US2973456A - Lamp flasher with daylight-responsive inhibiting means - Google Patents

Description

Feb. 28, 1961 H. L. R. SMYTH 2,973,456

LAMP FLASHER WITH DAYLIGHT-RESPONSIVE INHIBITING MEANS Filed May 6, 1957 I3 FEE- F 18 '8 12I9TR3 CUT-OFF 20 I9 20 W {5:5 '5 TIME g IO E E I6 22 23 24 m 3 TIME F1 L 5.0K lopx 5 TOTAL RESISTANCE (R4+Ra) a IA 15.2.

INVENTOR HENRY LYALL ROSS SMYTH United States Patent F LAMP FLASHER WITH DAYLIGHT-RESPONSIVE INHIBITING MEANS Henry Lyall Ross Smyth, Ottawa, Ontario, Canada, as-

signor to National Research Council, Ottawa, Ontario, Canada, a body corporate of Canada Filed May 6, 1957, Ser. No. 657,365

2 Claims. (Cl. 315-159) This invention concerns switching circuits for intermittently flashing navigational marker lights, and in particular relates to a novel circuit employing transistor devices for controlling the length and periodicity of flashes of electric lamps and including light responsive devices for discontinuing the flashing during daylight hours.

In general the intermittently illuminated types of navigational marker lights carried on buoys or sited at unattended shore installations are most reliably operated from a battery source of electric supply. Such lights usually operate with a very low duty cycle, for example flashes having durations of less than a second are emitted at the rate of one every three to six seconds. Heretofore the replacement of batteries at remote locations, and the initial cost and maintenance of motor driven switch devices, have contributed to the relatively high annual expense of operating a marker. The unreliability of impracticality of existing photoelectric controls has not assured of realizing economies of supply by shutting down the operation during daylight hours. It is understood that for efficiency a photoelectrically controlled switch must itself not draw an appreciably large current from the battery and in addition the light-responsive control should have fail-safe characteristics. In other words, if the photoelectric element becomes defective, the flashing circuit must continue to operate just as well as before.

The type of sunlight responsive switch most commonly in use comprises a pair of differential expansion mechanical elements, and depends for its operation on the absorption of heat from the suns rays by a blackened element, with resultant expansion exceeding that of a non-absorbing element, whereby to produce a magnified displacement of a lever arm coupled between them to open electrical contacts in the supply circuit. This type of switch has been found critical of adjustment and troublesome to maintain. It functioned positively only on bright days, and in the transition between fully closed contact and fully open contact states was susceptible of erratic operation and gave unreliable switching performance. Such device was therefore far from serviceable when used anywhere but on fixed shore lights with stable platforms and failed to perform properly when used on buoys. Accordingly the important group of buoy-mounted navigational marker lights have required much more frequent replacement of batteries since no reliable and effective daylight control was .available.

The present invention is directed in part to the satisfy- .ing of the requirement for a reliable, eflicient and relatively low cost light-responsive control for limiting lamp operation to periods of darkness and accordingly has as its principal object the provision of a daylight-responsive control for initiating flashing shortly after sunset and terminating the operation shortly before full daylight.

A concomitant object is the provision of a daylightresponsive flashing control circuit which does not affect the flasher cycles in the event that the photosensitive deice,

vice fails. It is a concomitant purpose of the invention to substitute novel cyclic flash controlling arrangements employing transistors in special circuit relation for stably controlling the duration and periodicity of lamp flashes, including means responsive to daylight to discontinue the flashing.

In accordance with the invention, a navigational light flash control includes a free-running multivibrator timing circuit employing transistors whose output controls a switching transistor device, which in turn drives means closing a supply circuit for a navigational light, and further includes a photoconductive light cell associated with the circuit closing means to eifectively disconnect the lamp circuit when the intensity of daylight exceeds a predetermined value. In one embodiment the switching transistor device drives supply circuit closing means which take the form of an electromagnetic relay whose arma ture operates lamp circuit contacts, the relay winding being energizable in parallel from a photoconductive cell in series with the battery supply.

In carrying the invention into effect, a pair of transistors have their bases and collectors cross coupled by resistive and capacitive elements in a multivibrator type of circuit whose output controls the base current of the switching transistor, and accordingly controls the current flowing in the collector circuit. In a first embodiment a relay winding energized by the current controls contacts to prevent the light from going on when the switching transistor is biassed on, and a photoconductive cell such as a cadmium sulphide type is connected in parallel with the emitter-collector circuit whereby to hold the relay operated and thereby disable the lamp circuit when daylight intensity exceeds a given level.

For a more complete understanding of the invention the following description is to be read in conjunction with the accompanying drawing wherein Fig. 1 is a schematic circuit diagram of flash timing system employing an electromagnetically operated flasher and a daylight responsive control;

Fig. 2 is a graph showing the control of the flash periodicity by adjustment of certain values of the circuit elements of Fig. 1; and,

Figs. 3a and 3b respectively refer to the voltage waveforms at the collector terminals of the two stage multivibrator circuit.

The development of transistors has progressed to a stage where they have satisfactory reliability for use in navigational aids and relatively low cost. However the temperature dependence of certain parameters has presented difficulties in realizing practical circuits to obtain a flash cycle in which the periodicity and duration of the fiash do not change appreciably over a range of ambient temperatures experienced at the sites at which markers are installed. The switching performance is generally required to be satisfactory over a range of ambient temperatures from about -40 degrees to +100 degrees F., and operation down to F. is required in airborne applications.

Referring to Fig. 1 a complete switching circuit exemplified in this diagram comprises a pair of like transistors of the PNP junction type labelled TRl and TR2 interconnected in what may be generally referred to as a freeby assuming that the circuit is suddenly energized by the application of battery potentials, for example derived from 6 volt D.C. battery supply (not shown) used for lighting the marker lamp, to connect positive supply to terminal 13 and negative to terminal 14. ResistorsRl and R6 in the collector circuits respectively of TR and TR2 are of substantially equal value, and capacitor C1 is several times larger than C2. Base current will accordingly tend at the first instant to flow in each transistor, the current flow through R2 and C1 gaining control while the base current of TR2 is quickly checked and ceases to flow until capacitor C1 is charged to a certain fraction of total battery potential. At this time the base current in TRl falls to a low value. The capacitor C2 now begins to charge, and due to the potential rise at the collector end of resistor R1 the direct base current through this resistor falls rapid y to zero. At this stage TRl is cut off and TR2 has become fully conducting. As a consequence the potential of the collector end of R6 becomes substantially that of the positive supply, and transistor TR3 has its base current cut off. Accordingly lamp l2 flashes on in response to de-energization of relay winding M. Since capacitor C2 is the smaller, the duration of the on time for TR2 is relatively less than the on time for TRl, hence the off time for TR3 is shorter than its on time. It will be apparent that the on condition of TR3 corresponds to the de-energized condition of lamp 12. Capacitor C1 which has previously been charged discharges through the resistor chain R1, R3, R6, while C2 is charging; the charging current of the latter is poled in the same direction as the discharge current of C1 through resistor R1. Therefore when equal potentials exist across each capacitors terminals further discharge cuts off TR2 and C1 begins to charge. The multivibrator system continues to run as described, TRl having longer duration on states.

In general the duration of the length of the flash is modifiable by adjustment of C1 and R3 while the Olf time is a function of the values of C2 and R4. A rheostat R8 smaller than R4 is included in series therewith to permit control over a small range of the Oh? time. Resistors R2 and R5 are relatively low value resistors for the protection of the transistors against damage by transients which occur as the circuit flips from the flash to the off-time state and vice versa. Resistor R7 is likewise employed to protect TR3 and reduces the loading of TR3 on the timing circuit, being relatively smaller than either R2 or R5. The Off time can be increased substantially by increasing the value of R7. The use of low value collector and biassing resistances throughout the circuit and the returning of bias resistors R3 and the pair R4, R8 directly to the collectors of their respective transistors rather than to the supply bus provides sufficient degeneration of the circuit to compensate for extreme temperature variations. By this provision it has been made possible to stabilize the flash cycle to substantially constant duration and period of flash over the temperature range of about F. to about +100 R, and in fact to attain satisfactory operation somewhat beyond the extremes of the range.

The relay M is chosen to have resistance and operative current characteristics such that a dissipation of only a few milliwatts in the winding suffices to hold contacts 10 and 11 open, and to release on a current difierential which is a small fraction of a milliampere, while its contacts are rated to interrupt resistive lamp currents of several amperes at 6 or 12 volts DC. The photocell P which may preferably be of the passive type is connected in series with the winding of M across the supply. The cell resistance-versus-incident light intensity characteristics are preferably such that it presents an extremely high resistance for the dark state but at a certain light level the resistance falls to a value sufficiently low to pass enough current through the winding to operate the relay, holding the :lamp circuit open. The current drain of the relay therefore is added in this embodiment to the current drain of the transistors which continue to operate in cyclic manner as described. Should the cell P fail, the operation of relay M by the multivibrator and TR3 is not affected and the device therefore fails safe. In such event the lamp will flash continuously until the situation is corrected.

Fig. 2 diagrams the relationship between the total resistance offered by R4 and R8 and the number of flashes per minute produced by the circuit. The solid line 15 extends over a range in which the adjustment of flash rate is preferably made by resistance control at all ambient temperatures normally likely to be encountered. Dashed line 16 extends over that part of the range in which the low-temperature operation tends to be unstable with the large values of resistance required,

hence the variation of size of C1 is preferred for achieving the lower rates. At these slower rates of flashing the sum of R4 and R8 is kept relatively low and C1 may be very considerably increased.

Referring now to Fig. 1 and Figs. 3a and 3b, the voltage variations with time at the respective collectors of TR2 and TRl may be compared. From the cut-off condition the potential rises steeply at 1% to substantially full positive value and remains steady at 18. Above a certain level indicated at 29 the relay current through TR3 drops to close contacts 10, 11 and flash the lamp. As C2 begins to discharge the collector of TR2 rapidly falls to negative supply value and remains steady as indicated by the portion 17. The slowly rising potential curve 22 of the collector of TRI is due to the finite charging time of C1 through its resistive series charging circuit, and terminates at 23 as TR2 goes on and C2 commences to charge. The TRl collector potential drops more rapidly from full positive along curve 24 and terminates at a less positive level at 2-5, when TRl again switches on.

In specific embodiments of Fig. l, the values of components found effective were as follows:

R1, R6 3.3 kilohms.

R2, R5 l kilohm.

R3, R4 33 kilohms.

R7 120 ohms.

R8 67 kilohms variable.

Cl microfarads.

C2 25 microfarads.

TRLTR2, and TR3 Type 2N130 PNP transistors made by Raytheon Mfg. Corpn.

Cell P Cadmium sulphide photoresistive cell, manufactured by the Marconi Company.

RelayM Potter 8: Brurnfield type 225D with windings paralleled for 2500 ohms.

The foregoing elements in circuit arrangement according to Fig. 1 operated a light having a flash current up to 1.5 amperes at 6 volts with an average supply current drain apart from the lamp of 3 milliamperes in daylight and 1.5 milliamperes at night. The circuit was turned off by cell P about 15 minutes before sunrise and went on about 15 minutes after sunset. Such units operating in the summer months showed a duty factor of about 30% On-time out of 24 hours, and under winter operating conditions were On for about 65% of the day.

While the use of a passive photoresistive type of light cell has been indicated as preferable, the operation of the system is by no means restricted thereto, and cells manitesting a decrease of resistance while at the same time generating a voltage with increase in incident illumination may be employed, the cell being poled with respect to I supply in voltage-adding relation.

erated with lights which are interrupted for relatively short periods and remain On during the greater part of the time, by coupling to the collector of TR1 rather than to TR2 as shown.

I claim:

1. A daylight responsive control for a lamp energizing circuit comprising a circuit closing means and a source of electrical supply for energizing said lamp, a load, a switching device comprising a junction transistor having base, emitter and collector elements, an emitter-to-collector output path connected in series with said load and a voltage source and a base input circuit adapted to be fed by switching voltage pulses, said circuit closing means being responsive to energization of said load to open said lamp energizing circuit and to close said lamp energizing circuit whensaid load is de-energized, and a photosensitive resistor exposed to ambient light connected in shunt with said emitter-to-collector path having a resistance value decreasing with intensity of incident illumination and effective to disable said lamp circuit when said illumination exceeds a predetermined intensity.

2. Daylight-responsive control means for a lamp energizing circuit comprising a load, a switching transistor having base, emitter and collector elements, an emitterto-collector path connected in series with a polarizing source of electrical supply and said load, a base input circuit, a source of switching signal connected to said base input circuit for controlling the conduction state of said emitter-to-collector path, said load comprising the winding of an electromagnetic relay having an armature carrying normally-closed contacts actuatable on energization of said winding to open said lamp energizing circuit and to close said lamp energizing circuit on deenergization of said load, and a photoconductive device whose resistance decreases with increase of incident illumination exposed to incidence of ambient illumination and connected in shunt with the emitter-to-collector path of said switching transistor effective to open said contacts when the incident illumination exceeds a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Transistorized Headlight Dimmer," by Paul Penfield, in, Radio & Television News, August 1955.

High Speed Counter Use Surface-Barrier Transistors, Electronics (pp. 174-178), March 1956.

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