US 3735342 A
An alerting signal responsive to the receiving of a signal characterized by a sequence of tones. There is a plurality of frequency-selective circuits each operative in response to the receiving of a respective preselected one of the tones and sequence-detecting means is responsive to operation of the frequency-selective circuits in predetermined sequence for providing an output signal indicative of the receiving of said signal. Circuitry is provided for resetting the sequence-detecting means at predetermined regular periodic intervals so that the output signal is not provided unless the respective preselected tones are each received in the predetermined interval between resetting.
Description (OCR text may contain errors)
ited elliker et a1.
atent 1 [451 May 22,1973
 Inventors: Conrad Dean Helliker, Tulsa; Terry E. Wilkinson, Bixby both of Okla.
 Assignee: La Barge, llnc., St. Louis, Mo.
 Filed: Mar. 10, 1971  Appl. No.: 122,939
3,568,144 2/1971 Streb ..340/34 Primary Examiner-William C. Cooper Att0mey-Koenig, Senniger, Powers and Leavitt [5 7] STRACT An alerting signal responsive to the receiving of a signal characterized by a sequence of tones. There is a plurality of frequency-selective circuits each operative in response to the receiving of a respective preselected one of the tones and sequence-detecting means is responsive to operation of the frequency-selective circuits in predetermined sequence for providing an output signal indicative of the receiving of said signal. Circuitry is provided for resetting the sequence-detecting means at predetermined regular periodic intervals so that the output signal is not provided unless the respective preselected tones are each received in the predetermined interval between resetting.
20 Claims, 4 Drawing Figures PATENTED V 2 2 I913 SHEET 2 BF 3 ALERTING SYSTEM RESPlDNSIVlE TO A PL i TONE SIGN BACKGROUND OF THE INVENTION This invention relates to tone-responsive circuits and more particularly to an alerting system responsive to the receiving of a signal characterized by a predetermined sequence of tones.
It has been heretofore proposed to provide a system for alerting the driver of a vehicle of the existence of the sound of a siren exteriorly of the vehicle. Such systems are disclosed, for example, in US. Pat. Nos. 2,545,218, 2,931,020, 3,014,199, 3,182,288 and 3,412,378. Such systems have typically amplified sounds which are present exteriorly of the vehicle and have either used a simple filtering network or employed such apparatus as a tuned reed relay to select those frequencies which are picked up which are most likely to characteristic of the sound of a siren, for example. In one such prior art system, a single filter having a passband relatively low in the audio spectrum passes all sounds of this low frequency range to an amplifier. A detector then operates in response to signals passed by the filter to trigger an alarm. Circuits of this type are not truly selective so as to operate solely in response to the receiving of a sound of a siren, i.e., a signal characterized by a predetermined sequence of tones.
Tone decoder systems have been disclosed in the prior art which are responsive to the receiving of tones in a predetermined sequence. Circuitry of this type is shown, for example, in US. Pat. Nos. 3,355,709, 3,447,133 and 3,465,294, each disclosing decoder circuitry for use in a receiver or the like. Such systems employ a plurality of tone-responsive channels each having a reed filter. The channel circuits are interconnected in a fashion such that if the reed filter detects its characteristic frequency and enables its respective channel, that channel enables the operation of successive channel for a short interval. Systems of this type are not well-suited to responding to the sound of a si' ren, inasmuch as the circuitry is adapted to respond to tones received in a relatively quick sequence, as opposed to the tones of a siren which is a signal characterized by a sequence of tones which occur over a relatively longer period of time, such as several seconds. Moreover, reed filters are relatively sensitive to shock and hence may cause erroneous operation.
SUMMARY OF THE INVENTION Among the several objects of the invention may be noted the provision of a system selectively responsive to the receiving of a signal characterized by sequenced tones; the provision of an alerting system responsive to the receiving of sound of a siren; the provision of such a system for a motor vehicle which is operative to alert the occupant of the motor vehicle of the presence of the sound of a siren exteriorly of the vehicle; the provision of such a system which is responsive solely to the sound of a siren and which is not responsive to extraneous and miscellaneous noise; the provision of such a system which is responsive to a signal only when the respective preselected tones of the signal are each received in a predetermined time interval; the provision of such a system wherein said predetermined time interval may readily be changed; the provision of such a system which provides the occupant of the vehicle with a characteristic aural alerting signal; the provision of such a system which allows the occupant of the vehicle to selectively listen to the amplified sounds picked up exteriorly of the vehicle; the provision of such a system which is relatively immune to shock, which is highly reliable in use and which is relatively simply and economically constructed. Other objects and features will be in part apparent and in part pointed out hereinafter.
Briefly, the present invention constitutes an alerting system for a motor vehicle and is responsive to the receiving of the sound of a siren. The system includes means for sensing sound which is present exteriorly of the vehicle and a plurality of frequency-selective circuits interconnected with the sensing means and each operative in response to the receiving of a respective preselected tone of the sound of a siren. Sequencedetecting means, preferably constituted by a plurality of triggerable semiconductor current switching devices, is responsive to operation of the plurality of frequency-selective circuits in a predetermined sequence for providing an output signal. Signalling apparatus is operative in response to this output signal for providing an alerting signal interiorly of the vehicle indicative of the receiving of the sound of a siren. Means is provided for resetting the sequence-detecting means at predetermined regular periodic intervals whereby the output signal is not provided unless the respective preselected tones are each received in the predetermined interval between resetting.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of a display and control unit of the invention shown positioned on the instrument panel of a vehicle;
FIG. 2 is a block diagram of circuitry of the invention; and
FIGS. 3A and 38 together constitute a schematic circuit diagram of the FIG. 2 circuitry, interconnections between FlGS. 3A and 3B being indicated by roman numerals.
Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, illustrated generally at l l is a display module of the present system. Module 11 is adapted to be suitably mounted on or under the dashboard or instrument panel of an automobile or other vehicle where it may be seen and operated by the vehicle operator. Module 11 includes a yield signal light 13 adapted to provide a flashing red signal serving as a visual alerting signal for warning the vehicle operator that a siren is being sounded exteriorly of the vehicle, as by an emergency vehicle, so that the driver may yield right-of-way to the emergency vehicle. In this way, the operator is warned even though he might not otherwise hear the siren because of noise within the vehicle or because the windows thereof are closed, shutting out exterior noises.
Module 11 also includes a selector switch 15 which may be placed in either a monitor position (so indicated), enabling operation of the system for causing operation of light 13 in response to the sound of a siren; in a listen position (similarly indicated), enabling the operator to listen to amplified sounds exteriorly of the vehicle through a loudspeaker 17 (see FIG. 2).
The system also includes a provision for generating a characteristic two-tone aural alerting signal which is delivered by loudspeaker 17 simultaneously with flashing operation of light 13 in response to the sound of a siren when switch 15 is in the monitor position. Once sounded, this characteristic signal can be terminated by placing switch 15 in the listen position.
Referring to the FIG. 2 block diagram, the system includes a suitable transducer 19 such as a microphone element suitably mounted, for example, at the rear of a vehicle such as between the taillights of the vehicle. Transducer 19 constitutes means for sensing or detecting sound which is present exteriorly of the vehicle. Transducer 19 is connected to a transistor amplifier 21 which is in turn connected to three frequency-selective circuits, viz., bandpass filters BPl, BP2 and BP3 having respective center or pass frequencies of 600 Hz, 900 Hz and 1,200 Hz, for example. The output of each bandpass filter BPl-BP3 is connected to a respective amplifier 23, 25 and 27 in turn connected to the gate or triggering electrode of a respective triggerable semiconductor current switching device constituted by a socalled SCR or silicon-controlled rectifier. Each of the three SCRs, which are respectively designated SCRI, SCR2 and SCR3, is thus adapted to be triggered by an amplified output from the respective bandpass filter BPl, BP2 or BP3. These SCRs have their cathode and anode terminals, i.e., their main electrodes, connected in a series circuit such that the SCR s may only be triggered in a predetermined sequence, viz., SCRl, SCR2 and then SCR3. Load resistors R1, R2, R3a and R3b are connected between the cathodes of the respective SCRs and ground. Those skilled in the art will recognize this to be a cascade arrangement. The three SCRs constitute sequence-detecting means responsive to the operation of the filters BPl-BP3 in a predetermined sequence for providing an output signal, constituted by a voltage developed across load resistor R3b when the SCRs have been triggered in sequence, as is later more fully explained.
Connected to a solid state switching circuit 29 is a timer 31. Switching circuit 29 is in turn connected to the anode of SCRI. Circuit 29 is controlled by timer 31 so as normally to provide operating voltage across the series-connected SCRs but to remove or interrupt this voltage at several-second intervals (e.g., every seconds) for a brief period such as milliseconds. Timer 31 and switching circuit 29 together constitute means for resetting the SCRs at predetermined regular periodic intervals. The purpose of such resetting is explained later.
Ifthe SCRs are sequentially triggered, and thus each rendered conductive, a voltage is developed across resistor R3b. This voltage in effect constitutes an output signal indicative of the fact that transducer 19 has picked up the characteristic sequenced tones of a siren. This voltage across R3b is applied via a lead 33 to one input of a two-input logic AND gate 35, lead 33 being connected between that input and the junction of R3a and R3b. A pulsed signal is applied to the other input of gate 35 by a timer 37. This pulsed signal may have a voltage waveform which is alternately high" for about l second and then low for about 1 second. The output of gate 35 is connected to an amplifier 39 whose output is connected to one terminal of an incandescent bulb constituting signal light 13. One side of the bulb is connected to a voltage source designated V+. When the voltage across R3b is applied to gate 35, the latter delivers the pulsating output signal from timer 37 and which is efiectively amplified by amplifier 39 to cause flashing operation of light 13.
A lead 41 also supplies the voltage (when present) across resistor R3b to one input of a two-input logic AND gate 43, the other input of which is continuously supplied with a two-tone signal generated by circuitry including a pair of oscillators 45 and 47 having respective frequencies of oscillation of 1,000 Hz and 2,000 Hz, for example. The output of oscillator 45 is connected to one input of a two-input logic AND gate 49 and the output of oscillator 47 is similarly connected to one input of a further two-input logic AND gate 51. A timer 53 is constituted by a multivibrator with a switching interval of about 1 second, for example. Timer 53 has a pair of output terminals for delivering pulses which are of opposite phase. The terminals are each respectively connected to one input of the respective AND gates 49 and 51 so as to cause alternate gating by AND gates 49 and 51 of the two frequencies (i.e., 1,000 Hz and 2,000 Hz) to AND gate 43.
When AND gate 43 is supplied with an input resulting from voltage developed across resistor R3b, this two-tone or two-frequency signal is delivered as an output signal from gate 43, amplified by an amplifier 55, and delivered through switch 15 (when the latter is in the monitor position, as illustrated) to a further amplifier 57. The output of the latter is supplied to loudspeaker 17, thereby providing the characteristic twotone aural alerting signal noted previously.
When switch 15 is moved to the listen position, the connection from the output of amplifier 55 to the input of amplifier 57 is opened and a connection is instead completed from the output of amplifier 21 to the input of amplifier 57 to cause the amplified output of transducer 19 to be supplied to loudspeaker 17.
Referring to the left-hand side of FIG. 3A, the circuitry of the present system is supplied through a diode D1 with a source of dc power, designated V+, of a voltage suitable for semiconductor circuitry. This source may be constituted by the vehicle storage battery and may be connected by operation of the vehicle ignition switch. A lead 59 supplies the unregulated voltage V+ to certain portions of the system, and a voltage regulating circuit including an NPN transistor Q1 and a zener diode D2 provides a regulated voltage (determined by the characteristics of D2) to other portions of the circuitry via a lead 61. A second voltage regulating circuit including an NPN transistor Q2 and a zener diode D3 provides a second regulated voltage to still other portions of the circuitry via a lead 63.
Amplifier 21, to which transducer 19 is connected, comprises a pair of commercially available integrated circuit amplifiers A1 and A2 interconnected by a capacitor C1. Signals resulting from sounds sensed, or picked up, by transducer 19 are coupled through a capacitor C2 to the input of amplifier A1. The amplified signal is delivered to a contact 65 of switch 15 by amplifier A2 through a capacitor C3. Various conventional feedback or frequency compensation circuits are connected with amplifiers Al and A2 and will be understood immediately by those skilled in the art.
When switch 15 is in the monitor position shown, contact 65 is connected to a further contact 67 by a first slider 69a of the switch. A second slider 69b ganged with slider 69a similarly bridges another pair of contacts 71 and 73 for purposes explained below. A lead 75 connects terminal 67 with respective inputs of filters 8P1, BP2 and BP3 (see FIG. 3B). The latter are preferably passive L-C bandpass networks having respective center frequencies of 600 Hz, 900 Hz and 1,200 Hz. Such networks provide relative immunity from shock, as opposed to reed filters, for example.
The respective outputs of filters BP1, BP2 and BP3 are connected to amplifiers 23, 25 and 27 through respective capacitators C5, C6 and C7. Amplifiers 23, 25 and 27 are identical and constitute, in effect, solid state switches each comprising an NPN transistor Q3, Q4, Q5 connected in common-emitter configuration. The collector of each such transistor is connected through a respective diode D5, D6, D7 to the gate electrode of its associated SCR. Transistors Q3-Q5 are normally biased into saturation by respective biasing networks. Taking amplifier 23 as an example, this biasing network includes a pair of resistors R5 and R6 providing sufficient potential for normally biasing Q3 into saturation. An output signal from filter BPl momentarily causes Q3 to be cut-off and thus its collector is momentarily high, causing triggering of SCRl. In this way, an output signal from filter BP1 is effectively amplified through switching action to cause triggering of the SCR. Amplifiers 25 and 27 operate in identical fashion. As noted, the SCRs may be triggered only in succession since each successive SCR depends, for having necessary operating potential across its cathode and anode, upon triggering of the preceeding SCR.
Referring now again to FIG. 3A, lead 33 supplies the potential developed across resistor R3b upon triggering of all three SCRs to one input of AND gate 35. Timer 37, which is connected to the other input of gate 35, comprises a pair of cross-coupled logic inverter amplifiers, or gates, 77 and 79, each having its output interconnected by a respective capacitor C9, C with the input of the other gate to provide an astable or freerunning multivibrator constituting, in effect, a very low frequency oscillator.
Lead 63 supplies voltage to the inputs of gates 77 and 79 through respective resistors R7 and R8. The values of R7, R0,C9 and C10 are chosen so as to cause the outputs of gates 77 and 79 to alternate, in opposite phase relationship, between high and low states at switching intervals of about l second. The output of gate 77 constitutes the output of timer 37 and is connected to AND gate 35, providing a signal which is alternately high for 1 second and then low for one second. Accordingly, when the voltage across resistor R311 is also supplied to AND gate 35, the output of the latter provides this pulsed signal to amplifier 39 through a resistor R10 thereof.
Amplifier 39 includes a first NPN transistor Q7 connected for controlling the conduction of a further NPN transistor 00 whose collector-emitter terminals are connected in a series circuit with signal light 13. Each time the output of gate 35 becomes high, transistor O7 is driven into saturation, causing transistor Q0 to conduct for energizing light 13. Hence, the output of gate 35 is effectively amplified through switching action by amplifier 39.
Referring now to FIG. 3B, oscillators 45 and 47 are each constituted by a pair of cross-coupled inverter gates connected as an astable multivibrator. For example, oscillator 45 includes such gates 01 and 03 crosscoupled through capacitors C12 and C13. The supply voltage input to the gates is supplied through respective resistors R12 and R13. The values of these resistors and the capacitors C12 and C13 may be such as to provide an oscillatory output signal of a frequency of about 1,000 Hz. This output signal is delivered to one input of AND gate 49 through a resistor R14.
Similarly, oscillator 49 includes inverter gates 05 and 07 cross-coupled through capacitors C15 and C16 and supplied with power through resistors R15 and R16, these components being of a value causing an output signal of about 2,000 Hz which signal is delivered to one input of AND gate 51 through a resistor R17.
Timer 53 includes components connected identically with oscillators 45 and 47, viz., inverter gates 89, 91 cross-coupled by capacitors C10, C19 and supplied with power through resistors R10, R19 to provide an astable multivibrator. However, the values of these capacitors and resistors are selected to cause output signals to be alternately supplied by gates 09, 91 at about one-second switching intervals. The outputs of gates 89, 91 are respectively connected to one input each of AND gates 49 and 51 to cause alternate gating by the latter of the 1000- and 2000- Hz signals. The outputs of gates 49 and 51 are each connected through a respective diode D9, D10 and by means of a lead 93 to one input of AND gate 43 whose other input is connected by lead 41 to the junction of resistors R3a an R3b.
Amplifier 55 comprises a pair of inverter gates 95, 97 connected end-to-end to provide buffering of the twotone signal gated by AND gate 43 when so permitted by potential applied to the input thereof by lead 41 resulting from triggering of the three SCRs. Other types of logic gates may be used in place of gates 95, 97 to provide this buffering function.
With switch 15 in the monitor position, the twotone signal delivered at the output of gate is delivered via switch contact 73, slider 69b, and contact 71 to amplifier 57, which comprises a commercially available integrated circuit A3. Suitable feedback and frequency compensation networks are connected between the input and output of amplifier A3, as is conventional. The output of amplifier A3 is connected through a capacitor C20 to loudspeaker 17. It may be noted that switch 15 includes a contact 99 which, when the switch is moved to the listen position, is bridged with contact 65 by slider 69a to connect the output amplifier A2 to the input of amplifier A3.
Referring once more to FIG. 3B, timer 31 comprises components connected identically with timer 53 to provide an stable multivibrator having inverter gates 101, 103 cross-coupled by capacitors C21, C22 and supplied with the power supply voltage through resistors R21, R22. The values of these capacitors and resistors are selected to cause alternate switching between gates 101 and 103 at switching intervals of about five seconds, for example. The output of gate 101 (which is shown as constituting the output of timer 31) is thus alternately high for five seconds and then low" for five seconds.
Switching circuit 29 includes a PNP transistor Q7 whose collector-emitter terminals are connected in a series circuit with the cathode-anode terminals of SCRs 1-3 between ground and the power supply lead 61. Hence, the supply potential is effectively connected across the three SCRs when transistor Q7 is conductive (i.e., driven into saturation) but this potential is effectively removed when Q7 is cut-off.
Connected to the base of Q7 through a resistor R23 is the collector of an NPN transistor Q8 whose emitter is grounded. The base of transistor Q8 is biased to the positive supply potential via a pair of resistors R24 and R25. Thus transistor Q8 is normally biased into conduction to supply sufficient base drive to transistor Q7 for causing the latter to be driven into saturation. The output of timer 31 is connected through a capacitor C24 to the junction of R24 and R25. Each time the output of gate 101 goes low, a pulse is developed by capacitor C24 which is applied to the base of Q8. This momentarily cuts off not only Q8 but Q7 as well, thereby briefly interrupting or removing the potential across the SCRs. Since the output of gate 101 goes low approximately every 10 seconds, the SCRs are thereby reset every 10 seconds.
It may be noted that, by simply changing the values of capacitors C21 and C22 and resistors R21 and R22 of timer 31, the switching rate of this multivibrator circuit may readily be varied so that resetting of the SCR's may take place at a shorter or longer interval, as desired.
The various circuits and logic gates described may advantageously be in the form of monolithic integrated circuits. Logical equivalents of the logic gates may, of course, be used.
Operation of the system is effected by moving switch to the monitor" position. In the absence of the sound of a siren, transducer 19 receives only miscellaneous, extraneous and random noises which are amplified by amplifier 21. However, unless the noise received includes a frequency falling within the passband of filter BPl, SCRl is not triggered and the system is thereby not affected by the noise. Of course, should such a frequency be present, filter BPl will pass that signal, causing triggering of SCRl. It may be observed, however, that the voltage across the SCRs is briefly removed, causing commutation or resetting of these devices, at predetermined regular periodic intervals (e.g., every 10 seconds). SCRl thus becomes nonconductive once more. Hence, such chance or random noises, regardless of intensity, will not result in alerting operation of the system, since three successive tones of frequencies within the respective passbands of filters BPl-BP3 must each be received in the predetermined time interval between resetting of the SCRs. Even if random noise should result in successive triggering of SCRl and SCR2, periodic resetting of the devices will return them to their nonconductive states, and spurious operation is thus prevented.
If, on the other hand, transducer 19 receives the sound of a siren, such as being sounded by a police or emergency vehicle, the amplified tone of the siren, as it sweeps upward over its range of from approximately 400 to 1,500 Hz, will be successively passed by filters BPl-BP3 causing successive triggering of SCRl-SCR3. Upon gating of the SCR3, the potential developed across R3b is applied. The potential across R3b is also supplied to one input of AND gate 43, causing gating of the two-tone signal to amplifier 57, resulting in a characteristic two-tone easily recognized aural signal from loudspeaker 17 for alerting the vehicle operator. The potential across R3b also results in energization of signal light 13 for a flashing visual alerting signal.
Having thus been alerted that a siren is being sounded, the driver may yield right-of-way to the overtaking or approaching vehicle carrying the siren. The operator may then also move selector switch 15 to the listen" position so that he may hear over loudspeaker 17 the amplified exterior sounds picked up by transducer 19 (such as a police officers verbal commands over the police vehicle public address system). When switch 15 is moved to the listen position, slider 69b is moved to break the circuit between contacts 71 and 73 and thereby cutting off the two-tone signal from loudspeaker 17. However, when switch 15 is returned to the monitor position, normal siren monitoring operation of the system is resumed.
While the present system is described as being responsive to the sound of a siren, it should be observed that this invention may advantageously be utilized to provide a tone decoder function as, for example, in conjunction with a radio receiver. As noted, the system readily facilitates adjustment to vary the response interval between resetting of the SCRs, so as to provide a shorter listening interval, for example, as may be desired in such a tone decoder function.
In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.
As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
1. An alerting system responsive to the receiving of a signal characterized by a sequence of tones, comprising:
a plurality of frequency-selective circuits each operative in response to the receiving ofa respective preselected one of said tones;
sequence-detecting means responsive to operation of said plurality of frequency-selective circuits in a predetermined sequence for providing an output signal indicative of the receiving of said signal; and
means for resetting said sequence-detecting means at predetermined regular periodic intervals whereby said output signal is not provided unless the respective preselected tones are each received in the predetermined interval between said resettings.
2. An alerting system as set forth in claim 1 wherein said sequence-detecting means comprises a plurality of triggerable semiconductor current switching devices the main electrodes of which are all connected in a series circuit and each adapted to be triggered by a corresponding one of said frequency-selective circuits, the resetting means being operative to apply a voltage across said series circuit and to remove said voltage at said predetermined regular periodic intervals for resetting of said devices, whereby when said devices are triggered in sequence, current flows in said series circuit to provide said output signal.
3. An alerting system as set forth in claim 2 wherein the resetting means comprises a timer.
4. An alerting system as set forth in claim 3 wherein the resetting means further comprises a switching circuit, including a transistor having electrodes connected in said series circuit, said transistor being controlled by said timer.
5. An alerting system as set forth in claim 4 wherein said timer is constituted by an astable multivibrator operative to cause said transistor briefly to become nonconductive at said regular periodic intervals.
6. An alerting system for a motor vehicle and responsive to the receiving of the sound of a siren, comprising:
means for sensing sound which is present exteriorly of the vehicle;
a plurality of frequency-selective circuits interconnected with the sensing means and each operative in response to the receiving of a respective preselected tone of the sound of a siren;
sequence-detecting means responsive to operation of said plurality of frequency-selective circuits in a predetermined sequence for providing an output signal;
signalling apparatus operative in response to said output signal for providing an alerting signal interiorly of the vehicle indicative of the receiving of the sound of a siren; and
means for resetting said sequence-detecting means at predetermined regular periodic intervals whereby said output signal is not provided unless the respective preselected .tones are each received in the we determined interval between said resetting.
7. An alerting system as set forth in claim 6 wherein said means for sensing sound comprises a microphone positioned for receiving sound which is present exteriorly of the vehicle and amplifier means for amplifying the output of said microphone.
8. An alerting system as set forth in claim 6 wherein said signalling apparatus comprises a loudspeaker and oscillator means for supplying an oscillatory signal to the loudspeaker in response to said output signal thereby to generate an aural alerting signal.
9. An alerting system as set forth in claim 8 including switch means selectively operable interiorly of the vehicle for connecting said amplifier means to cause the amplified output of said microphone to be supplied to said loudspeaker.
'10. An alerting system as set forth in claim 8 wherein said signalling apparatus further comprises a signal light operative in response to said output signal for providing a visual alerting signal.
1 ll. An alerting system as set forth in claim 10 further comprising a timer and logic gate means operative in response to said output signal to effect flashing energization of said signal light under control of said timer.
12. An alerting system as set forth in claim 11 wherein said timer comprises an astable multivibrator and said logic gate means comprises an AND gate connected for gating an output signal from said multivibrator in response to the first-said output signal.
13. An alerting system as set forth in claim 8 wherein said oscillator means comprises a first oscillator providing a first tone frequency, a second oscillator providing a second tone frequency, first and second logic gate means respectively connected with the outputs of said first and second oscillators, and a timer connected with each said logic gate means and operative to cause alternate gating by said logic gate means of the output signals of said first and second oscillators thereby to provide an alternating two-tone aural alerting signal.
14. An alerting system as set forth in claim 13 wherein each of said oscillators and said timer is constituted by a respective astable multivibrator.
15. An alerting system as set forth in claim 13 including further logic gate means having a first input interconnected with the output of the first-said logic gate means and a second input interconnected with said sequence-detecting means, and operative to gate the output signal of the first-said logic gate means to provide said aural alerting signal in response to the output signal from said sequence-detecting means.
16. An alerting system as set forth in claim 15 wherein each of said logic gates means is constituted by a respective AND gate.
17. An alerting system as set forth in claim 6 wherein each of said frequency-selective circuits comprises a bandpass filter.
18. An alerting system as set forth in claim 17 wherein each of said filters is constituted by a passive L-C network.
19. An alerting system as set forth in claim 17 wherein said sequence-detecting means comprises a plurality of triggerable semiconductor switching devices each adapted to be-triggered by an output signal from a respective bandpass filter, said devices being connected for being triggered only in said predetermined sequence.
20. An alerting system as set forth in claim 19 wherein each of said semiconductor switching devices comprises an SCR.
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