY OF THE I~VENTION
Accordingly, an object of my invention is a passive ~ .
circuit network fcr controlling a vital relay which serves as the track relay means for alternating current track circuits in electrified railroads.
Also an object of my invention is a vital optical coupler circuit arrangement responsive only to dual input signals of the same preselected frequency and within a predatermined phase relationship to provide a usable output signal.
Another object of the invention is an optical coupler circuit arrangement to replace the frequency and phase re-sponsive track relay in vital track circuits for electri-fied railroads.
A still further object of the invention is an improvedfrequency sèlective track relay means for alternating current track circuits in electrified railroads.

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It is also an object of my invention to provide a track circuit arrangement for electrified railroads which includes a tuned optical coupler circuit networX at the receiving end of the track section to energize a vital track relay only in response to received energy of the preselected track circuit frequency.
StiIl another object of the invention is a half wave synchronized rectifier circuit network, including an optical coupler, usable in alternating current railroad track circuits for detecting ~he presence of two signals having the same frequency and within a predetermined phase relationship.
Still another object of the invention is a frequency -~
selective synchronous rectifier network including an optical oupler to replace the centrifugal relay in alternating current track circuits for electrified railroads.
A further object of my invention is an optical coupler circuit network responsive only to two source related input signals of the same frequency and having a predetermined~ -phase relationship for producing a registerable output signal indicating the presence or condition of one or both of the input signals.
A still further o~ject of the invention is an alternating current track circuit, for an electrified railroad track section with impedance bonds for propulsion current return, which includes a source of alternating current energy, having a frequency different from the propulsion power frequency, connected to the rails at one end of the section and a train 1~31~

detector means with a ~irst input coupled to the track circuit source and a second input connected across the rails at the other sec*ion end, second input signals ~eing rectified by a ;~
photo resistive element controlled by a light emitting diode actuated by the first input signal through a tuned filter com~ ) ~7~ Of ~c element, the rectified~output signal of the photo resistive element energizing the vital D. C. track relay to register an unoccupied track circuit condition only if both input signals are present and are within a predetermined p~ase relationship.
lG Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims w~en taken in connection with the accompanying drawings.
- According to the invention, a vital circuit arrangement has a first input means, tuned to a preselected frequency, to activate a light emitting diode (~ED) element when supplied with an alternating current signal of that frequency. The pulsed emission from the LED controls a photo re~istive cell, periodically reducing its resistanca to produce an averaged `
direct current output from another A. Cn signal having the same frequency and a related phase angle applied thr~ugh a second input means of the circuit arrangement. The averaged Do C. output signal produced by this optical coupler energizes a direct current registry relay which picks up only when the two input signals have the same fre~uency and are within a predetermined phase relationship. The vital circuit arrange-ment is substituted for a dual input, frequency responsive _ D~ _ 33~

track relay in an alternating current track circuit for an electrified railroad and maintains the required inhibition of response to the propulsion current, of a different but closely spaced frequency, flowing in the rails to prevent the improper registration of an unoccupied track section when a train is present or a braken rail exists.
In the specific practice of the invention, the illustrated vital circuit arrangement is a dual input detector means in-tended for use as the receiver or registry element in an al-ternating current track circuit for an alectrified railroadwhich uses alternating current propulsion powerO The input means for the circuit arrangement are coupling transformers with a first transformer co~nected to receive input signals direct from the track circuit alternating current energy source. This track circuit source has a frequency differ-ent from but close to the frequency o~ the propulsion cur-rent. The secondary output of this transformer is applied ~-to the Light emitting diode element through a filter circuit path tuned to the track circuit frequency. The LED is thus turned on during each positive half cycle of the local input signal. The ~ED is positioned to illuminate a photo resistiva element, during each period when the LED is turned on, to thus actuate periodic reductions in the resistance of this photo re--sistive element. The second input transformer is a saturable type connected across the rails to receive the track current signal transmitted from the track energy source connected at the other end of the corresponding track section. The ". ' ,r~
. . ~, ~3~
secondary of this second transformer is connected in series with the photo resi,stive element and a biased direct current vital relay which serves as the track circuit registry relay.
~he periodic actuation of the photo resistive cell by the LED
produces a modified or pulsed wave output which has an averaged D. C. component somewhat equivalent to that of a half wave rectified alternating current. When both inputs are present at normal levels~ are relatively in phase, and of course of the same frequency, the level or magnitude of the D. C. com-ponent in the output signal sufficiently energizes the registry relay to pick up, indicating the presence of both signals with proper characteristics and thus an unoccupied track section with no broken rails. Fre~uency and phase characteristics of the vital network are sharp enough to reject signals at other requencies and/or out of phase, that is, beyond the accepted phase relationship limits.
BRIEF DESGRIPTI:Ol!l OF THE DRAWI~GS
-- -- --I shall now describa in greater detail the specific embodiment o~ my invention as illustrated and characterized in the accompanying drawings, in which:
FIG. 1 is a circuit diagram of a vital circuit arrange-ment including an optical coupler for reyistering the presence or absence of two input signals by the position of a relay element.
FIG. 2 is a chart illustrating the wave form of the output signal from the circuit arrangement illustrated in FIG. 1 which is used to energize the registry relay.

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FIG. 3 illustrates schematically the use of the optical coupler circuit arrangement of FIG. l as a dual input track relay means in an alternating current track circuit.
FIG. 4 is a chart illustrating the relative D. C. output level of the FIG. 1 circuit arrangement as a function o the pha~ relationship between the input signals.
FIG. 5 is another chart showing the D C. output of the FIG. l circuik arrangement as a function of the frequency of the track input signals. ~-In each of the drawing figures, similar reference charac-ters designate the same or similar apparatus or functions. ~;
DESCRIPTION OF T~ ILLUSTRATED EMBODIME~T ~:
Referring to FIG. 1, a vital optical coupler circuit arrange-ment lS shown with its two principal or controlLing elements, the light emitting diode D2 and the photo resistive cell or device PRC. ~he LED element D2 is shown by conventional symbol and has the normal expected characteristic of being actuated to emit light when current flows in the conventional positive direc-tion through the u~it. The photo resistive cell PRC is also shown by a conventional symbol and has the characteristic of reducing its series resistance when illuminated by a light source, the reduction being to a relatively low level to allow a greater magnitude of current to flow through the corresponding circuit.
As to physical mounting, the LED D2 may be either a single element, as shown in the drawings for simplicity, or a series connected cluster of high intensity diode emitters. In either arrangement, the LED'S are focused on device PRC so that the ,~t - 7 ~ 3L33~
emitted light actuates the photo resistive cell to lower its ~ -resistance. Although not specifically shown in the drawings, the LED element is preferably a cluster of such uniks so that the photo resistive cell is actually overdriven to assure com-plete response. This over-driving compensates for aging of the LED units, for output variations due to temperature changes, and provides some voltage regulation of the input signals. All sub-sequent references to a light emitting diode element, including those in the claims, refer to either arrangement.
The entire circuit arrangement is designed to detect or indicate the presence of both of two input signals or the absence of at least a selected one of the two, each supplied through an input means from a selected source. These input ~means are the transformers Tl and T2. Transformer Tl is a step-down transformer while transformer T2, of the saturable type as shown, is a step-up transformer so that the two secondary outputs will be o the same general voltage range under normal conditions. The corresponding instant polarity of the various windings of these two input transformers is desig-nated in the conventional manner by the dot symbols.
The primary winding of transformer Tl is connected to alocal source of A. C. signal energy to receive input signal VL
which has a preselected frequency, for example, 100 Hz. The step-down characteristic of transformer Tl supplies a secondary ~5 voltage signal VL2 which i5 of a comparable level with that supplied by the other transformer, to be discussed shortly.
The secondary of transformer Tl is connected, in series with a noise rejecting band pass filter comprised of capacitor C

~31 33~0~ :
and inductor L and series tuned to the frequency of signal VLl, to supply signal VL2 to LED D2. A resistor R in this circuit, together with the impedance of inductor L, limits the current flowing through diode D2. A conventional diode ~ -Dl is connected in parallel with diode D2 but with opposite polarity to protect the LED against excess reverse polarity voltage in the circuit. When signal VL2 is present, which is normally continuously, diode D2 is turned on to emit light every positive half cycle, that is, when current flows through diode D2 in its low resistance direction. This LED, either a single element or a cluster, is positioned so that its output is focused on the photo resistive c~ll PRC. Thus the resis-tance of unit PRC decreases periodically to a relatively low -leve-l during each positive half cycle of signal VL2.
The primary winding of transformer T2 is connected to a second source of alternating current, having the same freque~cy ~ .
as and synchronized with the local source previously mentioned to receive ~he second input signal VTl. In the specific example herein, this primary winding LS connected to a trans-mission channel, e. g., the track rails, which is supplied with energy from the same central source as the local energy supply connected to transformer Tl. As mentioned, transformer T2 is a step-up transformer and has saturable charackeristics to provide an amplitude limiting feature for the circuit arrangement. In the principal use of this arrangement in trac~ circuits, the saturable characteristic of transformer T2 limits excessive voltage levels of signal VTl when an 3~
insulated joint failure allows the transmitted signal from the adjoining track section to feed direct into this receiver apparatus.
The secondary of transformer T2 supplies a signal VT2, whic~ is of the same order of magnitude normally as signal VL2~ ~o the circuit network consisting of device PRC and a biased direct current vital relay TR. This relay as shown is c~nnected between the output terminals ~+ and R-, for the circuit arrangement, with the polarity such that conventional current ~lowæ in the proper direction through the relay winding as designated by the small arrow therein. A current arrow I
is shown ass~ciated with this circuit network in order to provide a reference for correlation with the charts in the other figures. When signals VLl an~d VTl are in phase, and of course are of the same freguency, the current I flowing through the network from the secondary of transformer T2 is shown by the solid line in FIG. 2. This is a modified alter-nating current of the frequency of signal VT2 and its wave form is determined by the periodic change in the resistance through photo resistive cell PRC. This current has a direct -~
current component shown by the dash line designated as the AVERAGE DC LEVEL, which energizes relay TR. Thus this optical coupler network acts as a synchronous rectifier to provide the D. C. component in current I which is o~ the propex polarity to enexgize relay TR when the input conditions or signals have the proper characteristics.

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A principal use for the optical coupler circuit arrange-ment of FIG. 1 is in a railroad trac~ circuit as illustrated in FIG. 3. In this drawing, a track section T of a stretch of electrified railroad is shown with its rails 1 and 2 illus-trated by conventional single line s~mbols. The rails ofsection T are electrically insulated from the rails of the adjoining sections by the insulated joints 3, also illustrated by conventional symbols. In order to provide a return circuit for the propulsion current, lmpedance bond windings 4 are con- :
nected across rails 1 and 2 at eac~ end of section T and the : ;
associated ends of the adjoining sections. Center taps of each associated pair of bond windings 4 are connected by a lead 5 to provide a conventional circuit path through section T for propulsion current. It.is here assumed that the fre-quency of the A. C. propulsion power is the ommonly used 25 ~.
or 60 Hz.
A signaling system for this stretch of railroad is based on continuous train detection using an A. c. track circuit for each track section such as section T. Signaling energy for the track circuits lS provided from a central source S, shown conventionally at the lower left and having a preselected frequency, for example, 100 Hz, and is distributed along the stretch of railroad by the line wires 6 and 7. Energy is supplied across the rails of section T at the left or trans-mitting end through a track trans~ormer TT from line circuit6, 7. The supply connections include an adjustable resistor Z which limits the current flow when a train shunts the rails ~33~

at the transmitting end. Track transformer TT may be incorpor-ated as part of the impedance bond but is preferably a separate transformer~ as shown. At the other or receiving end of sec-tion T, the optical coupler circuit arrangement of FIG. 1 is connected across the rails and to the line circuit. This circuit is illustrated by a dashed block with input and output terminals designated by-~he same references as in FIG. 1. For example, the terminals VLl are connected across line wires 6 and 7 to receive energy direct from source S at this local location.
Terminals VTl are connected across rails 1 and 2 at the same point as bond winding 4 at ~his end of the tracX section. ~he txa~k relay TR, which is of the same biased vital type as in FIG. 1, is connected across terminals R~ and R- with proper polarity for energizing the relay when output is pxesent.
Considering now the operation of the track circuit, it is ;;
to be re~embered that the ~ital optical coupler circuit network ~onnected within t~e track circuit of FIG. 3 acts as a two ele- `~
ment A. C. txack relay means to register the absence or presence of a train within section T. This device will also detect the presence of a broken rail within the section which interrupts the normal flow of track current~ The track circuit is adjusted with minimum ballast conditions Iwet weather, low resistance) so that the track and local signals ~VTl and VLl~ at the receiver end are in phase. Under these conditions, track current is approximately at the minimum level which will s~ill pick up relay TRo In other words, the averaged D. C. output is at the relative 1.0 level of the charts shown in FIGS. 4 ~L~33~
and 5. Under dry weather conditions, with high ballast resis-tance, the track current is at a maximum level. ~e phase of the rack signal VTl, under these maximum current conditions, .
leads the p~a~e of the local signal VLl. In FIG. 4, the chart illustrates that the output D. C. of the optical coupler network is attenuated fxom its maximum value 1.0 under phase shift conditions. However, with maximum track current, t~e larger VTl input signal compensates for the reduced multiplier function, from the phase shift curve, so that sufficient output remains to energize track relay TR. In other words, the out-of-phase attenuation is counterbalanced by the higher level of the track current. Of course, the biased relay TR ~:~
. will not respond to a reverse polarity output, i. e., phase shifts beyond ~ or -.90. Thus any extreme phase shift, or a moderate shift without increased rail current, due to a fault condition, results in the release of relay TR.
When a train occupies section T and shunts the rails, the reception o~ rail current at the receiver end is inhibit~d.
With no input signal VTl and thus signal VT2 absent, no energy is available to supply the current through the device PRC f relay TR network, even though LED D2 is periodically activated by the continuously supplied local signal VLl. Track relay ~-TR i~ thus deenergized and releases to register the tracX
occupied indication. In other words, the circuit detects and 5 registers the absence of the one selected signal, i. e., VT
q~ afc which may also be caused by~ broken rail condition.

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This optical coupler circuit arrangement has extremely sharp rejection of signals at o~her than the track circuit frequency~ This is illustrated in the chart o~ FIG. 5 for the spec fic assumed example of a track circuit frequency of 100 Hz. It is to be noted that the circuit arrangement rejects large undesired propulsion current signals of either 25 or 60 Hz frequency, and the common second and third haxmonics thereof, without any additional filtering. Even on the amplified ~ertical scale used in FIG. 5, the averaged D. C.
output at these unwanted ~requencies is not measurable. This results from the natural or inherent synchronous filter charac-teristics of the disclosed circuit arrangement. In other words, the synchronous rectifier action of LED D2 and photo resistive cell PRC results in a registerable output from the arrangement only when both input sic~nals are at the track circuit frequency.
This circuit arrangement is al~o vital, that is, fail-safe, since any failure in the LED drive circuit connected to the secondary of transformer Tl, either an open or a short circuit, results in a decreased light pulse level. If the photo resistive cell opens, the xelay current ceases. Further, should this photo cell short out, the relay will receive an alternating current and will not respond since it is a biased D. C. relay. A short or open circuit failure in transformer TR2 will also result in a fail-safe condition, that is, the track relay releasing to indicate an occupied section. Thus any failure or circuit element fault within the arrangement _ 14 -~31~ ;

re~ults in the release of track relay TR to indicate an occup~ed track section, which is a safe condition.
The optical coupler circuit arrangement of my invention thus provides for an improved track circuit for electrified railroads. The two element circuit network uses passive circuit elemenks except for the final registry track relay which is a conventional and readily availa~le type of high reliability and low maintenance requirements. Preventative maintenance for thP entire track circuit is ~hus reduced and the reliability increased. The excellent phase angle and frequency response of prior art apparatus is retained so that broken rails can be detected and high level propulsion current signals rejected. This results in an efficient and economical track circuit apparatus.
Although I have herein shown and described but a single optical coupler track circuit arrangement ambodying the inven~
tion, it is to be understood that various changes and modifi-catlons therein may be made wi~hin the scope of the appended claims without departing from the spirit and scope of my inv~ntion.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A vital optical coupler circuit arrangement for registering the presence or absence of a selected one of two input signals, comprising, (a) a first transformer with its primary winding coupled for receiving a first signal from a first source of alternating current energy having a preselected fre-quency, (b) a saturable transformer with its primary winding coupled for at times receiving a second signal, from a second source of alternating current energy having said preselected frequency and a variable phase relationship to said first source, and responsive to said second signal for producing in its secondary winding an amplitude limited signal, (c) a filter circuit path tuned to said preselected fre-quency, (d) a single light emitting diode element coupled by said filter circuit path to the secondary winding of said first transformer and responsive for emitting light radiation during each of selected half cycles of said first signal, (e) a photo resistive device coupled to the secondary wind-ing of said saturable transformer and positioned to be actuated by radiation from said light emitting diode element for producing a direct current output signal of at least a predetermined magnitude only when both said first and second signals are present, are of said preselected frequency, and are within a predeter-mined phase relationship, and (f) a registry means coupled for receiving said output sig-nal from said photo resistive device and operable for registering the presence or absence of said second signal as said direct current output is at least equal to or is less than said predetermined magnitude, respectively.

2. An optical coupler circuit arrangement as defined in claim 1, in which, (a) said second source is a transmission means over which said second signal is supplied from a remote location by said first source and which also carries energy having a different frequency characteristic, (b) the presence and absence of said second signal repre-sents a first and a second condition of said trans-mission means, respectively, which is to be registered, and (c) said registry means normally occupies a first position to register a second condition of said transmission means and is operable to a second position, to register a first condition of said transmission means, only when said output signal equals or exceeds said predetermined magnitude.

3. An optical coupler circuit arrangement as defined in claim 2 in which, (a) said registry means is a vital, biased direct current relay operable between a first released position and a second energized position, (b) said photo resistive device is actuated by the pulses of light radiated by said light emitting diode element to periodically reduce its resistance in series with said registry relay for producing a modified alter-nating current signal of said preselected frequency having an averaged direct current component, (c) said relay operates to its second position only when said averaged direct current component equals or exceeds said predetermined magnitude.

4. An optical coupler circuit arrangement as defined in claim 3 in which, (a) said transmission means comprises the rails of an insulated section in an electrified railroad, said rails also carrying alternating current propulsion current of a different frequency, (b) said first and second conditions of said transmission means are said section unoccupied and occupied by a train, respectively, (c) said registry relay registers an unoccupied section only when operated to its second position, and which further includes, (d) another diode connected with opposite polarity in parallel with said light emitting diode element for protecting against excessive voltage signals during the negative half cycles of the signal induced in said first transformer secondary winding.

5. A track circuit arrangement for an insulated section of electrified railroad having a propulsion power source with a first preselected frequency and a return circuit through the section rails, comprising in combination, (a) another source of alternating current energy of a second preselected frequency coupled to the rails at one end of said section, (b) a first transformer with its primary winding coupled for receiving a first signal direct from said other source, (c) a saturable transformer with its primary winding coupled to said rails at the other end of said section for receiving a second signal from said other source through said section rails only when the rails are intact and said section is unoccupied by a train, (i) said saturable transformer producing an amplitude limited output signal in response to the reception of said second signal, (d) a band pass filter element tuned to said second pre-selected frequency, (e) a single light emitting diode element coupled to the secondary winding of said first transformer by said filter element and responsive to a received first signal for emitting a light radiation each half cycle of current flow in a forward direction through said diode, (f) a photo resistive device coupled to the secondary wind-ing of said saturable transformer for receiving said amplitude limited output signal therefrom and positioned to be periodically illuminated by said light emitting diode for producing a direct current component signal within said output signal when said first input signal is also present, both input signals are of said second frequency, and are within a predetermined phase rela-tionship, and (g) a registry means coupled in series with said photo resistive device and said saturable transformer second-ary winding for receiving said output signal and respon-sive thereto for registering an unoccupied or occupied track section as said direct current component is pro-duced or is absent, respectively.

6. A track circuit arrangement as defined in claim 5 in which, (a) said registry means is a biased direct current relay connected in series with said photo resistive device to the secondary winding of said saturable transformer and responsive only to a direct current component sig-nal of predetermined polarity for registering an un-occupied section indication.

7. Synchronous rectifier detection apparatus for an alternating current track circuit for a track section of an electrified railroad, said track circuit supplied with operating energy by an alternating current source connected to the rails at the transmitting end of the track section and having a pre-selected frequency different from the frequency of the propul-sion current also flowing in the rails, comprising, (a) a first transformer with its primary winding coupled directly to said track circuit source, (b) a filter circuit path tuned to said preselected fre-quency, (c) a single light emitting diode element coupled in series with a secondary winding of said first transformer and said filter circuit path for receiving a first signal of said preselected frequency only and responsive there-to for emitting a pulse of light radiation during each half cycle of that signal that flows through the diode in the low resistance direction, (d) a photo resistive cell positioned to be actuated by pulses of light from said diode for substantially reducing the resistance of said photo resistive cell during each pulse period, (e) a registry means normally occupying a first position indicating the corresponding track section occupied, and operable to a second position to indicate an un-occupied section only when energized by a direct current signal of at least a predetermined magnitude, and (f) a saturable transformer with its primary winding coupled to the rails at the other end of said track section for receiving from said track circuit source, through said section rails when said section is unoccupied by a train, a second signal having a variable phase relationship with said first signal, (g) the secondary winding of said saturable transformer connected in a series circuit network with said photo resistive cell and said registry means for supplying thereto an amplitude limited form of said second sig-nal in response to the reception of a second signal, (h) said series circuit network responsive to said amplitude limited signal for generating a modified signal having an averaged direct current component produced by the periodic reduction of the resistance of said photo resistive cell by said light pulses from said diode, (1) said direct current component signal having suf-ficient magnitude for operating said registry means to its second position only when both said first and second signals are present, are of said pre-selected frequency, and are within a predetermined phase relationship.

8. Synchronous rectifier apparatus as defined in claim 7 in which, (a) said registry means is a biased direct current relay having a predetermined minimum operating level equal to said predetermined magnitude and connected with selected polarity in series with said photo resistive cell in said network, (b) said registry relay normally occupying a first released position for indicating an occupied track section and operable to a second energized position for indicating an unoccupied track section only when said direct current component signal is equal to or exceeds said predetermined magnitude and has said selected polarity.