US2941084A - Receiver for light-flash signaling system - Google Patents

Description

June 1960 F. FRUENGEL 2,941,084"

RECEIVER FOR LIGHT-FLASH SIGNALING SYSTEM 7 Filed Dec. 12, 1952 2 Sheets-Sheet 1 INVEN TOR. Frmw: FRUENG'EL Am/ZNQL June 14, 1960 F. FRUENGEL 2,941,084

RECEIVER FOR LIGHT-FLASH SIGNALING SYSTEM Filed Dec. 12, 1952 2 Sheets-Sheet 2 FIG. 3

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United States Patent RECEIVER FOR LIGHT-FLASH SIGNALING SYSTEM Frank Fruengel, 251 Tinsdaler Kirchenweg, Hamburg-Rissen, Germany Filed Dec. 12, 1952, Set. No. 325,511

Claims priority, application Germany Jan. 2, 1952 8 Claims. (Cl. 250-214) The present invention relates to a receiver for light flash signals and particularly to an electronic apparatus for receiving and converting into distinctly visible or audible signals the light flashes sent out by an electric impulse lamp as described and claimed in my companion application, Serial No. 325,512, filed December 12, 1952 which matured on June 5, 1956, into Letters Patent 2,749,482. The system referred to, while it can be used for communication at relatively short distances between moving stations anywhere, is especially destined to serve as safety means in road traflic and may be termed overtake signal system, since it enables motor cars on the road to signal and thereby caution vehicles, especially trucks, ahead before overtaking.

It is an object of this invention to provide a receiver which is particularly adapted to receive the light impulses of spark discharge character, as transmitted by a cooperating impulse lamp, and to convert them into readily perceptible signals. Therefore light flashes of spark discharge character or spark discharge flashes to be understood as clearly defining in this specification and the claims such light flashes as having each one the extremely short duration between 10' and second and a steepfronted wave shape giving rise to a momentary increase in luminous flux at a rate of more than 10 lumens per 1 microsecond.

Another object is to provide a light flash receiver which is so sensitive and reliable that all advantages rendering the light flash signaling system superior to acoustic systems are fully utilized, and this chiefly on account of the fact that the receiver responds only to the light flashes transmitted to it and does not react upon light impressions that are happening in nature or are incidentally made by men.

For carrying the invention into effect, the receiver for the particular light flashes sent out by the impulse lamp is provided with a pickup or converging lens before a blue light filter upon which the light of the impulses falls. Behind the filter there is a phototube having preferably a cesium-antimony cathode and being particularly sensitive to light in the blue and violet region of the spectrum, because it is especially such light that emanates fiom an electric spark being the light source of the electric impulse lamp serving as transmitter. The light filter provided serves to retain red and yellow light which would harm the phototube by heating when direct sunlight would fall on it. I

The voltage impulses caused in the phototube by the incident light flashes are conducted to electronic amplifier stages, whereby a circuit of properly related capacitance and resistance, according to this invention, admits only impulses of a duration of less than about 5 X 10 second for amplification, whereas impulses of longer duration can penetrate only weakened or not at all. This fact renders the receiver insensitive. to strange light impulses "ice p in receiving only the light impulses sent out by the transmitter.

so tuned that preferably only frequencies between 100 incidentally occurring in its vicinity, and thus very reliable and 1,500 kilocycles are amplified, the voltage impulses reach the grid of the first tube of a relaxation type oscillator'having two tubes. This relaxation circuit is so designed that during the relaxation period a relatively high anode current flows through the second tube of the oscillator, which suflices to cause a glow lamp to light, or a relay to respond. The relay in turn may close the circuit of an incandescent lamp or a buzzer, which then gives a clearly perceptible signal.

Since the relay actuated by the anode current in accordance with this invention, can be only very small, its contacts are able to switch only minute currents; therefore a second relay in a separate circuit is provided to be actuated by the relay in the anode circuit. This second relay can be retarded in its action by an auxiliary condenser and can so effect that after each energizing impulse the indicating lamp or the buzzer remains on for about one second. If in such arrangement the light impulses of the transmitting impulse lamp occur at a higher rate than one per second, the second relay will remain continuously closed for the whole period the light flash transmitter is in action and thus the indicating lamp or buzzer will also be on continuously throughout such period.

Since the light flash receiver as provided by this invention is primarily intended for operation on motor trucks or the like, the pickup lens must be accessible for the light flashes sent out by the car behind, and therefore must be located at the rear of vehicles of above type where it is difficult to protect the receiver from weather and other external influences. In order to obviate the danger of damage, this invention provides that in certain cases only lens and phototube as a separate unit are mounted at the rear end of the vehicle and that the receiver is placed remote therefrom in a safe location near the drivers cab. The connection between both units is then in the form of a cable of low capacitance such as a coaxial cable.

For a clearer understanding of the foregoing and further novel features of the present invention, some preferred embodiments will now be described by way of example in connection with the accompanying drawings in which:

Fig. 1 represents a complete circuit diagram of a light flash receiver according to this invention;

Fig. 2 illustrates a modified relay circuit for actuating the output signaling means;

Fig. 3 shows a partial circuit diagram indicating the manner of connecting the pickup or phototube unit with the amplifier unit in an installation where these two units are remote from each other;

Fig. 4 shows another partial circuit diagram indicating modified means for connecting the two units; and

Fig. 5 illustrates still another modification of the connections shown in Figs. 3 and 4.

Referring to the circuit diagram Fig. 1, there can be seen at the left the phototube 1 which is reached by the incident light impulses of the transmitter through a converging lens 29 and a blue filter 30. The sesiumantimony cathode 1k of the phototube 1 has its optimum sensitivity of about 7 quanta per electron in the blue and violet region of the spectrum, which is the spectral region of an electric spark forming the light source of the transmitting impulse lamp. The incident light fiash causes a negative voltage impulse on the anode 1a otfl the phototube 1.- This. negative voltage impulse'fpro- Patented June 14, 1960 gresses through the anode connection and through the 7 coupling condenser 2 of about 50 micromicrofarad to the grid 4g of the first amplifier tube 4. A grid leak resistor 3 of about 0.1 megohm is connected to a junction point in the connection between condenser 2 and grid 4g, The combination of the selected capacitance (50 micromicrofarad) and resistance (:1 megohm) results in a time constant of 5() 10- l0 =5 10* second. This time constant has the'efiect that substantially only voltage impulses of a duration of less than 5 l0- second can reach the grid of 4g of the amplifier tube 4. Longer lasting impulses are markedly weakened or cannot pass at all. By exact design calculation of the constants in the circuitshown and by proper choice of components particularly the capacitive coupling means, it has been achieved that the receiver responds to, and amplifies only light flashes lying in the blue and violet spectral region and having a steep gradient, i.e., being suchlthat their luminous'fiux rises at a rate steeper than 100 lux per microsecond. For light impulses with such steep gradient, impulses as sent out by the impulse lamp described in my companion patent application previously referred to, the receiver is particularly sensitive. The anode resistors 10, 11, 12 of the first threeamplifier tubes 4, 5, 6 have relatively low resistance, only a few thousand ohms. Thisis necessary in order to'obtain uniform amplification of the high-frequency voltage impulses in spite of the fact that the tube capacitances are bridged by these resistors. Amplification of only high-frequency impulses, or impulses of steep-fronted wave shape, is important, because no benefit can be had from amplification of lowfrequency voltage changes. On the contrary, such amplification would render the receiver less disturbancepro0f, since it could then react to light effects other than thetransmitted flashes. The restriction to only highfrequency amplification necessitates operation withlow amplification factors of the order of m 30 per stage. In the frequency range between 100 to 1,500 kilocycles,

amplification is then substantially linear. At higher fre-' quencies, the amplification decreases and is at 3,000 kilocycles already 2' to 3 neper lower than maximum amplification. 7 i The voltage impulse, which occurs negative at the anode 1a of the phototube 1, becomes positive behind the first amplifier tube 4, negative behind the second amplifier tube 5, and again positive behind the third amplifier tube 6. The stages 4 and 5, and 5 and 6, respectively, are capacitively coupled by coupling condensers 8 and 9, respectively. The tube 6 operates with a high anode current. Its cathode 6k is connected with cathode 7k of tube 7 and is not bridged by a condenser. Now, when a negative voltage impulse arrives at grid 6g of tube'6, the anode current of this tube Willdecrease and consequently the cathode 6k becomes more negative. Simultaneously therewith, tube 7 becomes conductive due to the fact that a positive voltage impulse reaches its grid 7g. 'In'consequence thereof, the voltage at the cathode resistor 13 is now raised more than it had been decreased by the current drop at tube'6, and this because a stage of amplification lies therebetween. Increase of voltage on cathode resistor 13 renders the cathode 6k more positive and the grid 6g in relation thereto more negative, whereby this negative potential is additive to the original small negative impulse received from the second amplifier tube 5. Thus, the negative charge of grid 6g increases by cumulative action until the tube 6 is completely cut oil, whereas tube 7 becomes fully conductive. Return to the original condition occurs then in accordance with the time constant established by grid condenser 14 (50 micromicrofarad), interposed between tubes 6 and 7, and the grid leak resistance 15 (1 megaohm) inSOX 10- X10 =50X 10* second; During this relaxation period of about second, there flows through tube 7a relatively high anode current (about 10 milliamp'eres) which is'sufijcient to energize relay coil 16, or to light a glow lamp which may be inserted instead of the relay. The relaxation type oscillator circuit just described is so sensitive that an impulse of about $4 volt and a duration of 10- second applied to grid 6g is able to start a relaxation cycle of the tubes 6, 7.

Since the energy that can be drawn from the anode output of tube 7 for energizing relay coil 16 is very low, its contacts 17 can handle only small currents which in some cases are not suflicient to give positively perceptible signals. For example, if it is desirable to light one or several indicating lamps, including a return-signal lamp for giving a confirming signal to the car in rear, or to sound a'buzzer, it has been found advisable to interpose another type of relay circuit between the points A, B and C in the anode connection. Such a circuit, as indicated in Fig. 2, includes the relay coil 16 which in this case actuates a normally closed contact 18 interposed in a battery circuit which therefore normally serves to energize a second relay coil 19 which, when energized, holds its contact 20 open. 'The contact 20 is inserted in a battery circuit providing current for the signaling means shown in the figure as an indicating lamp 22, a buzzer 23 and a return-signal lamp 37. It willbe noted that when relay 16 is energized, it opens contact 18 and de-energizes thereby the second relay coil 19, which in turn releases its armature and closes thereby contact 20 and the circuit for the signaling means. This second relay coil 19 can be considerably larger and its contacts can handle currents which are adequate to energize most effective signaling means. In the arrangement just described it is possible to delay the action of the second relay by interposing a condenser 21 (about 4 microfarads) in the auxiliary circuit and achieve thereby that this relay remains closed for about one second after each impulse from the relaxation circuit, and thus causes that also a signal lamp lights for such a prolonged period. In this manner not only the single impulses are made to cause a perceptible signal but it is also achieved that whenever the sequence of the light flashes of the transmitter impulse lamp is faster than one flash during each second, the signal-giving lamps 22, 37 lights or/ and the buzzer 23 sounds continuously. during the whole onperiod of the light flash transmitter.

In order that it is readily accessible for the light flashes transmitted from a car following behind, the receiver, if constructed as one unit, .must be located at the rear end portion, for instance, of a motor truck or the like. Here, as a rule, it is'exposed to the weather and other unfavorable influences. To better protect the receiver, it is advisable to dispose it in a safe place near the drivers cab and to mount only the pickup lens and the phototube at the rear of the vehicle. The connection between such pickup unit, which serves actually as a light antenna, and the receiver proper is, according to this invention, in the form of a cable of low capacitance in order to prevent weakening of the incoming signals as much as possible. The phototube 1, as shown in Fig. 3, is here placed within the focal length of an optical system, preferablya lens system 31, in such a way that the phototube can receive the light from a relatively large space angle toward the rear. In a practical embodiment the phototube is mounted in a Searchlight-shaped housing having a suitable converging lens system as front covering. Such a unit can be made very sturdy and can be readily mounted by means of a support which may be adjustable in height and flexible in such manner that it will yield when striking an obstruction Without being damaged. A suitable cable for the connection to the receiver is an antenna cable 36 as used in auto-radio installation. Such cable has generally a capacitance as low' as 12 micromicrofarads per meter, which is low enough to not materially impair the sensitivity of the light flash receiver.

Where no loss of incoming signal strength can be tolerated, it is, in accordance with this invention (Fig; 4),

possible to insert adjacent to the phototube 1 a pulse formers having cores of laminated material, which have the advantage of transmitting such single impulses in a considerably broader frequency band than so-called highfrequency transformers with high-permeable cores. This surprising effect is due to the fact that extremely short impulses cause magnetization of the iron at one flank of the hysteresis loop only. Thus the magnetization is merely pushed upward, whereas return to normal is left to take place gradually withoutrapid decay.

For obtaining highest sensitivity of light flash reception it is advisable to amplify the voltage impulses directly at the phototube in somewhat similar manner as in a condenser microphone. However, the application of an electronic tube for this purpose is not practicable, since such tube is rather delicate for the expectable rough service conditions and it requires too many connections for the various tube elements, whichit would be necessary to accommodate in the connecting cable to the pickup unit. For this reason, the present invention contemplates to' employ a semi-conductor triode or transistor 35 (Fig. 5) placed adjacent to the phototube 1 and designed for amplification of the current value of the voltage impulse by operating with battery voltage or equivalent as bias Voltage for the transistor. In such an arrangement it is essential to correlate the output impedance of the transistor and the wave impedance of the connecting cable 36 for most eficient passing of the signal impulse. For such proper correlation it may be advisable to interpose in this circuit a pulse transformer between the transistor and cable as indicated at 32 (Fig. 5) and in a manner somewhat similar to the arrangement shown in Fig. 4.

In order to prevent disturbances which could be introduced into the receiver system through the connecting cable from faulty ignition systems of passing cars or the like, the conductors of the cable must preferably be arranged symmetrically with respect to the phototube and the whole cable must be provided with a grounded shielding as indicated in the Figs. 3 to 5.

The power for the receiver, particularly the plate current of about 200 volts D.-C., can be supplied by any suitable well-known means. It is advisable, as shown in Fig. 1, to convert direct current of, say, 12 volts by means of a double-acting vibrator 24 and inverter transformer 25. Such double-acting arrangement gives the assurance that the current supply is sustained even then when one-half of the vibrator contacts fails. Dry-disk rectifier 26 serves to complete the power source in a generally known manner. Indicating lamp 27 shows when switch 28 is closed and the receiver is in operation.

Since the receiver in accordance with this invention amplifies only high-frequency voltage impulses, it cannot be disturbed by A.-C. hum resulting from insufiicient anode voltage filtering. A simple vibrator interference suppressor, which eliminates high-frequency distrubances and furthers interference-free car radio reception too, will generally suflice.

The circuits herewith described are only a few examples of several ways in which this invention may be carried into eflect, and it is obvious that many changes in circuits and application of elements can be made without departing from the spirit and scope of this invention. It is, for instance, possible to employ either less or more amplifier stages and it is also possible to combine the tubes 6 and 7 to a double tube in a single envelope. A modification is also feasible in which the amplified voltage impulses are applied to raise the potential of a control grid of a discharge tube which thereby is made conductive for current actuating the relay 16. Such arrangement, however, has the drawback that it requires relatively high amplification and it has been found that the relaxation method as described is to be preferred since it is simpler.

When speaking of light flashes in this specification and particularly in the claims, this expression is intended to be interpreted as including also flashes of light in the infra-red and ultra-violet spectral region.

What I claim is:

l. A receiver for light flash signals of spark-discharge character, comprising a photosensitive device having a photocathode sensitive to radiations near the upper frequency boundary of the visible spectrum and operative in converting incident spark light flashes into voltage impulses by photoelectric action, electronic amplifying means coupled to said photosensitive device for amplifying said voltage impulses, timing filtering means interposed between said photosensitive device and said amplifying means and having a time constant of somewhat longer duration than the duration of the light flashes to be received, thus being adapted to pass substantially only voltage impulses resulting from spark light flashes having a rise in luminous intensity steeper than lux per microsecond and to prevent the passage of voltage fluctuations caused by light effects of non-spark-discharge character to said amplifying means, and signal means connected to said amplifying means to be actuated by amplifier voltage impulses.

2. A receiver as in claim 1, wherein said filtering means includes a coupling condenser and a grid leak resistor forming a circuit member whose time constant is of somewhat longer duration than the duration of the light flashes to be received.

3. A receiver for light flash signals of spark-discharge character, comprising a photosensitive device having a photocathode sensitive to radiations near the upper fre quency boundary of the visible spectrum and operative in converting incident spark light flashes into voltage impulses by photoelectric action, a network connected to said photosensitive device including at least two stages of electronic amplifiers, capacitive coupling means, each including a coupling condenser and a grid leak resistor, in circuit between said photosensitive device and the amplifier stage and between the amplifier stages, the electrical parameters of said coupling means being such that substantially only voltage impulses originating from light flashes having a rise in luminous intensity steeper than 100 lux per microsecond are amplified, and signal means connected to the last amplifier stage to be actuated by said voltage impulses.

4. in a light flash receiver responsive to light flashes of spark discharge character, in combination, a photosensitive device having an anode and a photocathode, electronic amplifying means including at least one amplifier tube having a plate, a cathode and a control electrode, said amplifying means being capable of amplifying voltage impulses resulting from spark light flashes impinging upon said photocathode and having a rise in luminous intensity steeper than 100 lux per microsecond, coupling means interposed in circuit between said photosensitive device and said amplifier tube, said coupling means comprising a condenser connected between said anode and said control electrode, a ground connection for said photocathode, a grid leak resistor joined at the junction between said condenser and said control electrode, the electrical parameters of said condenser and said grid leak resistor being such that these elements form a resistance-capacitance circuit having a time constant of the order of 10- second, a source of D.-C. potential connected to the junction of said anode and said condenser, and output means connected to said amplifying means. 7

5. A receiver for light flash signals of spark-discharge character, comprising a photosensitive device having a photocathode sensitive to radiations near the upper frequency boundary of the visible spectrum and operative in converting incident light flashes into voltage impulses by photoelectric 'action, a network connected to said photosensitive device including at least two stages ofeleetronic amplifiers, filtering means interposed between said photosensitive device and the first stage of said amplifiers including a coupling condenser and a grid leak resistor forming a circuit member adapted to pass substantially only voltage impulses resulting from spark light flashes and to prevent the passage of voltage fluctuations caused by light effects of non-spark-discharge character to said amplifiers, an anode output circuit in the last stage of said amplifiers, a relay having an energizing winding and first normally closed contacts, said energizing winding interposed in said output circuit, an auxiliary power source, a relay having an energizing winding and second normally closed contacts, circuit'means connecting said power source'through said first normally closed contacts of said first-named relay to the energizing winding of said last-named relay for holding said second normally closed contacts in open position, signaling means, second circuit means connecting said power source through said second normally closed contacts of said last-named relay to said signalling means, whereby the contacts of said first-named relay are opened and the contacts of said last-named relay are released to close when a voltage impulse passes said anode output circuit, and a condenser bridging the energizing winding of said last-named relay and being effective in slowing the action of said last-named relay for keeping its contacts continuously closed when said first-named relay operates at a higher than a predetermined rate. V

6. A light flash receiver responsive to light flashes of spark discharge character substantially as described, comprising in combination a photosensitive device having a photocathode sensitive to radiations near' the upper frequency boundary of the visible spectrum and operative in converting incident spark light flashes into steep-fronted voltage impulses by photoelectric action, a converging lens in front of said photocathode for concentrating incident spark light thereon, a blue light filter interposed between said lens and photocathode said blue light filter being adapted to augment the radiation pattern inherent in spark light for which said photocathode is particularly sensitive and to retain substantially red and yellow radiation, electronic amplifying means coupled to said photosensitive device for amplifying said voltage impulses, filtering means interposed between said photosensitive device and said amplifying means adapted to pass substantially only steep-fronted voltage impulses converted from spark light flashes, said filtering means including a coupling condenser and a grid leak resistor forming a circuit member Whose time constant is of somewhat longer duration than the duration of the spark light flashes to be received, and signaling means connected to said amplifying means to be actuated in response to spark light flashes received.

7. A receiver as in claim 1, wherein said electronic amplifying means include a relaxation type oscillator circuit tuned for a predetermined relaxation period of suflicient duration to cause rise of said voltage impulses to adequate intensity for actuating said signal means.

8. A receiver as in claim 1, wherein said relaxation type oscillator circuit includes a pair of tubes having each anode, cathode and control'grid, a grid condenser and a grid leak resistor interposed in circuit between said tubes, a signal means energizing means connected to the anode. of one of said tubes, 'said grid condenser and grid leak resistor being tuned for a time constant substantially equal to said predetermined relaxation period.

References Cited in the file of this patent UNITED STATES PATENTS 1,859,822 Engl May 24, 1932 1,876,109 Van der Pol Sept. 6, 1932 1,976,120 Francis Oct. 9, 1934 1,988,349 Beggs Jan. 15, 1935 2,032,958 Stogofi Mar. 3, 1936 2,066,680 Gieskieng et al. Jan. 5, 1937 2,140,350 Dawson- Dec. 13, 1938 2,140,355 Gulliksen Dec. 13, 1938 2,228,560 Cox Jan. 14, 1941 2,234,011 Shepard Mar. 4, 1941 2,499,996 Kelsey Mar. 7, 1950 2,521,946 Rathje Sept. 12, 1950 2,556,961 Feigal June 12, 1951 2,559,173 Shawhan July 3, 1951 2,588,368 Edgerton Mar. 11, 1952 2,596,591 Packard et a1. May 13, 1952 2,608,128 Kelsey Aug. 26, 1952 2,632,040 Rabinow Mar. 17, 1953 2,632,855 Bendz Mar. 24, 1953 2,686,266 Pringle et al Aug. 10, 1954 FOREIGN PATENTS France 2. June '18, 1934 OTHER REFERENCES Zworyzin et al.: Photoelectricity and its application, 1949, John Wiley and Sons, Inc., New York, pages 55 and 114.

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