Abstract:
A water leak detector system with alarm can be used to monitor inaccessible areas under various household appliances and warn of water leaks. A leak detection mat, having base layer of varying and appropriate sizes has disposed on it one or more pairs of electrodes arranged in a pattern around the upper surface of the base mat such that the two wires of each pair are parallel to the other and generally separated at a constant distance, and together the pair is positioned in a pattern across the surface. An absorbent layer is affixed to the base layer, covering and in contact with the electrodes. The absorbent layer may be impregnated with a soluble ionic salt to increase electrical conductivity when the adsorbent layer is wetted. A sensing and transmitting circuit having two input connections is affixed to the base substrate, with one end of each electrode attached to one of the input connections of the sensing and transmitting circuit. A plurality of leak detection mats may be used in a single system, each mat located under a potential water leak source. Upon water falling upon and being adsorbed by the adsorbent layer, the electrical resistance between the two electrodes at points nearest the leak will decrease. This decrease in resistance is detected by the sensing and transmitting circuit, which transmits a signal. An alarm unit receives the signal and activates an audible and/or visual alarm, the signal being sent either by radio waves or by an electrical signal through wires. The alarm unit is capable of monitoring a plurality of leak detection mats.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of water leak detection systems utilizing a pair of electrical conductors or probes to detect the presence of an undesirable accumulation of water by changes in the electrical impedance between the two electrodes. 
     2. Description of the Related Art 
     Water leakage from various appliances and plumbing fixtures in residential and commercial buildings has often been a problem and has caused significant damage to a building&#39;s structure, trim and cabinetry. Many appliances in the home use or contain water. These include washing machines, dishwashers, hot water heaters, refrigerators, including those equipped with ice makers, and air conditioners. Any water leakage from these appliances could flow onto the underlying floor surface and thence down into the underlying floor structure. Flooring and floor support structures manufactured of wood are susceptible to rot if left in a wet or damp condition for an extended length of time. Even a slow or intermittent leak of water from under an appliance can, over enough time, cause sufficient damage to require the replacement of the flooring, subfloor, and supporting joists. These leaks, especially smaller ones, often cannot be detected visually due to the low clearance between the bottom of the appliance and the floor. 
     In addition to appliances, certain plumbing fixtures and devices located within cabinetry or vanities under kitchen and bathroom sinks are susceptible to leakage and are not readily visible. These may include compression joints used in drain fittings, garbage disposals and water filters. Slow or intermittent leakage from these joints and devices may go unnoticed and cause damage to expensive cabinetry as well as to the floor systems. Even a large, substantial leak may not be apparent from outside the sink cabinetry. 
     Several leak detection systems to detect and warn of water leaks are available in the prior art. These include systems utilizing a pair of electrodes which detect the presence of water by a means which utilizes a change of electrical resistance between the two electrodes. The presence of water or moisture, which is capable of conducting electricity, between the two electrodes lowers the electrical resistance between the two electrodes. The reduced resistance can be detected by an appropriate electrical circuit and used to activate an audible or visual alarm. 
     These leak detection systems generally work well for certain applications, particularly those for protecting large areas against large volumes of water leakage. However, their ability to detect slow or intermittent leaks is limited. For example, some systems utilize short lengths of electrodes, which are posited vertically and capable of detecting a volume of water of at least a minimum height located at the electrodes. Small leaks, or leaks which do not produce pools of water which reach the electrodes, will not be detected. 
     Other systems use a length of paired conductors or electrodes, mounted in parallel on a long substrate, such as a strip or tape. Some of these type incorporate an absorbent layer between the electrodes which aids in the detection of smaller volumes of leakage. To use these systems to protect a certain area, the strips must be manually positioned either in a circumferential pattern around the perimeter of the area or in an anfractuous or sinuous pattern across the area. The former pattern would not detect small or intermittent leaks which did not flow laterally from under the appliance. The latter requires the appliance first to be moved and then the strip or tape is manually configured and posited in the desired pattern. Moving the appliance is often impractical or inconvenient, while bending, folding and otherwise manipulating the tape or strip into the desired pattern may damage the tape or short-circuit or break the electrodes, as might happen as well when the appliance is moved back into its original position. In either case a qualified professional, rather than the homeowner, would usually be required to properly lay out and install the system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a leak detection mat and system which satisfies the need for detecting slow or intermittent leaks from under appliances with limited accessibility and which may be installed quickly and easily. The leak detection mat comprises a thin base substrate of dimensions proportional to the area to be monitored or the size of the appliance, upon which a pair of electrodes is affixed in a pattern, such that substantially all points of the base substrate area are located between some point on each of the two electrodes. A layer of an absorbent, porous material is posited on top of the pattern of electrodes and is adhered to the base layer. 
     The leak detection system includes the leak detection mat, wherein one end of each electrode is connected to a sensing means which utilizes the electrical resistance between the two electrodes. When a leak occurs, water dripping on the absorbent layer would diffuse through the absorbent layer and reach the two electrodes, reducing the resistance between the two electrodes and thereby creating an electrically conductive path. The sensing means, comprised of an electrical sensing and transmitting circuit, is connected to the ends of each of the electrodes which could detect the change in resistance between the two electrodes and transmit a signal. The sensing and transmitting circuit may be posited directly on the base substrate. In one embodiment of the invention, the sensing and transmitting circuit would be powered by a small battery. The signal is transmitted to an alarm means, being a remote receiving and alarm station in the preferred embodiment, either by wires in one embodiment or by wireless means, such as radio waves, in the preferred embodiment. The receiving and alarm station is mounted in a more visible and accessible location, thereby giving timely notice to the building&#39;s occupants of a potentially destructive leak. 
     One object of this invention is to provide a leak detection device which can be easily installed under home appliances having a narrow clearance between its bottom surface and the floor. 
     Another object of this invention is to provide a leak detection device capable of detecting a small leak originating from anywhere under an appliance. 
     Another object of this invention is to provide a leak detection device capable of monitoring the entire area under an appliance, but which can be installed simply by sliding the detector under the appliance. 
     Another object of this invention is to provide a leak detection system capable of monitoring a plurality of leak detection devices located in various locations in the building. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the leak detection mat. 
     FIG. 2 is a sectional perspective drawing of the leak detection mat. 
     FIG. 3 is a diagram of the sensing and transmitting circuit. 
     FIG. 4 is a diagram of the circuits comprising the receiving and alarm station. 
     FIG. 5 is a diagram of the display-enable flip-flop (FF) circuit in the receiving and alarm circuit 
     FIG. 6 is a diagram of the multivibrator and the oscillator circuits in the receiving and alarm station. 
     FIG. 7 is a diagram of a typical leak detection system in use, showing a plurality of leak detection mats, each with a sensing and transmitting circuit, and a central receiving and alarm station. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention disclosed herein is a leak detection system for use in residential, commercial and other buildings to detect leaks of water from appliances containing or generating water. As illustrated in FIGS. 1 and 2, the system includes a leak detection mat  101 , which is constructed with a base substrate  102 , upon the upper surface  109  of which arc disposed first and second electrodes  103 ,  104 . An thin, absorbent layer  105  is disposed above and adhered to most of the upper surface  109  the base substrate  102  and the two electrodes  103 ,  104 . As shown in FIGS. 1 and 2, the absorbent layer  105  is partially cut away to show the underlying base substrate  102  and two electrodes  103 ,  104 . Connected to one end of each electrode is a sensing and transmitting circuit  106 , which transmits a coded signal upon a significant decrease in electrical resistance between the pair of electrodes  103 ,  104 . A receiving and alarm station, not shown on FIG. 1, receives the coded signal, sounds an audible alarm and displays an identification number transmitted by the sensing and transmitting circuit  106 . 
     The base substrate  102 , having length and width, is preferably constructed from a flexible, electrically non-conductive material, such as a polymeric resin, plastic, rubber or composites of laminar films, and preferably of plasticized polyvinyl chloride sheeting. The base substrate  102  should be sufficiently stiff to allow sliding the leak detection mat horizontally across a floor surface, without gathering or wrinkling, by applying manual pressure to an edge of the leak detection mat  101 . 
     Affixed to the upper surface  109  of the base substrate  102  are two metallic electrodes  103 ,  104 . The electrodes  103 ,  104  may be wires or flat strips, and may be made of copper, aluminum or other conductive metal. The electrodes  103 ,  104  are affixed to the base substrate by adhesive, heat or other suitable means. The electrodes  103 ,  104  have sufficient thickness to effect a definite contact with the absorbent layer  105 . 
     The two electrodes  103 ,  104  are affixed to the base substrate  102  in a pattern in which, to the extent desired or practical, provides any point on the base substrate  102  is located between points on either electrode  103 ,  104 . In such a pattern, a minimal amount of water falling on the mat will then have the highest probability of creating an conductive bridge between the two electrodes  103 ,  104  and activating the sensing and transmitting circuit  106 . The maximum distance between the two electrodes in the pattern may vary, dependant upon the sensitivity of the sensing and transmitting circuit.  106 . 
     In the preferable pattern, each electrode  103 ,  104  would have a proximal end disposed along a first edge  112  of the base substrate, proximate to the other as necessary or appropriate to facilitate connection to the sensing and transmitting circuit. A main lead  110  of each electrode would first traverse along the first edge  112  in directions opposite the other electrode, then along the edges adjacent to the first edge  112  and opposite that of the other electrode. From each main lead  110 , a plurality of branches  111  emanate perpendicular to the main lead, spaced at regular intervals and traversing the interior of the base substrate  102  towards the other main lead  110  and interspersed between two adjacent branches  111  of the other main lead. 
     In an alternate embodiment, the two electrodes  103 ,  104  are linear strips or wires and are disposed parallel to each other in an anfractuous or sinuous pattern across the upper surface  109  of the base substrate  102 . With this pattern, however, certain points on the base substrate  102  are between the same electrode, such as within the concavity of a curve in an electrode, and small leaks in these areas may remain undetected. 
     In any embodiment, the proximal ends  107  of either electrode  103 ,  104  would be disposed along the same first edge  112  of the base substrate  102  and within a distance of the other appropriate for connection of the sensing and detecting circuit  106 . Each proximal end  107  would have attached to it means for connecting to the sensing and transmitting circuit  106 , such as soldering lugs, terminal jacks, screw connectors or similar type connectors. In one embodiment, the distal end  108  of each electrode  103 ,  104  would terminate on the edge opposite the first edge of the base substrate  102  within proximal distance of the other. From the distal ends  108 , a second leak detection mat  101  may be connected by attaching wires, jumpers or other electrical connectors to the proximal ends  107  of a second detection mat  101 . 
     On top of the base substrate  102  and the pair of electrodes  103 ,  104  is affixed an absorbent layer  105 . The absorbent layer  105  may be fabricated of a woven cloth or felt, made with cotton, wool, linen, flax, jute or their blends with synthetic fibers. The absorbent layer  105  may also be made of a layer of polymeric foam, such as polyurethane. The absorbent layer  105  is affixed to the base substrate  102  using, for example, contact adhesive. The absorbent layer  105  is adhered to the base substrate  102  such that the absorbent layer  105  is in firm contact with the electrodes  103 ,  104 . 
     When water drips onto the absorbent layer  105 , the water is absorbed into the fibers or material of the absorbent layer  105 , thereby increasing the electrical conductivity of the dampened area. As water continues to drip or leak onto the absorbent layer  105 , the interstitial and free water migrate radially away from the initial contact point by diffusion and hydraulic flow, until the dampened area extends over some point on each of the two electrodes  103 ,  104 . At this point, electrical current may conduct between the two electrodes  103 ,  104  through the dampened material of the absorbent layer  105 . As a result, the resistance between the termini  107  of the two electrodes  103 ,  104  will decrease, which can be sensed or measured by an appropriate electrical circuit and an alarm thereby triggered. 
     In one embodiment, the absorbent layer  105  is impregnated with a aqueous-soluble salt, thereby increasing the electrical conductivity of the dampened fiber. The absorbent layer  105  may be impregnated by saturating it with an aqeous salt solution, then drying the absorbent layer  105 . Suitable salts for impregnating the absorbent layer  105  include sodium chloride and potassium chloride. 
     The sensing and transmitting circuit  106 , for sensing decreases in the electrical resistance across the two electrodes and transmitting a signal, is preferably mounted on the base substrate  102  near to the proximal ends  107  of the two electrodes  103 ,  104 . As shown in FIG. 3, the subcircuits within the sensing and transmitting circuit sense a decrease in resistance between the two electrodes  103 ,  104  by impressing a voltage on electrode  103  through a built-in pull-up resistor in an encoder chip  305 , while electrode  104  is connected to ground. The encoder chip  305  is a Holtek Semiconductors, Inc. Model HT12E, having a Transmit Enable (“TE”) input  309  internally connected to the source voltage  311  through pull-up resistance. The pull-up resistance with the encoder chip  305  is high relative to the resistance between the electrodes  103 ,  104  when the absorbent layer has been wetted. In operation, when no leak is present and the leak detection mat is dry, the voltage at the first electrode  103  equals the supply voltage  311 , also known as the ON state. Once a leak occurs, the absorbent layer is wetted and electrical continuity between the two electrodes  103 ,  104  is established, permitting electrical current flow from voltage source  311 , through encoder chip  305 , and across the electrodes  103 ,  104  to ground  315 . The current flow permits a voltage drop across the built-in resistance of encoder chip  305  that results in having near zero voltage at chip at TE  309 , also known as the OFF state for the terminal The relatively high voltage drop across the built-in resistance and near-zero, or low, voltage drop across the conducting electrodes  103 ,  104  maintain the OFF state so long as electrical continuity exists between the two electrodes  103 ,  104 . 
     Upon switching to the OFF state at TE  309 , a 12-bit code is transmitted. To generate the 12-bit code, the encoder chip  305 , also has 8 address inputs  302 , 4 data/address inputs  304 , and a data output  308 . In operation, when the voltage at TE  309 , which is connected to the first electrode  103 , switches to the OFF state, the encoder  305  communicates a series of 12 binary pulses on the data output  308 , the pulses of which corresponding to the states of the 8 address inputs  302  concatenated with the 4 data inputs  304 . The states of the 12 inputs are controlled by an 8-position DIP switch  301  and a 4-position DIP switch  303 , the jumpers of which are manually positioned. The 8-position DIP switch  301  sets a verification code, which must be identical in all other sensing means and in the alarm means used in the system. The 4-position DIP switch  303  sets an identification code unique for each individual sensing means. While a 4-bit binary number could represent one of 16 combinations, only up to 10 are used in the preferred embodiment, as the identification code is processed as binary coded decimal (“BCD”) in the receiver. Thus, the permissible settings for the 4-position DIP switch are from 0000b to 1001b. 
     The transmitter chip  306  may be a commonly available chip, such as a Reynolds Electronics TWS-434 transmitter. When a 12-bit pulse code is received on its input terminal, the 12-bit code is transmitted by radio through an antennae  307  attached to the RFOUT port of the transmitter chip. Dependent upon the required range, the antennae  307  may be up to 35 cm in length and an impedance of 50 ohm for optimal range. 
     The encoder chip  305  continually outputs the 12-bit code, which is continually transmitted by the transmitter chip  306 , so long as voltage at TE  309  remains in the OFF state. Once the absorbent layer  105  of the leak detection mat  101  dries, and resistance across the two electrodes  103 ,  104  increases and current flow between the electrodes  103 ,  104  diminishes, thereby restoring voltage at TE  309  to the ON state, disabling TE  309  and ceasing transmission. 
     Power to the encoder chip  305  and the transmitter chip  306 , along with supply voltage  311  to the detection mat first electrode, is provided by a battery in the preferred embodiment. Power may also be provided through a power supply using common 110-volt alternating current. Such power supplies are well known to those skilled in the art. 
     An alarm means  400 , shown in FIG. 4, is provided as part of the leak detection system to receive the 12-bit signal transmitted from the transmitter chip  306  and to provide an alarm. In the preferred embodiment, as shown in FIG. 4, included in the alarm means is a receiver chip  402 , such as a Reynolds Electronics RWS-434 receiver, which receives the signal through its antennae  401 . The receiver chip  402  operates at the same frequency with the transmitter chip  306 . When a signal is received by the receiver chip  402 , the signal is communicated to a decoder  405 , such as a Holtek Semiconductor Inc. model HT12D decoder. The decoder  405  has 8 address inputs  404 , 4 data outputs  409 , and a Verified Transmission (“VT”) output  406 . 
     An 8-position DIP switch  403  is connected to the 8 address inputs  404  of the decoder chip  405 . The jumpers on the 8-position DIP switch  403  must be positioned identical to the same positions as the jumpers on the 8-position DIP switch  301  in the sensing means  106 . When the receiver chip  402  communicates the same 12-bit code to the decoder  405  three times sequentially, the first eight bits parsed from the received code are compared by the decoder  405  with the values on the 8 address inputs  404 . If all eight bits match, a valid signal has been received, at which point the last 4 bits are communicated on the 4 data outputs  409  and the VT output  406 , which is normally OFF, is switched ON. The 4 data bits are internally latched within the decoder  405  and continue to be communicated on the 4 data outputs  409  until another 12-bit code is communicated three times sequentially from the receiver chip  402 . The VT output  406  switches OFF shortly after cessation of communication of the 12-bit code from the receiver chip  402 . Upon switching the VT output  406  to ON, the 4-bit identification code is displayed as a single decimal digit on an display  414  and an alarm buzzer is activated for approximately 5 seconds. 
     The display  414  is latched, displaying the identification code until a reset button  408  is manually pressed. As shown in FIG. 5, the display  414  is activated and latched by inputting the VT signal  406  to a display enable Flip-Flop circuit  507  consisting of first and second NOR gates  501  and  502 . This type of Flip-Flop circuit, commonly referred to in the art as an R-S Flip Flop, has a Set and a Reset input and a Normal State output and an Inverse State output. In an R-S Flip Flop circuit generally, when the set input changes to the ON state, the Normal State output turns ON and the Inverse State output turns OFF. The outputs remain in these states, even when the Set input switches to OFF, until the Reset input turns ON. Then, the Normal State output switches to OFF, the Inverse State output switches to ON, and each remains in that state even after the Reset input switches to OFF. 
     The VT signal  406  sets the display-enable Flip-Flop  407 , while the manual reset button  408  resets it. The inverse state output  415  of the display-enable Flip-Flop  407  communicates in parallel with a first input of each of four 2-input OR gates located in the quad OR chip  410 . The quad OR chip  410  is a Philips Semiconductors Model HEF4071B quadruple 2-input OR gate in the preferred embodiment. Within the quad OR chip  410 , the second input of each OR gate receives one of the four bits of the identification code communicated on the 4 data lines  409  of the decoder chip  405 . The four outputs  411  of the four OR gates in the quad OR chip  410  communicate with the inputs of a binary coded decimal (BCD) to 7-segment LED converter (the “BCD converter”)  412 , as shown in FIG.  4 . The BCD converter  412  may be a commonly available IC chip, such as a Philips Semiconductors model HEF 4543 latch/decoder/driver. The BCD converter  412  internally converts the four outputs  411  into 7 separate outputs that communicate across the display data circuits  413  to drive a 7-segment LED or LCD display  414 . The display  414  reveals the corresponding decimal digit. The display  414  may be one of many 7-segment LEDs or LCDs commonly known in the art. When the VT signal  406  switches OFF, the inverse output of the display-enable Flip-Flop  407  remains OFF, which, once processed through the four OR gates in the quad OR chip  410 , will result in continued communication of the 4 bits of the identification code to the BCD converter  412 . Once the manual reset  408  is pressed, the display enable Flip-Flop  407  will reset, switching its inverse output  415  to ON. When this inverse output  415  is processed by the quad OR chip  410 , all four outputs  411  of the quad OR chip  410  will be ON. When this code, 1111b, is communicated to the BCD converter  412 , all 7 outputs on the display data circuits  413  are switched OFF, clearing the single-digit display  414 . Thus, the display  414  will continue to display the last received identification code until the manual reset switch  408  is pressed, after which the display  414  will clear until another valid code is transmitted. 
     The VT output  406  also activates the speaker circuitry, which activates a buzzer for about 5 seconds in the preferred embodiment. As shown in FIG. 4, this is accomplished using an astable multivibrator  416  which activates an oscillator  417  for about 5 seconds, producing an oscillating wave communicated to a speaker  418  with an audible signal of about 1 kilohertz. 
     As depicted in FIG. 6, the multivibrator  416  uses a flip-flop circuit  617 , comprised of two NOR gates  603  and  604 . The flip-flop circuit  617  is latched ON upon the VT signal  406  from the decoder  405  switching ON. The normal output  619  of the Flip-Flop  617 , which is normally OFF, is switched ON. The normal state output  619  of the flip-flop circuit  617  communicates with the base of an NPN transistor  609 , the collector of which is provided supply voltage. When the normal state output  619  of the flip-flop  617  switches ON, the base-emitter junction of NPN transistor  609  is forward-biased, and the NPN transistor  609  is turned ON. Current flows, gradually charging capacitor  611  and increasing the voltage in the circuit from the collector of NPN transistor  609 . The rate at which the voltage increases is inversely proportional to the resistance of resistor  610  and the capacitance of capacitor  611 . Once the capacitor voltage exceeds the threshold voltage of diode  617 , a minute amount of current will flow to the first input of NOR gate  601  allowing a high input. NOR gate  601  performs a logical NOR on the output of the diode  607  and the signal from the reset switch  408 . The output of NOR gate  601  is inverted by a second NOR gate  602  in series. The output of NOR gate  602  serves as the reset input for flip-flop circuit  617  by communicating with one of the inputs of NOR gate  604 . 
     The inverse output  618  of the flip-flop  617  in the multivibrator  416  communicates with an oscillator  417 , which is comprised, in part, of two NOR gates  605  and  606  in series. The inverse state output  618  of the Flip-Flop  617  serves as the first input to NOR gate  605 , while the output of NOR gate  605  feeds back to the second of its inputs through a resistor  612  . The output of NOR gate  605  also communicates with both inputs of NOR gate  606 , the output of which thus inverts the output of NOR gate  605 . A capacitor  613  communicates between the feedback loop of NOR gate  605 , after the resistor, and the output of NOR gate  606 . In this configuration, once the input to NOR gate  605  from the inverse output  618  of Flip-Flop  617  switches to OFF from its normal ON state, the output of NOR gate  606  will oscillate between ON and OFF over a period proportional to the capacitance of capacitor  613  and the resistance of resistor  612 . This oscillation is achieved by the feedback in NOR gate  605 . Once the inverse output  618  from Flip-Flop  617  turns from normally ON to OFF, the output from NOR gate  605  will switch from normally OFF to ON. However, due to resistor  612  and capacitor  613 , the voltage in the feedback to the second input to NOR gate  605  increases only gradually over a finite period of time to switch from the OFF state to the ON state. Once the voltage in the feedback to NOR gate  605  reaches the ON state, the output from NOR gate  605  will switch to OFF, and the charge stored in capacitor  613  will gradually reverse polarity by quickly charging from the output of NOR gate  606  and by slowly deischarging through resistor  612  into the output of NOR gate  605 , until the voltage in the feedback to the second input of NOR gate  605  reaches OFF state, and the cycle repeats itself. 
     The output of NOR gate  606 , being the inverse of NOR gate  605 , communicates with the base of PNP transistor  614 . The collector of PNP transistor  614  originates from a split-resistor circuit formed by resistors  621  and  622 , which communicates supply voltage to the base of PNP transistor  615 . The emitter of PNP transistor  614  communicates to ground through resistor  620 . The collector of PNP transistor  615  is provided supply voltage, and the emitter of PNP transistor  615  communicates with a speaker  616 , and thereafter to ground. 
     In the normal state, the output of NOR gate  606  is ON, which creates a reverse-bias across the base-collector junction of PNP transistor  614 , cutting off PNP transistor  614 . When PNP transistor  614  is OFF, the voltage to the base of PNP transistor  615  equals the supply voltage, which causes a reverse bias across the base-collector junction in PNP transistor  615 , cutting off PNP transistor  615 . If PNP transistor  615  is OFF, the speaker receives no current and is deactivated. Once the output of NOR gate  606  switches OFF, the base-collector junction of PNP transistor  614  is forward-biased and turns ON, which causes the base-collector junction of PNP transistor  615  to forward bias and turn ON, thereby activating the speaker. The 1-kilohertz frequency at which the output of NOR gate  606  cycles between the ON and OFF states will thereby produce an audible tone at 1 kilohertz in the speaker  616 . PNP transistor  615  serves to amplify the power output of PNP transistor  614 . 
     After a leak has been detected and the display and speaker activated, the alarm means  400  is reset by manually pressing the reset button  408 , which resets the display-enable Flip-Flop  503  and the Flip-Flop  617  in the multivibrator  416 . The system is then enabled to monitor for a subsequent water leak. 
     In use, as shown in FIG. 7, leak detection mats  101  may be placed under various appliances containing water, such as a washing machine,  702  and a hot water heater  701 . The alarm means  400  would be mounted in a more visible location convenient for the building occupants, and may be located in a room other than that in which the leak detection mats are located. Other leak detection mats, not shown, may be located in other rooms of the building and monitored by the same alarm means  400 .