Remote door controller

May 21, 1963

G. H. MARMONT

3,090,959

REMOTE DOOR CONTROLLER

Filed Aug. 6, 1956

2. Sheets-Sheet 1

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May 21, 1963 _G. h. marmont 3,090,959

REMOTE DOOR CONTROLLER

Filed Aug. 6, 1956 2 Sheets-Sheet 2

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United States Patent Office Patented May 21, 1963 such a manner as to provide a direct current which will be adequate to operate relay control means, when all the frequencies are being received, but which is inadequate to operate the control means when only one frequency is being received.

It is, therefore, an object of the present invention to provide radio control for a closure structure or other remotely operated device, wherein a sending station transmits a plurality of frequencies in timed sequence, these frequencies being combined at a receiving station to provide a direct current for the operation of a control relay.

A further object is to provide a transmitter wherein an oscillator circuit is arranged to generate two signal waves of different frequencies, these frequencies being alternately transmitted at a frequency rate less than either of the signal frequencies.

A further object is to provide a transmitter wherein a synchronous vibrator is utilized to switch the oscillator frequency as well as pulsate the direct current supply.

Another object is to provide a garage door control transmitter which may be mounted on an automobile having a radio receiver normally connected to a receiving antenna, and wherein switching means are provided to disconnect the radio and connect the garage door control transmitter to the receiving antenna during broadcast of a garage door control signal.

A still further object is to provide a radio signal receiver in connection with a remote door controller, wherein two radio frequency control signals in the same frequency band are combined to actuate control relay means, but neither of which frequencies by itself will actuate the relay.

Another object of the invention is to provide a receiver for a garage door actuator, in which means are provided for automatically compensating for variations in received signal amplitude due to variations in transmitting distance of the signal.

Still another object is to provide a signal receiver wherein two signals are received by a tuned signal selecting filter coupled with a single receiving antenna, and combined to provide a relay energizing current.

A still further object is to provide a signal receiver for a closure actuator, arranged to receive two radio, frequency signals over a single antenna, and selectively separate these frequencies and recombine them into a composite control signal.

A further object is to provide a garage door controller which incorporates a safety overriding control adapted to respond to the interruption of a light beam movable with the closure.

Further objects of the invention will be brought out in the following part of the specification, wherein detailed description is for the purpose of fully disclosing the invention without placing limitations thereon.

Referring to the accompanying drawings, which are for illustrative purposes only:

FIG. 1 is a schematic diagram of the signal transmitter of the present invention;

FIG. 2 is a schematic diagram of the radio signal receiver, and further showing the control circuit connections with the controlled actuator means; and

FIG. 3 is a schematic diagram of the safety control mechanism.

THE CONTROLLER IN GENERAL

Referring generally to the drawings, the remote controller of the present invention embodies three components, namely, a radio transmitter A, a signal receiver B and safety control mechanism C. The transmitter is

3,090,959

REMOTE DOOR CONTROLLER

George H. Marmont, Los Angeles, Calif., assignor, by mesne assignments, to The Dalton Foundries, Inc., War- saw, Ind., a corporation of Indiana

Filed Aug. 6, 1956, Ser. No. 602,336 3 Claims. (Cl. 343—225)

The present invention relates generally to remote control for actuating mechanisms, and may be used, for 10 example, for opening and closing garage doors and other closures.

A number of remote radio control systems have heretofore been devised and used for the opening and closing of closures, such as a garage door, from a remotely 15 located automobile as it approaches or leaves the garage. These known systems have only partially solved the problems inherent to such applications, and for the most part have been unsatisfactory.

Under regulations of the Federal Communication Com- 20 mission, only certain frequency bands are available for public use, so that the systems thus far considered have been limited to operation in these available frequency bands.

The highest available frequency band (460 to 470 me.) 25 is in the Ultra High Frequency range and is practically not very convenient to work with for signalling purposes in connection with door operators. As a consequence, this frequency is not often used for control of closure operating mechanisms.

Most of the present garage door control systems operate in the Very High Frequency range (260 me.). Such systems are under present regulations illegal to operate, althougth the Commission has no control over sales of such equipment. One difficulty of operating in ⁰⁵this range is that spurious operation and interference may result from frequencies utilized in this range by television, radar, airplane navigation, etc.

Another allowable frequency is in the “citizens band” (27.255 me.) in the High Frequency range. This frequency is legal provided the transmitter is crystal controlled. However, this frequency has several disadvantages. The necessity for crystal control makes the transmitter more costly, and coding is possible only, by audiofrequency-modulation. Since many electronic devices for scientific, industrial and medical uses operate at this frequency, there is the ever present possibility that radiations from these devices, as well as those from other electrical disturbances, both natural and man-made, may _gocause undesirable spurious and random opening and closing of the garage door.

The remaining frequency permitted is in the Low Frequency range, and if a frequency of 300 kc. is utilized., it is possible to obtain a maximum permitted transmission 55 distance, which will be substantially 500 feet. This frequency has been exploited but very little for remote control of garage doors, as there has heretofore been no satisfactory arrangement devised for coding the signals so as to provide selective operation and non-interference ₆₀with respect to neighboring receiving devices. Having in mind the inherent disadvantages of presently available remote radio control arrangements, the present invention contemplates a remote radio control for a garage door or other mechanism and devices, which operates in the ₆₅low frequency range in the region of substantially 200400 kc. A remote transmitter located on the automobile is arranged to repeatedly broadcast signal pulses of two or more different frequencies at a rate of approximately 115 cycles per second. A receiver for controlling the ₇₀door actuating mechanism, is arranged to pick up the broadcast frequencies and combine these frequencies in

5,090,959 arranged to be mounted on an automobile and is under the control of the operator who merely by pushing a push button may transmit a coded signal to the receiver located at and associated with a closure member such as a garage door so as to open the door as the automobile approaches. Likewise, upon leaving the garage, the operator simply by pressing a control button may transmit the coded signal and effect closing of the closure as the automobile is leaving. The safety control mechanism is associated with the closure, and is so arranged that it will reverse the operation of the door at any time that a person or object is positioned in the path of movement of the door, the safety mechanism being so arranged that it will operate without actually making contact with the person or object. The components of the invention will now be taken up in detail.

The Transmitter A

The signal transmitter A, as shown in FIG. 1, is arranged to be mounted on the automobile under the control of the operator and utilizes an oscillator circuit which broadcasts on two different frequencies, these frequencies being rapidly alternated at a predetermined rate, the frequency of which is less than the signal frequencies being broadcast.

The oscillator circuit comprises a duo-triode tube 10 in which a triode section 10^a forms an oscillator section containing a cathode ll^a, control grid 12^a and a plate 13^a; while a triode section 10^b forms a power output amplifier containing a cathode ll^b, control grid 12^,J and plate 13^b. The oscillator section has its control grid 12^a connected through a grid leak 14 with one end of a tuned tank circuit 15, the other end of the tank circuit being connected with the cathode ll^a which is grounded. The plate 13^ais connected into a plate circuit containing a winding 16 of a transformer 17 by which the plate circuit is closely coupled with the other winding IS of the transformer in the grid circuit to provide a feed-back.

Power supply is obtained from a conventional voltage doubler circuit 19 containing capacitors 20, 21, and selenium rectifiers 22, 23. The voltage doubler is fed from a secondary winding 24 of a small power transformer 25 which has a primary winding 26, the energization being controlled by an associated vibrator 27.

The vibrator 27 embodies a vibrating contact 28 which is grounded and arranged to alternately engage a pair of fixed contacts 29—29 respectively having connection with the extremities of the primary winding 26 of the power transformer. The vibrating contact is operatively associated with an actuating coil 30 connected between the vibrating contact 28 and one of the fixed contacts 29. A mid-tap 31 on the primary winding is connected through a conductor 32 and a transmitter push button switch 34 to the ignition switch circuit of the automobile, thus permitting the supply of electrical energy for the oscillator to be obtained from the storage battery of the automobile. A buffer capacitor 35 for the vibrator is connected across the ends of the secondary winding 24 of the power transformer.

The double frequency operation of the oscillator is obtained by alternately connecting into the tank circuit plugin units respectively embodying capacitors 36^a, 36^b and capacitors 37^a, 37^b. This is accomplished, by providing an additional pair of fixed contacts 38—38 which are alternately engageable by the vibrating contact 28 of the vibrator. The capacitors 36^a and 37^a are variable to permit tuning of the tank circuit in the radio frequency range of 200-to 400 kilocycles. By appropriately varying these capacitors with respect to each other, it is further possible to obtain a displacement of one of the frequencies with respect to the other frequency. For example, one of the frequencies may be displaced by 20 kilocycles with respect to the other (300 kilocycles and 320 kilocycles). Capacitors 36^b and 37^b provide temperature compensation. The action of the vibrator thus not only controls tire supply for the oscillator, but switches the frequency of oscillation '4 alternately at the rate of the vibrator, which may be at, for example, 115 cycles per second. The utilization of two frequencies, as explained above, permits variable signal coding and allows a sufficient number of combinations so that it would be unlikely that close neighbors would get the same code. Moreover, it provides an effective method of preventing other radio signals, including radio frequency noise from actuating the closure, thus providing an effective solution to the problem of discriminating against unwanted signals which is quite serious in devices of this character.

The power output amplifier section 10^b has its grid circuit connected to receive the oscillator frequency output, a damping resistor 39 being inserted in the connection to the control grid 12^b so as to prevent the generation of any spurious high frequency oscillations in the oscillation circuit. The plate 13^b is connected with the high voltage side of the voltage doubler, a choke 40 being connected in this circuit to provide load impedance for the plate circuit.

The plate 13^b is further coupled to an antenna 41 through a coupling circuit containing a blocking capacitor 42 and resistor 43 which acts to place the capacitor 42 at ground potential, and also offers an impedance to ground so that an A.C. oscillation or antenna swing is obtained with respect to ground.

In transmitters for radio remote control of closures such as garage doors, it is the conventional practice to utilize a separate antenna for broadcasting the control signal from the automobile. The present invention is arranged to make use of the antenna normally utilized for the radio receiver which is usually mounted in the automobile. This is accomplished by incorporating suitable contacts in the transmitter switch 34. Normally closed contacts 44 connect the radio with the antenna 41, and normally closed contacts 45 connect the ignition switch circuit through a resistor 46 to the plate 13^b. When the transmitter switch button is depressed, the above noted normally closed contacts will be opened, and normally open contacts 47 will be closed so as to connect the transmitter to the antenna 41 in lieu of the radio. At the same time, normally open contacts 48 are closed to connect the conductor 32 with the ignition switch circuit and energize the power supply for the oscillator, when the ignition switch is in the “on” position. Means are also provided to prevent noise being picked by the radio receiver, in the event it is turned on. For such purpose, auxiliary normally open contacts 44^a are arranged to close, as the contacts 44 open, and connect the radio antenna terminal connector with ground. Upon release of the push button, contacts 45 close to connect the resistor 46 and capacitors 20 and 21 in series across the automobile battery. A circuit is thus established which permits rapid discharge of the capacitors and stoppage of oscillations which might otherwise cause objectionable noise disturbances in the radio, should it be turned on.

The filament heaters for the cathodes ll^a and ll^b are connected through a conductor 49 with the ignition switch of the automobile so that, whenever the ignition is on, the heaters will be energized and the tube 10 will be ready for operation whenever the transmitter switch button is depressed. Depression of the transmitter switch button then acts to connect the vibrator and starts its operation, and simultaneously disconnects the antenna 41 from the radio and connect it with the transmitter output. The dual frequency coded signal is then broadcast over the antenna, during the momentary depression of the transmitter switch button, to be picked up by the receiver B, which will be described now in detail.

The Receiver B

The receiver, as shown in FIG. 2, is arranged to pick up the transmitted signal on the antenna 56. As previously explained, a signal composed of alternately transmitted radiations of different frequencies is conducted from the antenna through a wide band amplifier stage 51 which is

3,090,959 through bypass capacitors 85^a and 85^b which serve to filter out radio signal frequencies. By connecting the cathodes 84^a and 84^b to the —150 volt side of the power supply, through resistors 86^a and 86^b, a small voltage, which is positive with respect to ground and the grid, is placed upon the cathodes. Gain and current flow are thus stabilized and better operation obtained. Compensation for changes due to tube gain, etc. is thus automatically obtained.

The triode sections 72^a, 72^b are respectively controlled by grids 86° and 86^d. The grid circuits are such that, when no signals are being received, the grids will have a negative bias thereon such that the tube sections will not conduct. This bias will be of a value established by the series connected resistors 87, 88 which form a voltage divider between ground and the —150 volt side of the power supply unit.

Plate anodes 87^a and 87^b are connected through load resistors 88^a and 83^b in parallel plate circuits with the +150 voltage side of the power supply unit. The resistors 88^a and 88^b are respectively associated with capacitors 89^a and 89^b which are connected between the plate and cathode of the associated triode section so as to form a filter for eliminating radio frequency components from the plate circuit and leave square waves having an interrupted rate as determined by the vibrator of the signal transmitter.

The output amplifier stage 55 utilizes pentode tubes 90^a and 90^b having control grids 91^a and 91^b respectively, and plate anodes 92^a and 92^b. The grid 91^a is capacity coupled with the plate 87^a of the detector stage through a capacitor 93^a, while the grid 91^b is similarly coupled to the plate 87^b through a capacitor 93^b. The capacitors 93^aand 93^b act to discriminate against direct current flowing through the control grid as a result of the generation of unwanted continuous waves from aircraft beacons, etc.

Grid bias is applied to the grids 91^a and 91^b by a conductor 94 from the juncture of resistors 89 and 90 of the power supply, a connection being made to each grid respectively through a resistor 95^a and 95^b. The plates 92^a and 92^b are connected in parallel to a common plate circuit which includes an operating coil 96 of relay 56 having normally open contacts 97 and 98 which are adapted to energize, upon closure, the local actuator or other device 57. Closure of the contacts 97 and 98 operate to source source power coupled with separate radio frequency amplifier stages 52, each of these amplifying stages having its output connected with a tunable plug-in signal selecting filter network 53. The two frequencies of the signal are thus separated and respectively conducted through separate channels to associated detector stages 54 where the respective signals are transformed into square waves having phase displaced relationship. The square waves are then conducted to output amplifier stages 55 of a control circuit wherein the waves are combined to form a D.C. current capable of operating a control relay 56 for an actuator of the door closure or other device to be controlled, as generally indicated at 57, when both signal frequencies are being received, but which current is insufficient to operate the relay when only one signal frequency is being received. Power supply for the receiver is obtained from a power supply unit as generally indicated at 58.

More specifically, the antenna 5® is connected through a suitable antenna load resistor 59 to ground. The antenna side of the resistor 59 is connected through a resistor 59' with a control grid 60 of tube 61 which comprises the amplifier stage 51. Cathode 62 of the tube connects with ground. Plate anode 63 has a plate circuit including a load resistor 64 connected with a capacitor 65. The juncture between the resistor 64 and capacitor 65 is connected, through a resistor 66 and resistors 67 and 68 of the power supply unit, with the positive side of the potential source. The resistors 66, 67 and 68 cooperate with the capacitor 65 to provide decoupling and prevent feedback which might cause unwanted oscillation or degenerative feedback.

The plate 63 of the tube 61 is further coupled through a capacitor 69 with parallel tuned signal selecting channels which respectively include a radio frequency amplifying stage 52, a signal selecting filter network 53 v/hich is tunable to one of the two broadcast frequencies of the control signal, and a detector stage 54. The amplifying stage 52 and detector stage 54 for the respective signal channels are obtained by utilizing multisection tubes 70® and 7®^b. Each of these tubes has a pentode section 71^aor 71^b constituting the amplifier stage, and a triode section 72^a or 72^b constituting the detector stage.

Both amplifier and detector stages for each of the signal channels are similarly arranged. Grids 73^a and 73^bare respectively connected through damping resistors 74^aand 74^b directly with the coupling capacitor 69, and also through a resistor 75 to ground so as to provide grid control circuits for the amplifier stages of the two signal channels. Plate anodes 76® and 76^b are connected into paral- secondary winding 105 which supplies the plate potential, lei plate circuits containing primary windings 77® and 77^b —^J------^J-------.•-ji-- -----jof radio frequency transformers 78® and 78^b by which the plate circuits are respectively coupled with secondary windings 79^a and 79^b, in the grid control circuits of the detector stages 54.

supply 115 volts 60 cycle current from a power 10® over conductors 101 and 102. This power is also connected with a primary winding 103 of supply transformer 104, this transformer having a and a secondary winding 106 which supplies the cathode heating filaments of the tubes.

The relay 56 is set so that it will pick up when a 24 volt peak-to-peak signal is received from each of the detector 55 stages, that is, when both radio frequencies are received in

Tuning capacitors 80® associated with transformer primary winding 78^a, and tuning capacitors 80^b associated with transformer primary winding 78^b permit the output plate circuits of the two signal channels to be respectively tuned to the two code frequencies of the received transmitted signal. Feed back from the plate circuit to the grid circuit of each pentode section is accomplished by utilizing pickup coils Sl^a and 81^b in coupled relation with the transformer secondary windings. One side of the pickup coil is grounded, while the other side is connected to the grid control circuit through a feedback limiting resistor 82^a or 82^b, these resistors being shunted by suitable bypass capacities 83^a and 83^b, repectively.

In order to facilitate convenient and rapid change of the receiver unit to selectively accommodate it for a desired combination of two signal frequencies, it will be noted that the associated transformer tuning capacitors, pick up coil and feed back limiter have been embodied in a plug-in unit.

The cathodes 84^a and 84^b are connected to ground the transmitted signal from the transmitter, but will not pick up when only one of the frequencies is being received. Moreover, the capacitor 93^a and resistor 95^a, and the capacitor 93^b and resistor 95^b are respectively arranged to 6θ make the bias on the associated grids more negative when the signals from the detector stages exceed 24 volts.

It will be appreciated that in a radio receiver of this character, the receiver will be responsive to variations in the amplitude of the signal which is transmitted from the 65 sending transmitter. The transmitter and receiver are coordinated to operate within a transmitting range of for example, 100 feet, at v/hich distance an amplified signal of 7 volts peak-to-peak would be applied to the grids of the detector stage tubes. However, as the automobile carrying the transmitter approaches the closure, this signal may exceed 7 volts and might under some circumstances go as high as 150 volts. Obviously, provision must be made to stabilize the signal voltage in order to obtain reliable operation, and this is done by the provision of a resistor 107 and capacitor 108 which are connected in parallel in

3,090,959 a common conductor of tide grid circuits for controlling the grids 86^c and 86^d of the detector stage, thus providing an automatic volume control which will regulate the bias so as to limit the signal voltage. Moreover, as previously explained, when no signal is received, there will be 5 no potential induced across the secondary windings 7?^aand 79^b, and the bias on the grids 86^c and 86^d will under such circumstances be reduced to substantially 3.5 negative potential, at which bias the triode sections will not conduct. jq

As an added protection against abnormal operations, which might result from neighboring installations broadcasting a signal in an unused channel, means are provided to automatically control the gain of the amplifier stage 51. For such purpose, resistors 107^a and l®7^b are connected 45 in series to form a voltage divider between the grid connection side of resistor 107 and ground. The resistor 107^bis in parallel with a capacitor 108^a and the juncture of resistors 107^a and 107^b is connected to one end of resistor 59. 20

Terminal connections 109 are brought out to permit connection of a switch in the form of a push button, key switch or the like as generally indicated at 110, which may be operated to closed position, and in this position will ground the control grid of the output amplifier stage and 25 thus remove the negative bias and cause the relay 56 to operate. As an alternative arrangement, provision may be made for operating the relay directly from the power supply. To this end, one of the connections 169 may be broken at a point as indicated at 109' and a resistor 116' 30 connected between the switch 110 and the conductor interconnecting the plates 92^a and 92^b. With this arrangement operating voltage is applied directly to the relay, when the switch 110 is closed, so as to be independent of the tubes in the output stage 55. 35

As an additional safety feature, a capacitor 111 is connected across the operating coil of the relay. The purpose of this capacitor is to introduce a slight delay in the actuation of the relay and thus guard against abnormal operation of the relay which might be caused by 40 the flow of spurious currents, due to lightning or other causes, in the plate circuit of the output amplifier stage tubes.

It will be noted that the relay 56 is a normally open contact relay which closes its contacts when energized .. by a received signal. Reversal of operations in the case of a door closure are obtained by a cycling mechanism (not shown) which is included as a part of the actuator 57, and which is so arranged that upon alternate actuations of the relay 56, the closure will be alternately opened and closed.

In connection with the receiver described above, provision is made for optional connection of safety control mechanism. For such purposes the necessary interconnecting circuits are brought out to a terminal board 112^awhich is arranged for interconnection by means of conventional connectors with a companion terminal board 112^b of the safety control mechanism C, which will now be described.

Safety Control Mechanism C

OU

In conventional closure operating mechanisms, it has been the usual practice to provide auxiliary control on the closure which would operate to stop the closure movement in the event that an object is contacted by the door during its movement. These auxiliary safety device 65 depended upon actual contact in order to operate, and in the case of the door striking a person, injury might be inflicted before the actual movement of the closure could be terminated. The present safety control mechanism contemplates the establishment of a light beam along the 70 lower inside edge of the door, and the safety control is so arranged that interception of this light beam will operate to reverse the direction of travel of the door closure. Thus, actual contact of the door with an object or person is not necessary to initiate the safety operation. 75 .8

More specifically, the safety control of the present invention utilizes a duo-triode tube 113 having a triode section 114 and triode section 115. The triode section 114 has a cathode 116, a control grid 117 and a plate 118, while the triode section 115 has a cathode 119, a grid 120 and a plate 121.

Power supply is obtained through a transformer 122 having a primary winding 123 connected with supply conductor 124 and 125 w’hich connect with the 115 volt 60 cycle power source 100, as shown in FIG. 2. It will be noted that the conductor 125 is directly connected, while conductor 124 is connected only when the contacts 98 of relay 56 are in closed position. Thus, it will be appreciated that the primary winding of the transformer 122 will be energized while the door closure is being raised or lowered, and will be deenergized when the door closure is in either raised or lowered position. The transformer 122 has a low voltage secondary winding 126, for example, 28 volts A.C. One end of the secondary winding 126 is directly connected to a conductor 127 which is grounded and forms one side of a direct current supply circuit, while the other end of the winding 126 is connected through a rectifier 128 to a conductor 129 which forms the other side of the direct current supply circuit. The direct current supply is connected across a capacitor 136 which is in parallel relation with a resistor 131.

There is also connected across the D.C. supply of 28 volts a photo-conductive cell 132 in series with a resistor 133, the photo-conductive cell and resistor cooperating to form a voltage divider. The photo-conductive cell is mounted on the lower edge of the door closure, and is cooperatively associated with a light source shown as comprising two electric lamps .1.34 which are adapted to produce a light beam which is focused upon the photoconductive cell. This light beam falling upon the photoconductive cell reduces its resistance, and conversely when the beam of light is interrupted for any reason, for example, by an object in the path of movement of the garage door at right angles to the light beam axis, the photoconductive cell will increase its resistance. Advantage is taken of this characteristic to control the grid 117 of the triode 114.

The grid 117 is connected through a coupling capacitor 135 with the juncture connection between the photoconductive cell 132 and resistor 133. A grid leak 136 is connected directly between the grid 117 and the grounded conductor 127.

Plate potential is supplied to the plate 118 from the positive side of the 150 volt potential source through a resistor 137. The plate 118 is also connected to the negative side of the 150 volt potential supply through series connected resistors 138, 139 and 14®. The control grid 126 of the triode section 115 is connected at the juncture of the resistors 139 and 14®, whereas a point 141 at the juncture of resistors 138 and 139 is connected through a rectifier 142 with the conductor 129. The output circuit of the triode section 115 is formed by connecting the plate 121 by means of a conductor 143 to the side of the operating coil 96 of relay 56 which is connected with the plates 92^a and 92^b of the output amplifier stages shown in FIG. 2. With this connection, the relay 56 will be energized whenever the triode section 115 conducts.

The operation of the safety control mechanism will now be considered. Assuming the door is being raised or lowered, the transformer 122 will be energized and supply 28 volts D.C. to conductors 127—129 by way of rectifier 128 acting in conjunction with filter capacitor 13®. The door operation having been initiated, the door actuator will be locked in by interlocking switches (not shown) contained in the actuator 57, and the relay 56 will be permitted to open its contacts without interrupting the energization of the transformer 122. By virtue of the connection from point 141 through the rectifier 142, the point 141 can be at a voltage lower than the voltage of 28 volts D.C., but not higher than this voltage. Any

3,090,959 ⁹higher voltage will be discharged through the capacitor 130 to ground.

Assume now that during the door movement, the light beam falling upon the photo-conductive cell 132 is interrupted. The resistance of the photo-conductive cell in- 5 creases with the result that the change in potential drop thereacross will act to make the bias on the grid 117 more negative. The plate current will therefore be decreased and the grid bias on grid 120 increased to a value which will permit the tube to conduct and thus operate the relay 56 to reverse the door movement previously initiated. The use of the capacitor 135 in series with the grid leak across the resistor 133 provides an arrangement wherein slight changes will be immediately effective to change the bias on the grid 117, but if the changes persist then the voltage may be dissipated through the grid leak 136 to ground.

Since the transformer 122 is so connected that it will be deenergized when the door is in raised or lowered position, it will be appreciated that since the lamps 134 are connected directly across the secondary winding 126 of 20 the transformer, these lamps will be extinguished, and that this would normally have the same effect as if the light beam were intercepted by an object in the line of travel of the door. Consequently, provision is made for distinguishing this condition from the situation in which 25 the light beam is interrupted by an object. Thus, upon deenergization of transformer 122, the voltage across the capacitor 130 will be discharged through the resistor 131 so that conductor 129 will drop from 28 volts D.C. to ground potential. Since the point 141 cannot be at a higher potential than the conductor 129, the point 141 will likewise go to ground potential. Under such circumstances the resistors 139 and 140, acting as a voltage divider, will bias the grid 120 below cutoff potential, thus overriding the control effect from the photo-conductive 35 cell due to interruption of the light beam thereon. Operation of the relay under these circumstances will not be initiated.

Various modifications may suggest themselves to those skilled in the art without departing from the spirit of 49 the herein described invention, and, hence, I do not wish to be restricted to the specific form shown or uses mentioned, except to the extent indicated in the appended claims.