Patent Abstract:
A low battery detection circuit is disclosed for detecting low battery voltages in hearing aids and other battery operated devices. The low battery detection circuit operates reliably on a small amount of current, which does not significantly increase overall battery drain. An output signal is generated whose amplitude and frequency increase as battery voltage decreases below a predetermined level. The circuits minimal number of components and output signal characteristics make it particularly applicable as a low battery early warning device for hearing aid devices.

Full Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation of abandoned application U.S. Ser. No. 08/033,943 filed Feb. 16, 1993, entitled “LOW BATTERY DETECTOR AND INDICATOR”, which is a continuation of my abandoned application Ser. No. 07/416,703 filed Oct. 3, 1989, which is a continuation-in-part of my abandoned application U.S. Ser. No. 07/414,903, filed Sep. 29, 1989. The disclosures of said applications are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an alarm circuit for a hearing aid, which can provide advance warning that the battery is approaching its end of life, and/or function as an increased-audibility alarm to provide wake-up or other-purpose alarm signals to the hearing impaired wearer. Unlike other increased-audibility alarms for the hearing impaired, the alarm circuit of the invention is private and not annoying to those around the wearer. 
     2. Background of the Prior Art 
     There are many low battery detector circuits on the market. These typically include a plurality of discrete components or are contained in a separate integrated circuit designed for that purpose, and typically operate in a binary manner: No output until the battery voltage drops below the detector threshold, then full output in the form of a warning light, series of beeps, or the like. 
     The prior art also includes arrangements particularly for testing batteries used in conjuction with hearing aids. Oticon Corporation manufactures a behind-the-ear hearing aid incorporating a battery test switch, with an LED readout, eliminating the need for a separate battery tester. A low-battery indication is built into the hand-held remote control transmitter used with Widex Corporation&#39;s “QUATRO” remote-controlled hearing aids to monitor the battery in the transmitter although not the one in the hearing aid itself. 
     SUMMARY OF THE INVENTION 
     This invention was evolved with the general object of providing a simple and effective arrangement by which the user of a hearing aid might be alerted to low battery or other alarm conditions. 
     Important aspects of the invention relate to the recognition of problems with prior art arrangements and the causes thereof. The space and circuitry requirements of low battery detector circuits of the prior art are such that they cannot be practically included in a hearing aid. In my aforementioned application, of which this is a continuation-in-part, a circuit is disclosed which is such that it can be incorporated in a hearing aid, having the important advantage that it can use the existing audio amplifier circuitry of the hearing aid for producing an audible indication of the low battery. That circuit has the additional advantage of providing a warning whose loudness and signal frequency progressively increases as the battery falls lower and lower below the preset threshold. It is quite simple and can be added to a hearing aid integrated circuit amplifier with little additional area required on the chip. 
     It is has been found that there are instances in which a user may desire to disable the warning signal until such time as he may conveniently change the battery, or in which the user may desire additional features and, in accordance with an important feature of the invention, the user of a hearing aid is provided with a manual means of disabling the low battery warning signal once it has been heard. 
     Another important feature relates to an arrangement with provides the user with the option of using the on-off switch of the hearing aid as the disabling switch, in order to avoid the additional space and expense required by a separate disabling switch. 
     A further feature of this invention relates to the provision of an automatic means of limiting the percentage of time the low-battery warning is audible, in order that it not become annoying before he has time to change the battery. 
     Still another feature of the invention relates to the provision of a wake-up or other alarm that is easily heard privately by the hearing impaired wearer, but which is not disturbing to those around him or her. 
     The invention also provides automatic means of increasing the audibility of the warning or alarm signal until such time as the hearing aid wearer signals that he has heard the signal and turns off the alarm. 
    
    
     These and other objects, features and advantages will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates diagrammatically a hearing aid which is constructed in accordance with the invention: 
     FIG. 1A is a block diagram of an alarm system usable in the hearing aid of FIG. 1; and 
     FIGS. 2,  3 ,  4 ,  5 ,  6  and  7  show various forms of alarm systems usable in the hearing aid of FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a hearing aid  10  which is constructed in accordance with the principles of the invention and which includes a battery  11 , on-off switch  12 , microphone  13 , amplifier  14 , earphone  15 , earpiece  16  for coupling the hearing aid output to an ear, and an alarm system  20 . The battery  11  may be a battery of the disc type having a negative end engaged with a grounded terminal  11 A and with a positive end engaged with a terminal  11 B which is connected to the on-off switch  12 . 
     FIG. 1A shows an alarm system  20  as in FIG. 1, which alarm system may include battery level detector  21 , includes oscillator  22 , and may also include disabling means  23 , and timing means  24 . 
     FIG. 2 shows one preferred embodiment  20 A of alarm system  20  in which detector  21 A contains a voltage dependent triple of current sources operating as described in aforementioned patent application dated September 29, 1989, and current-dependent oscillator  22 A also functions as described in aforementioned patent application, with a switch means  23  connected to disable its operation. It should be understood that a low-voltage non-contacting touch-sensitive integrated circuit switch such as manufactured by Gennum corporation of Canada may be used as the switch means  23 . It should also be noted that for purposes of this description the word battery will be used to describe both multiple-cell and single-cell power sources, the latter being more typical in hearing aid applications. 
     The battery voltage of battery  11  is determined by the power requirements of the battery operated device which uses it. While the present invention is in no way limited to the type of battery operated devices which would benefit from the use of the illustrated low battery detection circuitry, the operation of the illustrated circuitry will be discussed with reference to a nominal single-cell battery voltage of 1.3 volts D.C. This battery voltage is typical of batteries used to power hearing aids, the application in which the low battery detection circuit  50  is presently used. The illustrated circuitry, however, is not limited to hearing aid applications and may be used in any battery powered electronic apparatus whose utility would be enhanced by the operation of the low battery detection circuit. 
     The low battery detection circuit  21 A is powered by three different voltage values, two voltages derived from battery  11  and the actual voltage of battery  11 . Detection circuit  21 A detects the difference between a regulated voltage and a reduced tracking voltage which tracks battery voltage as battery voltage decreases during operation. 
     In the present embodiment, a regulator  25  is used to develop a regulated voltage of 0.84 volts. This voltage level is chosen to be sufficiently below the minimum operating voltage at which the hearing aid effectively operates. This permits the low battery detection circuit to indicate a low battery state before actual device operation is impaired. It will be understood that proper operation can be obtained with a choice of regulator voltage somewhat different from 0.84 Volts, and the exact choice can be made depending on the nominal battery voltage, the desired low-voltage detection level, and other circuit requirements on the regulator. 
     The regulated voltage of 0.84 volts is developed from the battery voltage. The regulator input, indicated by line  25 A, is connected through the on-off switch  12  to positive terminal  11 B of battery  11 . The regulator common terminal  25 B is connected to the negative terminal  11 A of battery  11  which forms a circuit ground. The regulator output  25 C of 0.84 volts is connected to a line  54  which forms a regulated voltage input of a low battery detection circuit. 
     A line  58  forms a tracking input  58  of a low battery detection circuit and receives a reduced tracking voltage developed at common node  60  of resistor  62  and resistor  64 . The reduced tracking voltage is derived from the battery voltage of battery  11 . The anode of diode  66  is connected to positive terminal  11 B of battery  11 . The cathode of diode  66  is connected to one end of resistor  62 . The other end of resistor  62  is connected to one end of resistor  64 . The other end of resistor  64  is connected to ground. The reduced tracking voltage at common point  60  tracks the battery voltage as the battery voltage decreases during hearing aid operation. The values of resistors  62  and  64  are chosen such that the voltage derived at common point  60  is approximately 0.4 volts below the regulated voltage of 0.84 volts when the battery voltage drops to approximately 1.15 volts. It is the difference between the regulated voltage of 0.84 volts at regulated voltage input  54  and the reduced tracking voltage at tracking input  58  which controls the operation of low battery detection circuit  21 A. 
     A supply input  70  of low battery detection circuit  21 A is coupled through switch  12  to positive terminal  11 B of battery  11 . Supply input  70  receives the actual voltage of battery  11 . 
     The low battery detection circuitry develops an output signal which is coupled through output  72  to amplifier  14 . The output signal at output  72  is an AC signal having both amplitude and frequency characteristics which indicate the extent to which the battery voltage has dropped from nominal. This output signal reflects the voltage difference which occurs between regulated voltage input  54  and tracking input  58 . As the voltage at tracking input  58  decreases with decreasing battery voltage, the voltage difference between voltage input  54  and tracking input  58  increases. This voltage difference determines the frequency rate and amplitude level of the output signal at output  72 . 
     The illustrated low battery detection circuitry uses a three-stage oscillator driven by a voltage dependent triplet of current sources. The voltage dependent triplet of current sources includes three PNP transistors  80 ,  82 , and  84 . The current delivered by PNP transistors  80 ,  82 , and  84  varies according to the voltage difference between the voltages at regulated voltage input  54  and tracking input voltage  58 . The regulated voltage developed by regulator  25  is coupled to the emitters of PNP transistors  80 ,  82 , and  84  through regulated voltage input  54 . The tracking voltage developed at common node  60  is connected to the base of each of the PNP transistors  80 ,  82 , and  84  through tracking voltage input  58 . As battery voltage decreases the voltage level at the bases of the PNP transistors  80 ,  82 , and  84  also decreases. This creates an increasing voltage between the emitter and base of each PNP transistor. This action increases the current flowing through the collectors of PNP transistors  80 ,  82 , and  84 . 
     An oscillator of the circuitry is composed of three NPN transistors  86 ,  88 , and  90 . The emitter of each NPN transistor is coupled to ground. The collector of each NPN transistor is coupled to its corresponding current source, such that the collector of transistor  86  is coupled to the collector of transistor  80  at point  92 , the collector of transistor  88  is coupled to the collector of transistor  82  at point  94 , and the collector of transistor  90  is coupled to the collector of transistor  84  at point  96 . These collector-collector connectors provide the necessary current to drive the oscillator. 
     The oscillator is DC biased for stable operation by Schottky diode  98  whose anode is coupled to point  92  through a line  100  and whose cathode is coupled to the base of transistor  90  at point  102 . A positive feedback capacitor  104  is connected to the collector of transistor  88  at point  94  and the base of transistor  90  at point  102 . In the preferred embodiment, the value of capacitor  104  is 50 picofarads, a capacitance which is easily included in an integrated circuit chip. As shown, line  100  may be connected to ground through the disabling switch  23 . 
     An NPN transistor  106  is arranged in an emitter follower configuration to supply an output signal at output  72  to amplifier  14 . The collector of transistor  106  is coupled to the positive terminal  11 B of battery  11  through supply input  70 . The emitter of transistor  106  is coupled to output  72 . The base of transistor  106  is coupled to the anode of diode  98  at point  108  which in turn connects it to the collector of transistor  86  at point  92  through line  100 . 
     As previously stated, the frequency rate and the amplitude level of the output signal depends upon the voltage difference between regulated voltage input  54  and tracking input  58 . This voltage difference is applied across the base-emitter junctions of each PNP transistor  80 ,  82  and  84 . When the battery voltage drops to 1.15 volts, approximately 0.4 volts is applied between the base-emitter junctions of those PNP transistors which in turn supply approximately 2 nanoamps of current to the collectors of each NPN transistor  86 ,  88  and  90 . This current is available to charge capacitor  104 . As capacitor  104  is charged by the current provided by transistor  82 , transistor  90  conducts. The conduction of transistor  90  causes point  96  to be essentially at ground. Since transistor  88  is in a cutoff condition, the current delivered to point  94  by transistor  82  is coupled to the base of transistor  86 , turning transistor  86  on. At this point transistors  86  and  90  are turned on and transistor  88  is turned off. 
     Once capacitor  104  approaches a full charge, the current to the base of transistor  90  is reduced sufficiently to force transistor  90  into a state of cutoff so that current from transistor  84  flows into the base of transistor  88 . Transistor  88  then conducts, forcing point  94  to ground and driving transistor  86  into cutoff. Now, transistor  88  is turned on and transistors  86  and  90  are turned off. 
     Since capacitor  104  has one lead connected to point  94  and transistor  88  is turned on, and since transistor  86  is turned off so that the other lead of capacitor  104  is receiving current from transistor  80  thru Schottky diode  98 , capacitor  104  begins to discharge and then recharge in the opposite direction. Once capacitor  104  has recharged sufficiently in the opposite direction, transistor  90  begins to conduct using current supplied through diode  98 . Once transistor  90  conducts sufficiently to absorb essentially all of the current from transistor  84 , transistor  88  will return to the off state, whereupon the curent from transistor  82  will again be available to charge capacitor  104  in the original direction. At this point, both Schottke diode  98  and capacitor  104  supply current into the base of transistor  90 . Once capacitor  104  has charged sufficiently, current from transistor  82  will flow into the base of transistor  86 , turning it on. The circuit is now back to the state in which transistors  86  and  90  are turned on and transistor  88  is turned off. This cycle repeats, creating an AC output signal at the base of transistor  106 , which is amplified by transistor  106  and coupled to amplifier input  74  through output  72 . In the case of a hearing aid, an amplifier receiving the output signal is coupled to a receiver providing an audible warning signal to the user indicating low battery voltage. 
     As the voltage difference between regulated voltage input  54  and tracking input  58  increases due to reduced battery voltage, the collector currents of transistors  80 ,  82 , and  84  increase. Since nearly 80% of the drop in battery voltage is applied to the base-emitter junctions of the current source transistors, the collector current approximately doubles for each 20 millivolt drop in battery voltage. 
     With an initial 2 nanoamps of charge current and the value of capacitor  104  established at 50 picofarads, charging of capacitor  104  takes approximately 10 milliseconds. Therefore, a complete cycle takes approximately 20 milliseconds setting the oscillation frequency at approximately 50 Hertz. In this circuit, a doubling of the collector current produces a doubling in the frequency of oscillation. The base current supplied to transistor  106  is also doubled. The resulting output waveform at output  72  therefore increases audibility for two reasons: first, the audibility at a constant sound pressure level increases rapidly with frequency above 50 hertz for most people with a hearing loss except those having extreme forms of hearing loss at higher frequencies, and second, the sound pressure level generated at the output of the hearing aid is typically more than doubled for each 20 millivolt drop in battery voltage because of the increased pulse current and the rising gain-vs-frequency characteristic of a typical hearing aid. These results produce a highly useful circuit function wherein even a 10 millivolt decrease in battery voltage will create a readily noticeable increase in the apparent urgency of the warning signal. 
     As mentioned above, the audible alarm of the present invention is heard privately (e.g., only) by a hearing aid use or wearer. In addition, as is apparent from FIG.  2  and the associated description, for example, the audible alarm and the audio received by the hearing aid microphone can be heard simultaneously by the user. 
     FIG. 3 shows a different variation  30  of alarm system  20  of FIG. 2, in which low-battery-detection circuit  31  enables a capacitor charging circuit  35  which is configured to retain charge on hold capacitor  36  long enough for the battery to be disconnected and reconnected quickly (by operation of the on-off switch of the hearing aid or by opening the battery drawer) while still biasing logic switch  37  so that it disables alarm oscillator  22 A by providing a relatively low impedance path between output line  38  and ground  39  as soon as the battery is reconnected. In this manner, a manual shut off of the alarm is provided without requiring a separate switch in the hearing aid. It will be understood that any oscillator capable of being disabled by a logic signal may be substituted for oscillator  22 A. 
     FIG. 4 shows an alternate embodiment in which alarm system  40 , which may be substituted for alarm system  20  in FIG. 1, contains detector  21 , alarm oscillator  22 , and timer  23 . Timer  23  may be a low-battery-drain watch-circuit type of integrated circuit which has been preprogrammed to periodically enable the low-battery warning alarm for a desirable period of time such as 3 seconds and then disable the low-battery warning system for a desirable quiescent period such as 10 minutes. Alarm system  40  automatically renders the operation of the low-battery warning system less obtrusive, while still accomplishing its primary function of providing advance warning of the impending end-of- life of the battery. 
     FIG. 5 shows alarm system  150  which may be substituted for alarm system  20  in FIG.  1 . Alarm system  150  contains a watch/alarm means  153  such as is readily available in low-battery-drain digital watch circuits and can be set by means of setting switches  154 ,  155 , and  156  in the usual manner, but whose alarm output  157  is amplified by the hearing aid in an amount controlled by attenuator  158 . Attenuator  158  may be adjusted by the hearing aid dispensor or hearing aid wearer so that the alarm is readily audible to the wearer, regardless of the degree of his hearing loss, in order that he will not fail to hear the alarm. Setting the time and alarm may be accomplished in a conventional manner if a miniature watch LCD readout  159  and aforementioned push buttons  154 ,  155 , and  156  are incorporated into the hearing aid case. It should be understood that the recent development of remote control programmers for hearing aids now permits the convenient setting of the alarm function by remote control, avoiding any change in the external appearance of the hearing aid, providing only that switch means  154 ,  155 , and  156  are remotely programmed. 
     FIG. 6 shows alarm system  160 , which may be substituted for alarm system  20  in FIG. 1, and which incorporates timer  163  with setting switches  154 ,  155 , and  156  and readout  159 , but whose output  164  replaces switch  23  of FIG. 2 in disabling an alarm oscillator such as alarm oscillator  22 A as shown, so that only when an alarm signal is desired is output  164  in the enabling (effectively open circuit) condition. 
     The alarm system  160  also contains alarm oscillator adjustment means  165 , with the variable resistance  166  adjustable so that in cooperation with fixed resistance  167  the base-emitter voltage of large-area PNP transistor  168  and thus the current supplied by current sources  80 ,  82 , and  84  may be controlled, controlling in turn the amplitude and signal frequency of alarm oscillator  22 A as described in the aforementioned patent application. In this manner, the output of alarm oscillator  22  can be adjusted by the hearing aid dispensor or user for optimum audibility to the hearing impaired user. The advantage of the circuits of FIG. 5 and 6 is that the hearing aid wearer will be able to receive privately an alarm which might otherwise be so loud as to disturb others. 
     FIG. 7 shows alarm system  170 , which may be substituted for alarm system  20  in FIG. 1, and which incorporates timer  173  which in turn has two outputs, one output  174  acting as above to provide a timed alarm function, while the other output  175  acts to cause increasing audibility with time of the alarm signal. For example, if output  175  is programed so that every 10 seconds it provides a doubling in the control current drawn from PNP transistor  168  in alarm oscillator adjustment means  165  of FIG. 6, the audible output of alarm oscillator  22  will increase by approximately 20 dB in hearing level every 10 seconds until the alarm is heard, whereupon it may be disabled by the user with disabling means  23 . With alarm system  170 , no individual manual setting of the alarm amplitude or signal frequency needs to be made in order that all users will automatically hear the alarm sooner or later, regardless of their hearing loss. 
     It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the invention.

Technology Classification (CPC): 7