Abstract:
A lost item finding system including at least two nearly identical locators. Either one can be used to find the other and whatever items are attached to it. In the preferred embodiment, acoustic signals near 6500 Hz are used to broadcast a search signal from an available locator to a lost one. Preferably, the search signal includes a sequences of tones having predetermined frequency differences between them. The lost locator is usually in a sleep mode, in which it nonetheless is capable of recognizing the beginning of the search signal. The locator turns on to an active mode in which it consumes more power. It determines the baseline frequency and then identifies whether the signal it is receiving conforms to the predetermined frequency differences between the sequence of tones. If it does, the lost locator transmits a beacon signal that can be perceived by the user as he searches for the lost item. The beacon signal may include both an audible signal and a flashing light emitting diode. Another feature allows the number of false alarms to be reduced, particularly in a noisy environment, but also allows the locators to be operable in a noisy environment. Yet further capabilities include the selection of baseline frequencies or the temporary deactivation of additionally available locators. Advantageously, both the transmission and reception of the audio signal is accomplished with one piezoelectric transducer, and the efficiency of the transducer is increased by forming a resonant cavity having a cap both protecting the piezoelectric transducer and forming a side emitting annular port.

Description:
RELATED APPLICATION 
     This application claims benefit of U.S. provisional application, Serial No. 60/152,691, filed Sep. 7, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to systems for finding lost objects. In particular, the invention relates to paired devices, particular those employing acoustic search signals, for finding the lost one of the pair. 
     BACKGROUND ART 
     It is common in personal and business life to lose small items in a relatively small area. A common example is a set of personal keys including keys for a personal automobile and probably for home and business. Automobile manufactures invariably supply duplicate sets of keys. The car owner usually carries one set in a pocket or purse and leaves the other set in a known location such as a key rack or storage container. If the primary set is misplaced, typically in process of changing clothes or emptying pockets or purses, the other, secondary set is usually readily available. However, the key owner usually wants to find the primary set, both because of the other keys attached to the key ring but also to assure that the secondary set is not subsequently also lost. Typically, the lost set of keys is known to be in a relatively restricted area, for example, at home in one of two or three rooms, since the lost keys were most probably used to drive home and open the door. Thereafter, their whereabouts in the home may be uncertain. As a result, the search for the missing keys needs to cover only a limited area, but the owner is usually in a rush to leave and wants to find them immediately. Similar limited-area searching is often required for eyeglass cases, television remotes, security badges, and the like. 
     SUMMARY OF THE INVENTION 
     A lost item finding system may comprise two or more identical or nearly identical locators. Transmission of a search signal from one of the locators causes one or more of the other locators to emit a beacon signal, such as an audible signal and a flashing light, enabling the user to locate the lost locator and attached items. The locators may substantively differ only in a programmed identification code used for either transmitting and/or receiving the search signal. 
     A preferred embodiment uses an acoustic transducer, for example, a piezoelectric transducer, to receive an acoustic search signal, to transmit a corresponding acoustic search signal, and to transmit an audible beacon signal. In transmission mode, the piezoelectric transducer may be subjected to bipolar pulsing across its two inputs. In reception mode, one input is left floating and connected to amplifying, pulse shaping, and counting circuitry while the other input is held at a fixed potential. 
     The acoustic transducer may be coupled to a Helmholtz resonant acoustic cavity tuned to the resonant frequency of the piezoelectric element, preferably having an annular output port coupled to a cylindrical cavity. A cap may both define the end of the cavity and be spaced from the case enclosing the cavity to form the annular output port. 
     Preferably, each locator can be selectively disabled, allowing two locators to perform the responding and beaconing without interference from additional locators. 
     The acoustic search signaling transmission and detection may be arranged to minimize the effects of ambient noise to allow operation in particularly noisy environments. The search signal may consist of a sequence of tones having well defined frequency differences and separated by quiet periods. The detection of the search signal on one hand may require the detection of a quiet period following the tone sequence but on the other hand may respond to multiple transmissions of the tone sequences in a noisy environment. 
     The baseline frequency, the first tone of the tone sequence, may preferably be any frequency in a broad range, and the detection circuitry nonetheless can detect the tone sequence by determining the frequency differences of the tones relative to the baseline frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of one embodiment of the lost item locator of the invention. 
     FIG. 2 is a schematic representation of the operation of the paired locators of the invention for finding one of them attached to lost items. 
     FIG. 3 is a simplified schematic diagram of the electronic circuitry usable with an audio embodiment of the invention. 
     FIG. 4 is a partial plan view of the section of a case of FIG. 1 including the audio transducer cavity. 
     FIG. 5 is a cross-sectional view of the audio transducer cavity taken along view line  5 — 5  of FIG.  4 . 
     FIG. 6 is a timing diagram of the sequence of audio tones used for searching. 
     FIGS. 7 and 8 are flow diagrams of an algorithm usable with the electronic circuitry of FIG.  3  and primarily concerned with the operation of a lost locator receiving a search signal and responding with beacon signals. 
     FIGS. 9 and 10 are a flow diagrams of an algorithm complementary to those of FIGS. 7 and 8 usable also with the circuitry of FIG.  3  and primarily concerned with operation of an available locator. 
     FIG. 11 is a schematic diagram of a locator using an RF search signal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective illustration of an embodiment of a locator  10  to be used in the lost item finding system of the invention. The locator  10  includes a molded plastic case  12  enclosing the electronics and audio transducer to be described later. The case  12  is approximately 5 cm long, 3 cm wide, and 1 cm thick so that it can be easily carried in a pocket or purse. The case  12  includes a loop  14  at one corner around which can be wrapped a key ring  16  holding one or more keys  18 . However, other items may be attached to the case  12 , such as a television remote or security badge among many other possible items which tend to be misplaced. A light emitting diode (LED)  20  protrudes from another corner of the case  12  and selectively emits a visible signal. A switch button  22  attached to a normally open switch is accessible on one flat side  24  of the case for convenient finger activation. A disk-shaped cap  26  is spaced by a small distance from the flat side  24  of the case  12  and is supported thereon by three unillustrated legs. The cap  26  protects an acoustic transducer under it and also forms one side of an acoustic resonator to be described later. 
     Two such locators  10   a  and  10   b  illustrated in perspective in FIG. 2 form a lost item finding system. One locator  10   a  is assumed to be misplaced along with its attached key or similar item while the other locator  10   b  is assumed to be readily available from, for example, a key rack or security box. The attached items are not shown, but they may be the primary and spare key set or a house key and a television remote, either of which is subject to being misplaced. When the user decides that the first locator  10   a  and attached item is lost, he finds the second locator  10   b  and presses the button  22  with his finger  30 . The second locator  10   b  emits a coded acoustic signal  34 , which is detected by the lost first locator  10   a . In response, the lost first locator  10   a  emits another audible acoustic signal  36  and a visible optical signal from its LED  20 , which enable the user to locate the lost first locator  10   a . Preferably, the two locators  10   a ,  10   b  are nearly identical so that either one can be lost and then found by activating the other one. 
     A simplified schematic diagram of the electronics contained in the case  12  is illustrated in FIG.  3 . The illustrated items are soldered to a printed circuit board shaped to fit inside of the case  12 . A programmable microprocessor  40 , such as a 12C509A available from Microchip Technology, forms the controller of the locator  10 . Two serially connected 3V batteries form a DC power supply  42  connected to the power input V CC  of the microprocessor  40  through the LED  20 . The microprocessor  40  is designed to operate from a DC power source providing between 3 and 5.5V. In order to provide a voltage in the middle of the operating range, the LED  20  drops the 6V of battery voltage to 4V at the power input V CC  of the microprocessor  40 . In sleep mode, the microprocessor  40  consumes only about 1 μA of current while in active mode it consumes about 1 mA of current. The locator  10  stays in sleep mode until its button  22  is activated or it hears an acoustic signal. The microprocessor  40  has three outputs OUT- 1 , OUT- 2  and OUT- 3 . The first output OUT- 1  connects to a load resistor  44  connected to ground. When the first output OUT- 1  goes high, current flows out of the microprocessor  40  through that output, and corresponding additional current flows into the microprocessor power input V CC  through the LED  20  causing it to emit additional light. The second output OUT- 2  is connected to the inputs of both a buffer  46  and an inverting tri-state buffer  48 , both of which may be formed from a 74HC125 quad tri-state buffer and a resistor. The buffer  46  simply outputs a high-power signal corresponding to its low-power input while the inverting tri-state buffer  48  either outputs a high-power signal inverted from its low-power input or presents a high-impedance output depending upon the signal from the third output OUT- 3  from the microprocessor  40 . A piezoelectric audio transducer  50 , such as available from Murata Electronics, is connected across the outputs of the buffer  46  and the inverting tri-state buffer  48 . Assuming that the microprocessor  40  has activated the inverting tri-state buffer  48  with its third output OUT- 3 , the outputs of the two buffers  46 ,  48  are complementary, either 0 or 6V, with the polarity determined by the signal from the microprocessor&#39;s second output OUT- 2 . The bipolar signal driving the transducer  50  generates a louder audio signal as the piezoelectric unit is driven in both directions. For audio signaling, this output OUT- 2  is switched at a frequency in the range of 5.5 to 7.5 kHz, with the effect that the audio transducer emits an audio signal in this frequency range. 
     However, in sleep mode or other periods in which the microprocessor  40  is awaiting reception of an audio signal, the microprocessor&#39;s third output OUT- 3  inactivates the inverting tri-state buffer  48  to its high-impedance output mode and its second output OUT- 2  goes low, thus grounding the other side of the audio transducer  50 . In this state, the audio transducer  50  can detect an audio signal and convert it to an electrical signal which is amplified by amplifier  52 . That is, the transducer  50  can act both as an audio transmitter and audio receiver. The detected audio signal, presumably also at 5.5 to 7.5 kHz, is amplified by amplifier  52  and then filtered by a low-pass filter including a serially connected capacitor  54  and a grounded resistor  56  which removes any DC bias. The audio signal with any DC bias removed is input to a comparator  58  having hysteresis so that any low-level noise is ignored and the generally sinusoidal detected audio signal is converted to a square wave more amenable to digital processing. Each cycle of the square wave amounts to a pulse within the digital circuitry. 
     The output of the hysteretic comparator  58  is input to a third input IN- 3  of the microprocessor  40  and to a divide-by-256 circuit  60 , which may be a 74HC393 dual 4-bit counter with the high-order output bit, which changes state every 128 pulses, treated as the output of the divider  60 . The divider output is connected to a second input IN- 2  of the microprocessor  40 . The third microprocessor input IN- 3  serves as a received data input that allows the microprocessor  40  to analyze the received audio signal and determine whether the audio signal is from the paired locator seeking to find the lost locator with the illustrated circuitry. The divide-by-256 circuit  60  provides one method of waking up the microprocessor  40  after 128 input pulses of the external audio signal (20 ms for a 6500 Hz signal) have been detected to further determine if the detected audio signal is coded to form a locating signal from the other locator. Using an algorithm to be described in more detail later, if it is determined that a correctly coded audio signal has been received, the microprocessor  40  causes the LED  20  to flash and the audio transducer to emit an audio signal that can be heard by the user. 
     The circuitry of FIG. 3 does not illustrate common elements, such as transistors, resistors, and capacitors that form the amplifier  52  and power supply leads connected to the non-processor active elements. 
     The microprocessor  40  is also woken up, if it is asleep, by a low signal applied to its first input IN- 1  produced when a normally open switch  62  mechanically connected to the button  22  is closed. The switch  62  is electrically connected between ground and the input IN- 1 , also serially connected through a resistor  64  and the LED  20  to the power supply  42 . When the input IN- 1  is activated, the microprocessor  40  causes the audio transducer  50  to emit a coded sequence of audio tones in the range of 5.5 to 7.5 kHz. However, as will be described in reference to the programmed algorithm, the button can be activated in particular ways to cause the locator to perform other functions. 
     In view of the limited power from small batteries and the possibly noisy ambient environment, it is important that the audio transducer be as efficient as possible both for transmission and reception. The bipolar pulsing with the complementary outputs of the two buffers  46 ,  48  increases the signal generating ability. Further efficiency is achieved with a Helmholtz resonator similar to that disclosed in the document published by Murata Electronics entitled “Piezoelectric Devices Application Manual,” pp. 84-91. As illustrated in the plan view in FIG.  4  and the cross-sectional view of FIG. 5 taken along view line  5 — 5  of FIG. 4, an upper half case section  70  is formed with a cylindrical cavity  72  between a downwardly projection tubular wall  74  terminating in a sharp annular ridge  76 . The piezoelectric transducer  50  includes an integral assembly of a 20 mm brass disk  80 , a piezoelectric ceramic layer  82 , and a flexible silver electrode layer  84  are fixed to the annular ridge  76  with a flexible adhesive. Unillustrated electrical contacts are made to the brass disk  80  and the silver layer  84  to allow the circuitry of FIG. 3 to apply a voltage across the piezoelectric ceramic layer  82 . The resultant lateral contraction or expansion of the piezoelectric layer  82  causes it and the brass disk  80  to flex alternately convexly and concavely, thus launching a sound wave. Alternatively, if an externally generated sound wave flexes the piezoelectric assembly, a voltage signal is generated between the two contacts in proportion to the intensity of the sound wave. The piezoelectric transducer  50  is mounted to the annular ridge  76  at a vibrational node of transducer  50 , at a diameter at which no vibration occurs. This mounting method causes the least mechanical suppression of vibration and thus provides for the highest efficiency of the transducer. 
     The cavity  72  together with the sound port  86  to the surrounding environment form a Helmholtz resonator. The dimensions of the cavity  72  and the sound port  86  determine the frequency of the resonator, here around 6500 Hz. The Helmholtz resonator is a harmonic oscillator and can be analogized to a weight attached to a spring, both of which can be described by the same equations. The air in the inner cavity  72  compresses and expands analogously to the motion of a spring, and the air in the sound port  86  is pushed back and forth and is analogous to the weight. Note that the Helmholtz resonator functions only when the cube root of the volume of the cavity  72 , the square root of the area of the sound port  86  at the interface to the cavity  72  and the effective length of the sound port  86  are all significantly less than the wavelength of the sound at the resonant frequency. 
     Prior-art Helmholtz resonators used in conjunction with piezoelectric transducers use a single circular port above the cavity  72 . Such a structure has two disadvantages for a lost item finder. If the single sound port for the Helmholtz resonator is lying on a surface face down, or for example, sandwiched between two pillows on a sofa, the resonant characteristics of the resonator will be adversely altered and the emitted sound will be significantly blocked. Further, the intensity of the sound directly in front of the sound hole of the conventional Helmholtz resonator is very loud, and it can even be painful to someone who might place it directly up to his ear. To overcome these problems, the resonant cavity  72  is preferably covered with a cap  26  spaced from the case half  70  by three unillustrated standoffs so as to form a sound port that is an annular channel  86  of predetermined height and length, where the height of the cap  26  and dimensions of the cavity  72  are chosen to form a resonant system that resonates at 6500 Hz. 
     I have derived an approximate formula for the resonant frequency of the cavity            f   0     =       c     2      π                4      g       1.3      d                     h        (     g   +       d                 g         )       .               ,                          
     where c is the velocity of sound (3.44×10 4  cm/s at 24° C.), d is the diameter of the cavity 72, g is the gap between the outer face of the cavity  72  and the cap  26 , and h is the height of the cavity  72 . Exemplary values are d=1.27 cm, g=0.102 cm, and h=0.38 cm, which imply that the resonant frequency f 0  is 6488 Hz. 
     With the structure for the Helmholtz resonator of FIGS. 4 and 5, the sound is much less likely to be blocked because it is emitted in 360 degrees pattern parallel to the face of the device. Further, the maximum sound intensity immediately in front of the improved resonator is significantly less than that immediately in front of a comparably efficient prior-art resonator. In one experiment the prior-art device and the improved device were found to generate the same intensity sound at a distance of 5 centimeters and beyond. At a distance of 0.5 centimeters, however, the improved device generated a sound pressure level that was 15 dB quieter than the prior-art device. Furthermore, the cap  26  protects the piezoelectric element  50  from being damaged by foreign objects. 
     The finding system of the invention can be programmed to operate in a number of different modes. The programming may be accomplished by coding the necessary instructions in the assembly language of the microprocessor and impressing those instructions in machine code form into the non-volatile memory of the microprocessor  40 . 
     One set of operating procedures will now be described based upon a coded sequence of tones schematically illustrated in FIG.  6 . The tone sequence is used to communicate a search signal from the available locator to a lost locator. Each audio tone is at a fixed frequency and of finite duration. A single such tone sequence can in many circumstances be sufficient for finding the lost locator, but it is anticipated that the user will repetitively activate the locator&#39;s transmitter for yet further such sequences. Once the lost locator has verified that the coded sequence has been received, it emits both an acoustic and an optical signal acting as beacons to the user attempting to find the lost locator. A baseline frequency f 0  is assumed to exist in the 5500 to 7500 Hz range. Audio signals in this range can be efficiently generated and received within a small case. Its precise value may depend upon environmental and aging effects of the transducer and its electronic controls. An inexpensive RC clock internal to the microprocessor  40  is accurate to no more than ±10% so timing between multiple locators must accommodate such variations. Further, a procedure is available to change the baseline frequency to one found to be most sensitive in actual use. The lost locator however, should be able to detect the coded tone sequence regardless of the baseline frequency f 0  as long as it is within the permitted range. 
     The tone sequence includes a first tone  90  at the baseline frequency f 0  enduring for 170 ms followed by a first quiet period  92  of 250 ms. A second tone  94  is emitted at a frequency that is 250 Hz below the baseline frequency f 0  for a period of 120 ms followed by a second 250 ms quiet period  96 . A third tone  98  is emitted at a frequency 50 Hz below the baseline frequency f 0  for a period of 120 ms followed by a third 250 ms quiet period  100 . A fourth and final tone  102  is emitted at a frequency 250 Hz below the baseline frequency f 0  for a period of 120 ms. A long quiet period  104  follows the tone sequence until another such tone sequence is transmitted as a further search signal. 
     This coded sequence is advantageous when operating in a noisy environment including, for example, music, conversation, or other combinations of noise. In such an environment, it is possible that almost any frequency is likely to be detected at some time. The precise frequency shifts, however, are unlikely to be duplicated by background noise. The frequency changes and the relatively long quiet periods also overcome interference resulting from the generated tones echoing for tenths of seconds or more in some rooms. 
     To further improve the reliability of the locator by accommodating situations where the tone sequence is duplicated by environmental noise, one feature requires that a quiet period be observed at the end of the tone sequence. The requirement that the tone sequence be followed by a quiet period all but eliminates the possibility of false alarms, but it also creates a problem. In noisy environments a locator will not respond to the tone sequence generated by another locator because the quiet period will not be observed. To resolve this problem, the requirement that a quiet period be observed may be temporarily suspended for approximately ten seconds when the tone sequence is recognized but the quiet period is not observed. Temporally removing the required observation of a quiet period does not create a problem, however, because it is very unlikely that any noise source will produce two valid sequences of the search signal in a ten second period. Thus, in a noisy environment the locator will not respond on the first try but it will respond on the second try and each try thereafter as long as each try is within ten seconds of the previous try. Having to press the signaling button more than once is not a problem because this is the normal mode of operation for a person anxious to retrieve a lost item. 
     The microprocessor  40  may be programmed to perform many different functions. However, an advantageous set of functions is described with respect to the following diagrams. 
     During periods in which the finding system is not being used, the microprocessor  40  remains in a SLEEP mode consuming very little power and doing little more than awaiting a Wake Up  110 , a form of processor reset, as illustrated in the flow diagram of FIG.  7 . Wake Up  110  occurs when, during the SLEEP mode, either the microprocessor&#39;s IN- 1  or IN- 2  input changes state in either direction, that is, either the button  22  is pressed or released, or a further  128  pulses have been detected by the divide circuit  60 . Signaling data on the IN- 3  input does not cause a Wake Up. The microprocessor  40  is also reset when the battery is first installed or replaced. Depending on whether the reset was the result of installing the battery or waking up as the result of an input changing state, different initializations are made to put the microprocessor  40  and other circuitry in the correct state. A test  112  is then made for whether the button  22  is pressed by determining if the IN- 1  input is low. If it is, processing is diverted to a BUTTON PRESSED routine  114  to be described later with respect to FIG.  9 . If it is not pressed, the time T is measured in step  116  for the receipt of five input pulses on the IN- 3  input. If test  118  determines that the time T is not between 650 and 950 μs, that is, an average frequency of between 5300 and 7700 Hz, then processing returns to the SLEEP mode  120 . 
     If the time T is within specified limits, an initial counting step  130  measures the number of pulses arriving on the third input IN- 3  over a 10 ms period. This 10 ms count occurs many times during the receiving algorithm. It is easily implemented knowing how long it takes to execute each instruction in the microprocessor  40 . A small loop is executed a fixed number of times while the number of high-low transitions on the third input IN- 3  is counted. For a 5500 to 7500 Hz audio signal, the number of counts would be between  55  and  75 . While counting the pulses, the time between successive pulses is also measured. At low levels, noise (either acoustic or electronic) can cause a pulse to be missed. Therefore, if the spacing between pulses is between 260 and 350 μs, twice the expected spacing, the algorithm assumes that a pulse has been missed and it adds a count. If during a 10 ms period the number of missed pulses exceeds seven, the process does not continue to add counts; the pulse train is either too corrupted to be accurately corrected or it is being generated from sounds other than the signaling tone. 
     Test  132  determines if an average count has already been determined. If it has not yet been determined, in step  134 , the average count is determined from the first ten 10 ms counts of step  130 . The averaging may advantageously discard the highest three and lowest three counts and average the remaining four counts. If at any time during the averaging process the highest and lowest count differ by more than two counts, a decision is made in step  136 , and the processing returns to the SLEEP mode. If a decision to start over is not made, the process returns to step  130  to determine the number of pulses that arrive during the next 10 ms counting period. 
     If test  132  shows that the average count has been determined, processing skip to step  138 . At this point, the first tone has been determined to have been identified, and the value of the average count is determined to correspond to the baseline frequency being used by the sender. Also, a time of so identifying the first tone is saved. Then, test  138  determines if the flag has already been set indicating the identification of the fourth tone. If the fourth tone has been identified so that the full tone sequence has been received, execution transfers to transfer point A in FIG. 8, the start of a routine that is concerned with features for dealing with background noise. If the fourth tone has not been identified, a test  142  determines if more than 500 ms has elapsed since the identification of the previous tone, specifically the first, second, or third tone. If such a long period has elapsed, then the anticipated tone is determined to have not been received and the process is returned to the SLEEP mode  120 . 
     If less than 500 ms has elapsed, then test  144  determines if the second tone has already been identified. If it has not, then step  146  attempts to identify the second tone. The second tone is a tone about 250 Hz lower than the first tone. Therefore, its 10 ms counts must be two or three counts less than the baseline count. The identification of the second tone is successful when four of five sequential 10 ms counts meet the criteria, whereupon a flag is set for the identification of the second tone and the time is saved for when this occurred. Whether or not the second tone has been identified in step  146 , processing returns to the 10 ms count entry point  148  leading to the initial counting step  130  to determine the number of pulses that arrive during the next 10 ms counting period. 
     If test  144  determines that the second tone has already been identified, test  150  determines whether the third tone has been identified. If the third tone has not been identified, step  152  attempts to identify the third tone. The third tone is a tone about 50 Hz lower than the first tone. Therefore, its 10 ms counts must be zero or one counts less than the baseline count. The identification of the third tone is successful when four of five sequential 10 ms counts meet the criteria, whereupon a flag is set for the identification of the third tone and the time is saved for when this occurred. Whether or not the third tone has been identified in step  152 , processing returns to the initial counting step  130  to determine the number of pulse that arrive during the next 10 ms counting period. 
     If test  150  determines that the third tone has already been identified, then step  154  attempts to identify the fourth tone following the same criteria as for the second tone identification  146 . When the fourth tone is identified a flag is set for its identification and the time is saved for when this occurred. Whether or not the fourth tone has been identified in step  154 , processing returns to the initial counting step  130  to determine the number of pulses that arrive during the next 10 ms counting period. 
     With reference to FIG. 8, transfer point A from FIG. 7 is followed by a test  160  for determining whether a background noise timer remains set, that is, has not expired by being decremented to zero. 
     If the test  160  determines that the background noise timer is no longer set, a test  162  determines whether a period of 1500 milliseconds has elapsed since the fourth tone has been identified. If 1500 milliseconds have not elapsed, step  164  attempts to identify a quiet period. A quiet period is identified when four out of five sequential 10 ms counts have count values of less than 25. Test  166  determines if a quiet period has been successfully identified. If it has not been, processing returns to the initial counting step  130  to determined the number of pulses that arrive during the next 10 ms counting period. Since the average count has already been determined and the fourth tone has already been identified, test  132  followed by test  138  of FIG. 7 will return to transfer point A with the required 10 ms count. 
     If test  166  determines that the quiet period has been identified, the search for the signaling sequence has been successfully completed, and step  172  performs the beacon signaling, which preferably includes sounding the audible buzzer implemented with the piezoelectric transducer and flashing the LED for a predetermined length of time of, for example, eight seconds. The audible beacon signal is generated by setting the third microprocessor output OUT- 3  to activate the tri-state buffer  48  and outputting a square wave on the second output OUT- 2  at the desired audio frequency. The visible beacon signal is generated by outputting a rectangular pulsed signal on the first output OUT- 1  at a frequency and duty cycle selected for the flashing LED signal. However, only one of the visible and audio signals are required for the beacon signaling. The frequency of the audio signal may be selected to be most audible to the user but should be within the range that can be handled by the piezoelectric transducer and its associated resonant cavity. After the beacon signaling of step  172 , the SLEEP mode  120  is entered. 
     Returning now to test  160 , if the test determines that a background noise time remains set, it is reset back to ten seconds at step  154 . Resetting the time back to ten seconds allows sequential search signals, spaced at intervals of less than ten seconds, to be recognized in the absence of a quiet period at the end of each. Thereby, this and subsequent quick button pushes on an available locator will cause the beacon signal from a lost locator to be generated. 
     If test  162  determines that more than 1500 milliseconds have elapsed since the identification of the fourth tone, the conclusion is that no quiet period has been observed in a reasonable time after the tone sequence has been identified and that the environment is very noisy so that no quiet period is likely to be observed. However, rather than assume that a valid tone sequence has nonetheless been received, the algorithm requires that the user again press the button within a 10 second period. The requirement that the user send out a second set of search tones further reduces the possibility of false recognition. To accomplish this, step  168  sets the background noise timer to 10 seconds and execution returns to the sleep mode  120  awaiting receipt of another tone sequence. Assuming the user does reactivate the identification procedure of FIG. 7 within ten seconds, when processing returns to the noise analysis of FIG. 8, the noise timer will again be reset to 10 seconds. 
     The process described above with respect to FIGS. 7 and 8 primarily concerns the operation of the lost locator that is being sought by the available locator. The description now turns to the operation of the available locator, the sole activation of which in most circumstances is the pushing of the button by the user. The available locator generates the search signal, but other types of operations are possible, as will be described. 
     As illustrated in FIG. 9, the process is initiated by the Button Pressed  114  corresponding to the user pressing the button  22  and closing the switch  62  of FIG. 3 to thereby pull the input IN- 1  low. This point may have been reached following the Wake Up  110  of FIG. 7 from the SLEEP mode or may be accomplished by a the button  22  being pressed with the microprocessor being in its active mode. Test  182  determines if a Single Press timer remains set and has not expired. The Single Press timer allows a circumvention of the usual system requirement that the button be pressed at least twice within one second to initiate the transmitting of the search signaling tone sequence. The requirement that the button be pressed twice prevents spurious signaling when the locator button is accidentally pushed, for example, as it is jostled in a pocket or purse. If the Single Press timer remains set, indicating that the button was double pressed within the preceding 15 seconds, step  184  generates the audio tone sequence represented in FIG. 6 that constitutes the search signal. 
     On the other hand, if the Single Press timer was never set or has expired, test  186  determines if the button has been doubled pressed, that is, pressed a second time within one second after the release of the first button press. If it was not again pressed in this short time, test  187  determines if the button is still held down from the first press. If the button is no longer pressed, operation is returned to the SLEEP mode  120 . If the button continues to be pressed, step  188  turns on the LED allowing it to be used as a search light to view objects in the dark such as keyholes and the like. Thus, if the user instead of doubling clicking the button to transmit a search signal, instead presses the button down and holds it down, the LED acts as a flashlight for the duration of the button being held down. In step  189 , when the button is released, the LED is turned off, and operation is returned to the SLEEP mode  120 . 
     Returning now to test  186 , if the button was pressed twice within one second, step  184  generates the audio tone sequence of the search signal. In step  190 , the Single Press time is set to 15 seconds. Step  192  is a wait state of up to 0.5 seconds if the button remains pressed. After the 0.5 second wait, test  194  determines if the button continues to be pressed. If it is not pressed, operation returns to the SLEEP mode  120 , during which a further pressing of the button will return operation to the Button Pressed point  114  at the beginning of FIG.  9 . 
     If the button continues to be pressed in test  194 , step  196  generates a short double beep tone to warn the user that he needs to make a decision during the next two seconds. Step  198  waits up to 2 seconds while the button continues to be pressed. Test  200 , illustrated in FIG. 10, then determines if the button continues to be pressed. If the user has released the button so it is not pressed, the locator is placed in a 90 second wait state  204  waiting for the button to again be pressed. During this time, the associated locator is inactive even to the receipt of search signals. This 90 second wait is useful in inactivating a third locator while a first locator is searching for a second locator. Responses from the third locator would result in two beacon signals from the second and third locator which would frustrate the search for the lost second locator. If at any time during the 90 seconds the button is pressed, test  206  determines the current state. If it is pressed, operation transfers to a Generate Tones point  208  just before the step  184  for generating the audio tone sequence of the search signal. This allows the user to use the unit before 90 seconds has elapsed. During those 90 seconds, it would be very disconcerting for the user to press the button and hear no response. The 90 second wait should be sufficient for the first locator to find the second locator. If the button, on the other hand, is not pressed during the 90 second deactivation period, operation returns to the SLEEP state  120 . 
     If the user has continued to press the button after the two second wait  198 , the test  200  is positive, and step  210  generates a short triple beep tone sequence. The short triple beep notifies the user that he needs to make a decision during the next two seconds in regards to a frequency calibration procedure for setting the baseline audio frequency f 0 . Step  212  waits up to 2 seconds while the button continues to be pressed. Test  214  then determines if the button continues to be pressed. If it is pressed, step  216  sets the baseline audio frequency to 6500 Hz, the middle of the resonant frequency range of the piezoelectric transducer and its associated resonant cavity. Then, step  218  generates a short quadruple beep tone to indicate to the user that the baseline frequency has been reset to the default value of 6500 Hz. A wait release state  220  is continued until the button is released, and thereafter operation returns to the SLEEP mode  120 . 
     If, on the other hand, the user released the button during the two second wait  212 , test  214  is negative resulting in step  226  setting the baseline frequency to an interim value of 5500 Hz. The tone at the interim baseline frequency is generated for up to 10 seconds or until the button is pressed. Test  230  determines whether the button is pressed. If it is not, operation returns to the SLEEP mode  120 , and the interim baseline frequency is ignored and the old value of the baseline frequency is retained. 
     If test  230  shows that the button is pressed, another wait state  232  is entered, following the cessation of the audio tone at the interim baseline frequency, where it will wait up to seconds while the button continues to be pressed. During this period, the user must decide if he is satisfied with the interim baseline frequency or wishes to cycle through the remaining available frequencies. Test  234  determines if the button continues to be pressed. It if continues to be pressed after two seconds, step  236  converts the interim baseline frequency to the permanent one. Operation transfers to the quadruple beep step  218  awaiting release of the button and transfer to the SLEEP mode  120 . 
     If, however, the user released the button during the 2 second wait period  232 , test  234  is negative and step  238  increments the interim baseline frequency by 125 Hz. Test  240  determines whether the incremented value is greater than the maximum permitted value of 7500 Hz. If it is not above 7500 Hz, operation returns to step  228  for generating the audio tone at the newly incremented value of the interim baseline frequency. If the increment value is above 7500 Hz, operation returns to the frequency initialization step  226 , which resets the interim baseline frequency to 5500 Hz and continues the frequency setting sequence. 
     The baseline frequency f 0  may be reset by this procedure to increase the sensitivity of the lost locator to the search signal. Note that if the piezoelectric transducer and its associated resonant cavity were guaranteed to exhibit peak performance around 6500 Hz and the clocks in the microprocessors where guaranteed to all run at close to the same frequency, the described calibration procedure would be unnecessary. In actuality, both the piezoelectric transducer and its associated resonant cavity as well as the clock in the microprocessor have tolerances no more precise than ±10%. When using the default baseline frequency of 6500 Hz, almost all units will work, however, the operating range in some instances can be improved by changing the frequency. Note that even the 6500 Hz default frequency will vary from locator to locator because the clocks in the microprocessors that control the tone generation will vary in frequency. 
     It is understood that the above described algorithm does not include many of the details of initialization, synchronization, and timing. However, these are standard issues easily addressed by a programmer of ordinary skill. 
     As described, any one of thousands of locators manufactured according to the described embodiment can be used in conjunction with any other of the many locators. Although this feature simplifies the manufacture, inventory, and distribution of the locators, an interference problem may arise if the locating system is widely used by many different parties in a close space. For example, multiple users using the system to locate coats in a restaurant coat room. The described system does not differentiate between different users. However, it is possible to set up more complex search signaling algorithms. For example, a longer sequence of tones may be used in which the frequency changes are programmed differently between a significant number of users. This could be accomplished by an 8-bit toggle switch embedded in each locator and changeable by the user to allow 256 different tone sequences. A means could also be provided to program different tone sequence codes into erasable non-volatile memory embedded in each locator. 
     The use of acoustic search signaling is advantageous in not needing to meet any government standards for electromagnetic emission. Such acoustic signaling can thus be advantageously applied to other types of finding systems in which the search unit and the beaconing unit have different designs with no ability to perform the other function. 
     The paired object locating system of the invention is not limited to using acoustic energy to signal a lost locator. Another approach has the paired locators using radio frequency (RF) signaling techniques such as are commonly used in garage door openers and automobile door remotes. During manufacture or in the field, the coding information for use by both the transmitter and receiver would be encoded into two or more matched locators so that an encoded RF signal emitted by one of them is recognized by any other of them. For example, as illustrated schematically in FIG. 10, an RF transceiver  250  is connected to an antenna  252  for both transmission and reception of a search signal. A controller  254  programmed with algorithms similar to those described for the acoustic finding system both controls the RF transceiver  250  and receives signals from it. Manual activation of the button controlled switch  62  connected to the controller  254  causes it to instruct the RF transceiver  252  to transmit an RF signal coded according to an identification code provided by the controller  254 . When such an RF search signal is received by an associated locator, its RF transceiver  250  transfers the coded message to its controller  254 , which checks whether the identification code is a match. If it is not, the search signal is ignored. If it is the same code, the controller  254  broadcasts beacon signals on the LED  20  and the audio transducer  50 . Of course, the RF identification codes may be more complex. Two different codes may be paired between two locators, or one locator may be selectively activated to find only one of several other locators. 
     It is of course appreciated that the locators can be incorporated as integral parts into other similarly sized items. For example, many automobiles are now sold with remote entry systems having a pocket remote key fob for locking and unlocking the car doors and the like, and usually two such remotes are supplied with the car. The locators of the invention can be incorporated into such remotes. Some car keys are intelligent in that they have embedded RF identification chips and RF transmitters for added security in starting the car. Again, the locator of the invention may be incorporated into the already enlarged head of the intelligent key. Also, the car key code may be used as the search code for the finding system. 
     The invention as described combines simple, compact, and inexpensive structure and circuitry with many programmable functions. Since the features are implemented in large part in software, the system is flexible and can accommodate additional functions. The described algorithm is illustrative only, and the same or similar functions can be programmed in different manners.