Abstract: An environmental sensor system that communicates sensor data to a receiving unit using wireless means such as a radio frequency signal. The receiving unit interfaces with a controllable system, possibly affecting its operation. This arrangement allows one or more sensor and transmitter units to be remotely mounted at a distance from the receiver, without regard to installation complications that often result with a hardwired type units. In the preferred embodiment, an irrigation system is interfaced wirelessly with an environmental sensor such as a rain sensor. The rain sensor is contemplated such that in the event of sufficient rainfall, a wireless signal is transmitted to the receiver unit, which in turn interfaces with an irrigation controller resulting in the cessation of watering cycles until the sensor system provides another wireless directive to resume watering.

Abstract: An automated device monitors moisture content in exterior sheathing of a building for detecting moisture intrusion into the sheathing walls. A plurality of moisture sensors are embedded in selected exterior walls of the building behind the cladding and are adapted to provide signals representative of the moisture content in the walls at each of the selected locations. A control unit is operatively connected with each of the plurality of moisture sensors for generating an alarm signal when at least one of the signals representative of the moisture content in the plurality of walls is indicative of a moisture content in excess of a predetermined threshold value.

Abstract: A detector unit (1) which, in the presence of moisture, operates a remote alarm via a wire-less link is contained within a sealed liquid-impermeable housing (2). The unit is powered by electrical charge stored in a capacitor, which may be rapidly recharged by inductive coupling. Remote sensing electrodes (69, 70) are formed by depositing an electrically conductive ink onto a disposable moisture-absorbent tail (65).

Abstract: The device for the detection of a wetting of a pane of glass, has an optical rain sensor which detects extraneous light and evaluates it for purposes of day-night detection. The amount of light striking the receiver (14) is integrated in an integration stage (16) and supplied to a comparator stage (20). The total light quantity formed by the light of a transmitter (12) and extraneous light and the extraneous light quantity detected when the transmitter (12) is switched off are measured in separate time intervals (Tg, Tf, Tf′). The time interval (Tf, Tf′) utilized for measurement of the extraneous light quantity is to be extended after a predeterminable number of measurement cycles (M) or after a predeterminable time period and/or the threshold (S2) is to be lowered for day-night detection.

Abstract: A vehicular rain sensor system for detecting precipitation on an exterior surface of a window includes an illumination sensor that is decoupled from the window. The illumination sensor is preferably an imaging array sensor which communicates a signal to a control which further determines whether rain is present on the window. The control includes an edge detection function for detecting edges of precipitation droplets on the window in response to the light values sensed by the sensor and activating the windshield wipers of the vehicle when the sum of the light values exceeds a predetermined threshold value. The rain sensor system may further include a polarizing filter and an illumination source, such that the rain sensor system not only prevents false signals of rain when only fog is present on an interior surface of the window, but also actually detects fog particles on an interior surface of the window.

Abstract: A sensing system for the detection and analysis of light and/or rain conditions affecting the vehicle has a mounting which can be installed within the scope of an assembly line production. The sensing system has a light sensor and a light sensor control unit and/or a rain sensor and a rain sensor control unit and a common carrier plate on which the light sensor with the light sensor control unit as well as the rain sensor with the rain sensor control unit can be mounted.

Abstract: An apparatus and a method for operating an optoelectronic rain sensor for detecting the wetting of a window (14) with moisture for wetting-dependent triggering of a system, in particular a windshield wiper system for motor vehicles, is proposed in which the component of the ambient beam (18) of the sensor signal (22) in a first interval is virtually eliminated and therefore need no longer be taken into account in the ensuing detection of the sensor signal (22) in a second interval relevant to triggering of the windshield wiper system.

Abstract: An apparatus and a method for operating a rain sensor (10) that outputs a sensor signal (18, 22) as a function of the wetting of a window (11) are proposed, having a controller (16) which as a function of the sensor signal (18, 22) outputs a control signal (28) to the rain sensor (10) for regulating the sensor signal (18, 22); for signal evaluation, the sensor signal (18, 22) and in addition the control signal (28) are used for tripping switching events of a device.

Abstract: A visibility sensor system includes a housing having a sensor head opening. A removable sensor head assembly is removably coupled to the housing within the sensor head opening. The sensor head assembly has a sensor enclosure and a connector. An electronics module is coupled to the sensor head through the connector. A rain sensor determines the presence of rain and causes shutters to cover openings in the sensor exclosure. In other embodiments, the sensor enclosure is configured to develop a pressure differential internally to minimize contamination of light source and light detector.

Abstract: A desiccant pack 1 with a humidity sensor comprises a pack body 10 formed by sealing the three opened sides of a package film having gas permeability and heat sealability, and a desiccant is filled inside the pack body 10. Humidity sensor substances are applied in a band or strip-shape to the surface of the pack body 10, by which humidity sensors 21, 22 and 23 are applied directly to the pack.

Abstract: For transmitting early warning signals of slick road conditions (1, 2, 3, 4), in addition to automatic measuring stations (AMS1-AMS5) that have probes for the weather conditions and the freezing point of liquid present on the road, virtual stations (VS1-VS 11) are created and used for which weather data, freezing point data, and parameters are taken from the automatic measuring stations in order to transmit an early warning of slick conditions for the virtual stations also, by means of an evaluating device (5).

Abstract: An apparatus and a method for operating a rain sensor (10) that outputs a sensor signal (18, 22) as a function of the wetting of a window (11) are proposed, having a controller (16) which as a function of the sensor signal (18, 22) outputs a control signal (28) to the rain sensor (10) for regulating the sensor signal (18, 22); for signal evaluation, the sensor signal (18, 22) and in addition the control signal (28) are used for tripping switching events of a device.

Abstract: A system for the detection of the presence of undesirable moisture and drops or droplets of liquid effusing from piping systems at the first stage of appearance, comprising a low-battery alert, provision for the device to remain operative for at least 48 hours before the battery power is depleted, and sensitivity such that the presence of any unwarranted source of humidity will activate the alarm, including moisture from the proximity of the sweat of humans and from unwarranted pipe sweating (such as the cumulative effect thereof may cause water damage) will activate the alarm, alert the operator of the system to the onset of the problem, save energy, water, pumps, boilers, pipes and property, from intrusion and damage, and allow for the restoration of plumbing systems in a minimum of time. The versatility, simplicity in operating, and miniaturization of the system is such that the device may be placed in multiple, remote areas of complex piping that confound access to a larger-sized system.

Abstract: A severe weather detection apparatus for detecting severe weather and method of detecting and warning of approaching severe weather conditions is provided. The severe weather detector apparatus includes a barometric pressure sensor and computerized means for calculating the rate of change in the barometric pressure. By monitoring and deriving a value representative of the rate of change in the barometric pressure over time, a threshold is established and compared to the current rate of change in the barometric pressure. The derived rate of change is compared to a value stored in the device which reflects approaching severe weather conditions. Rates of change which may indicate the approach of severe weather activate a voice-enunciated alarm that announces the potentially dangerous barometric activity. The severe weather detection apparatus is portable and suitable for battery power and installation in the same manner as a home smoke alarm or carbon monoxide detector.

Abstract: A water sensor, such as a rain or dew sensor, includes a light emitting device for producing sensing light to be reflected in a transparent plate such as a windshield glass panel of a vehicle, a light receiving device for receiving the sensing light, and a hologram sheet having at least one hologram for defining a light path of the sensing light between the light emitting device and the light receiving device. The hologram sheet may be attached to the inside surface of a windshield glass panel or interposed between inside and outside glass layers of a laminated glass panel.

Abstract: ThunderCall is a computer based system for delivering NWS(National Weather Service) severe weather warnings as a message (personalized information, if desired) via an automated dialing system to live persons and to automated recorders comprising the steps of: forming a database of names with their respective geographic locations; capturing a NWS Bulletin reciting the location of a severe storm; validating a relevant Bulletin into a Valid Bulletin by determining from the storm's location that it represents an imminent severe storm danger to at least one person of said database (capable of 10,000,000 or more persons) recording, if desired, an individual personal urgent warning to the name of the intended recipient of the Valid Bulletin; combining, if desired, said personal warning with a generic severe storm warning for said recipient; and, sending said Valid Bulletin with said combined warnings to said person whereby he is alerted within seconds of the release time of said National Weather Service bulletin to

Abstract: A device for determining the ambient heat index and communicating that information to a user, the device having a housing member, a control mechanism, and a power source. The control mechanism includes a microprocessor, an air temperature measuring device, and a relative humidity measuring device, wherein the air temperature measuring device and the relative humidity measuring device respectively determine the user's ambient air temperature and ambient relative humidity and communicate that information real time to the microprocessor for determination of the heat index. The control mechanism has a display mechanism for displaying the heat index, and another display mechanism for displaying the ambient temperature and the ambient relative humidity. Switches permit manual operation of activation of the display mechanisms.

Abstract: The invention is directed to a system for controlling lawn and garden watering by using electronic sensors to detect the moisture in a lawn. A probe determines the soil water content and powers a signaling device which operates switches for either signaling the need for more water or to stop watering to prevent over-watering. The status of the moisture in the soil is indicated by raising a flag or lighting a strobe light for a visual signal.

Abstract: A roof vent for use in a flat roof having a plurality of layers of roofing material including a waterproof membrane which includes a base flange dimensioned to fit in between two of the plurality of layers of roofing material, an upstanding hollow member coupled to the base flange, an electrical connector mounted mounted on said hollow member, and at least one moisture sensing element electrically coupled to the electrical connector and deployable on one of the plurality of layers of roofing material, wherein the moisture sensing element is in the form of a strip which is deployed like tape. All of the sensing elements are coupled via the electrial connectors of the vents to a central controller which has a display and a modem for signaling an alphanumeric pager of a leak condition.

Abstract: The device for controlling automatic operation of a windshield wiper in a motor vehicle includes a central electronic control unit provided with means for triggering wiping operations of the windshield wiper; a rain sensor including a detector for rain on the windshield and a device for generating sensor signals for the triggering of individual wiping operations according to detector response, which is connected to the control unit in order to furnish the sensor signals to the control unit, and a sensitivity control device for adjusting rain sensor sensitivity, which includes a display device for observing the rain sensor sensitivity or parameters controlling it optically and buttons operable to adjust the rain sensor sensitivity or at least one of the parameters controlling it.

Abstract: A humidity sensing device dimensioned to be received in a humidor for detecting an out of range ambient moisture condition and transmitting a signal to a remote monitor. The sensing device establishes an acceptable range of period for a humidity sensitive oscillator. The out of range condition causes a signal to be transmitted to the decoding monitor which provides an alarm signal.

Abstract: A meteorological information system useable by aircraft pilots and others. A receiver-calculator unit receives raw meteorological data and site-specific physical data from a ground-based weather station having a signal transmitter for broadcasting signals conveying data about weather conditions at the station. The transmitted raw data signals are received by the receiver-calculator unit in the aircraft. The receiver-calculator unit may feature an input with which the operator inputs information about the physical character, such as the heading, of the runway of interest. The receiver-calculator unit processes both received raw weather data and received site-specific physical data, and/or operator-input data, to derive additional meteorological information. The raw and derived weather conditions information is displayed by the unit to be available to the aircraft pilot during landing or take-off maneuvers.

Abstract: A system for detecting the presence of rain droplets on a windshield employs a source of radiant energy, such as an LED, and a photodetector installed on the instrument panel of a vehicle. The LED and the photodiode are arranged with respect to one another such that light from the LED cannot reflect directly off the windshield onto the photodiode. Only when the light from the LED is back scattered by an environmental condition, such as the raindrops on the exterior of the windshield, the condensation of moisture on the interior of the windshield, or the presence of smoke in the cabin, will the photodiode receive the light from the LED. Each of these environmental conditions has a predeterminable characteristic signature that permits the processing of data to detect same.

Abstract: A roof moisture detection assembly includes an imaging system for obtaining thermal and visible images of a roof surface, an imaging system support ructure for mounting the imaging system in a position elevated relative to the roof surface, a reference target mounted on the roof surface, and an image-processing system adapted to compare current thermal and visible images of the roof surface with previous thermal and visible images of the roof surface and detect shapes and areas of anomalous features, and to compare the current thermal and visible images with each other and detect shapes and areas of anomalous features.

Abstract: An optical device on board an aircraft for detecting icing conditions including an optical beam emitter to create at least one illuminated external volume of measurement through which there circulates a flow of air charged with water particles; a collector optical system for the collection of at least a part of the light beam back-scattered by the water particles (the external volume of measurement being located on the optical axis of said collector optical system); a photodetector for detecting back-scattered optical beams; a signal processor for processing the signal delivered by the photodetector to compute the severity of the icing conditions; an information provider for providing information on the calculated severity of the icing conditions. Advantageously, the optical means for the differentiation of device includes a differentiation for differentiating between liquid water particles and solid water particles, based on analysis of the polarization.

Abstract: An apparatus and method for controlling the operation of the windshield wipers of a vehicle in response to the deposition of water on the exterior of the windshield; the operation of the air conditioning system in response to the formation of water condensation on the interior of the windshield of the vehicle; and the operation of the ventilation system in response to the presence of smoke in the cabin of the vehicle.

Abstract: An arrangement for detecting moisture conditions on a pane, particularly a windshield of a motor vehicle, wherein radiation emitted by a transmitter is coupled by a radiation guide into the pane and is coupled out of the pane onto a receiver. With respect to the production and the dimensions, the design is supported by the measures which provide that the beam entry surface and/or the beam exit surface is/are arranged on the parallel side of the radiation guide facing the circuit board, and that the optical axis of the transmitter or of the receiver is oriented to be perpendicular to the beam entry surface or the beam exit surface and that the radiation guide has at least one input-end or output-end deviating surface with which the radiation that entered the radiation guide is guided at a right angle into the interior of the radiation guide or the radiation which is to be oriented out of the radiation guide onto the receiver is guided at a right angle out of the interior of the radiation guide.

Abstract: The intelligent alarm system of the present invention provides a reliable, low cost system for alerting a user to the presence of snow outside of a building. The system includes a sensor and an intelligent alarm clock for use inside a building or dwelling. The system can be set by a user to signal an alarm a programmable amount of time earlier or later than the user's normal waking time if an accumulation of snow is detected. The system is capable of processing preprogrammed data, user entered data and data accumulated from the sensor to determine whether a true snow condition exists and to generate an appropriate alarm signal.

Abstract: A vehicle (10) includes a windshield presenting an inner surface (14) including a test section (16) therein. A cooling device (24) is attached to the inner surface (14) for cooling the section (16) to a temperature a few degrees below the temperature of the inner surface (14) adjacent the test section (16). A sensor (26) is disposed adjacent the inner surface (14) for sensing condensation on the test section (16) and for providing a heating signal to a defogging control (28) in response to condensation on the test section (16) for initiating the heating of the surface surrounding the test section (16) by the vehicle heater (18) to prevent condensation from forming on the surface surrounding the test section (16).

Abstract: Roof vents include at least one moisture sensing element which is electrically coupled to an electrical connector in the roof vent. The electrical connector is located in the roof vent in such a manner that it is protected from the environment. A portable moisture sensing circuit or device with a moisture display for indicating a degree of moisture is provided with an electrical connector which mates with the electrical connector in the roof vent. According to the invention, the roof vents are installed in a roof system with the moisture sensors located below the roof insulation. When it is time to inspect the roof for leaks, a worker carries the portable device from one roof vent to another and plugs the device connector into the vent connector to take a moisture reading from the sensing element(s) associated with each vent. Since only one electrical circuit and display is used, the total cost of a system having many roof vents is significantly reduced.

Abstract: A tornado alarm that monitors humidity, static charge in the air and the barometric pressure. Nothing happens until the level of static electricity increases to a set amount over a stored base level and the humidity reaches 100 percent. At this point the first stage alarm is activated. The device sends out an initial blast followed by a report every 60 seconds during this mode of operation. At this point, the user is awakened by the alert. The user then can turn on news reports to verify danger. Once awake, if danger exists, the user can prepare for a tornado and be ready. When the barometric pressure reaches 28 millibars, the third condition is reached. Under most circumstances, a tornado is then immanent. This sets the device into the second stage alert mode. In this mode, a continuous alarm sounds.

Abstract: Disclosed is an interior space lighting unit for a car having a housing including a lower side and an upper side. The lower side is provided with a lower window behind which a first lamp is arranged, and the upper side is provided with an upper window behind which a second lamp is arranged. A switch is coupled to the current supply circuit for the lamps and is mounted to the housing. The housing is adapted to be mounted to a tiltable support such as the rear window on a vehicle tailgate, for illuminating the passenger or luggage space in case the tailgate is closed, or the loading area and/or luggage space in case the tailgate is open.

Abstract: The invention is directed to a system for controlling lawn and garden watering by using electronic sensors to detect the moisture in a lawn. A probe determines the soil water content and powers a signalling device which operates switches for either signalling the need for more water or to stop watering to prevent over-watering. The status of the moisture in the soil is indicated by raising a flag or lighting a strobe light for a visual signal.

Abstract: A rain drop detecting device includes an oscillation element disposed on a plate member such as a windshield for generating an ultrasonic wave on the plate member, an ultrasonic wave receiving element disposed on the plate member for receiving the ultrasonic wave generated by the oscillation element, a detecting device connected to the ultrasonic wave receiving element for detecting an amplitude of the ultrasonic wave, and an ultrasonic wave absorbing member disposed on the plate member. The absorbing member is designed to absorb a portion of the ultrasonic wave generated by the oscillation element to prevent the downwardly travelling portion of the ultrasonic wave from travelling beyond a desired region of the plate member. In the case of a windshield, the absorbing member can be used to prevent the ultrasonic wave from travelling to the resting location of the wiper so that inadvertent rain drop detection does not occur.

Abstract: A visibility sensor system includes a housing having a sensor head opening. A removable sensor head assembly is removably coupled to the housing within the sensor head opening. The sensor head assembly has a sensor enclosure and a connector. An electronics module is coupled to the sensor head through the connector.

Abstract: In apparatus for counting a succession of falling liquid drops having teardrop shape, with a downwardly convex down side, the combination that includes a beam provider for providing a beam of electromagnetic radiation to sidewardly pass into the drop at its convex lower side, to be refracted within the drop to pass out of the drop at its convex lower side, and a beam detector located to detect the refracted beam that has passed sidewardly out of the drop at its convex lower side.

Abstract: A food storage apparatus (22) comprises a container (26) to contain a food item (20), a humidity sensor (34) to sense a humidity within the container (26), and a transmitter (66) responsive to the humidity sensor (34) to transmit a signal based upon the humidity. A system for monitoring at least one food item within a storage place (24) comprises a receiver (88) to receive the signal, and an indicator (40) responsive to the receiver (88). Methods for monitoring at least one food item are also disclosed.

Abstract: The level of moisture in a buried splice case (14) is provided by a transmitter (26) that is energized from a power circuit (24) via a moisture detector (22) when the moisture detector detects a prescribed level of moisture in the splice case. Once erergized, the transmitter (26) transmits a coded word indicative of the prescribed moisture level for receipt by a receiver (30) located above ground. In this way, a technician, using the receiver (30) can directly determine the moisture level in the buried splice case without the necessity to excavate the case or to contact a member of the On-site Work Force at a communications facility.

Abstract: In a rain sensor system for a motor vehicle, a sensor element is provided as separate part for attachment in the vicinity of a vehicle window. The sensor element is wiped by a separate wiper blade which is fastened to a windshield wiper or driven by the latter. As a function of the a degree of wetness present on its top, the sensor element supplies electric signals to an evaluation electronics. Above a limit value of the signals, the evaluation electronics connects the windshield wiper.

Abstract: A compact moisture sensor for mounting on the inner surface of a windshield to detect moisture, such as rain, on the outer surface of the windshield and control the windshield wipers accordingly includes a coupler having a collimator and focuser and a housing for detachably covering the coupler. A planar circuit board disposed within the housing includes emitters and detectors mounted such that the axes of emission and detection are perpendicular to the windshield when the housing is secured to the coupler. The collimator and focuser are disposed adjacent the emitter and detector respectively such that the optical axes of the collimator and focuser form oblique angles with respect to the emission and detection axes. Two emitters and two detectors are used to form four optical paths of equal length and optical efficiency.

Abstract: Apparatus for lighting and/or indicating purposes, on a vehicle such as a motor vehicle that has a rear fog lamp. The apparatus includes an electrical supply and a control circuit connecting the electrical supply to the fog lamp and/or an indicator. The control circuit is responsive to activating signals from a fog detector that is adapted to produce the activating signal when a condition of reduced visibility exists in the environment of the vehicle. This activating signal, moreover, is generated in response to electromagnetic waves reflected to a receiver coupled to the fog detector.

Abstract: The present invention is a method and apparatus for controlling power supply to aircraft electronic systems based on ambient environmental conditions. The apparatus comprises a monitor for detecting an environmental condition, a power supply for providing power, a circuit which receives power from the power supply, and a switch coupled to the monitor. Upon detection of the environmental condition, the switch connects the power supply to the circuit, otherwise the switch disconnects the circuit from the power supply.

Abstract: The invented method can be applied to a house, building or other structure to detect the presence of a water problem, even in areas that are not visible to the naked eye. The method includes a step of changing the temperature of the structure relative to its temperature existing immediately before the start of the method. The method also includes a step of infrared scanning the structure. Because moist structure areas are slow to move to the changed temperature compared to dry areas due to the relatively high thermal capacity of water, the changed temperature enables the infrared scanning step to readily contrast moisture-suspect areas of the structure from dry areas thereof. The method can also include steps of determining whether a moisture-suspect area does in fact contain moisture, or is due to other causes unrelated to moisture.

Abstract: An apparatus for operating a windshield wiper having an automatic wiper control. The apparatus is provided with a rain sensor having a light-emitting transmitter and a receiver that responds to the emitted light by emitting a sensor signal, and an evaluation device for the sensor signal. The evaluation device has a microcomputer and a capacitor that is outside of the microcomputer and to which the sensor signal is supplied. Also included is a comparator connected to the capacitor for comparing the voltage built up at the capacitor by the sensor signal to a threshold voltage and for emitting an output signal if the threshold voltage is exceeded. The microcomputer forms a measured value from the output signal, the value of which correspond to a time at which the threshold is exceeded.

Abstract: A raindrop sensor of the present invention including light emitting part which emits a strip-shaped light; light receiving part which receives the strip-shaped light emitted from the light emitting means; and detecting device which detects a raindrop based on the amount of the light received by the light receiving device. The raindrop sensor of the present invention is applied to a wiper apparatus of a vehicle. The wiper apparatus includes a wiper for wiping a windshield of the vehicle, and a control unit for instructing a given wiper operation to the wiper in accordance with a given output signals from the raindrop sensor, the wiper being driven in accordance with the instruction from the control unit. Also, the wiper apparatus using the raindrop sensor is applied to a vehicle.

Abstract: A humidity sensing device comprises a sensor unit including a humidity sensitive capacitor having a capacitance which changes as a function of relative humidity, an oscillator circuit including the humidity sensitive capacitor wherein the output frequency of the oscillator circuit is a function of capacitance, and a sensor memory unit having stored calibration data for said humidity sensitive capacitor. A data processing unit is coupled to the sensor unit and includes a data processing circuit coupled to an output of said oscillator circuit, wherein said data processing circuit provides an output indicative of relative humidity as modified by the calibration data received by and stored in another memory unit as associated with such data processing circuit.

Abstract: A device for signaling a predetermined soil moisture condition includes a switching tensiometer that closes a switch when the root water tension reaches a predetermined level; a power source connected to the switching tensiometer; and a signal connected to the switching tensiometer so that when the switching tensiometer closes the switch, power source is connected to the signal operate the signal to indicate that the predetermined soil moisture condition has been reached. The signal preferably includes a spring-loaded flag that operates between a retracted position and an extended signal position; a latch for releasably securing the flag in its retracted position, and an electrically operated release that releases the latch to allow the flag to operate to its extended signal position.

Abstract: In a rain sensor, a sensor field is arranged between two windshield wiper blades which lie one above the other on a windshield. The sensor field is thus located outside of the field of view of the driver and is wiped by the lower windshield wiper blade. The sensor field is connected to an evaluation circuit arranged on a wiper motor by connecting lines which pass around the lower wiper blade.

Abstract: A roof moisture sensing system includes (1) a radio frequency pulse transter, (2) a moisture sensor disposed on a roof, and (3) a radio receiver adapted to monitor resonance of the moisture sensor activated by a pulse transmitted by the pulse transmitter. The receiver is adapted to analyze the resonance of the sensor to determine the presence of moisture in the sensor. The transmitter and the receiver can be remote from the sensor and the roof.

Abstract: The invention concerns an ultrasonic detection device intended to detect the presence of foreign bodies, on the surface of a window comprising a first sheet having a first thickness, comprising a transducer co-operating with a first face of said first sheet, said transducer being able to emit an ultrasonic signal of frequency f.sub.t and also able to receive a reflected ultrasonic signal on a second face of the first sheet, the variation in said reflected signal being representative of the presence or absence of foreign bodies on said second face facing said transducer. This device is characterized in that the frequency f.sub.t is selected to satisfy the following equation:f.sub.K /100.ltoreq..vertline.f.sub.t -f.sub.K .vertline..ltoreq.f.sub.K /10where f.sub.K =K.