Abstract: The present invention provides a locator/monitor capable of locating a boring tool and monitoring the progress of the tool for control purposes. The locator/monitor may be used in expedited locating methodology and straightforward calibration techniques of the present invention. A durable and cost effective pitch sensor is also provided by the present invention. In addition, the present invention provides a slotted transmitter housing formed of an electrically conductive material, where the magnetic field generated by the transmitter is capable of penetrating to the surface.

Abstract: Monitoring of the average dielectric constant in an extended volume of a medium such as soil is achieved using an oscillator incorporating a transmission line to be embedded in the medium. The transmission line is connected as the feedback loop of an amplifier, for one-way propagation of an electrical signal with a delay determined largely by the dielectric properties of the medium, such as its moisture content. The oscillation frequency is indicative of the dielectric constant of the medium.

Abstract: Three-axis particle impact probes detect particle impact vectors along x-, y-, and z-axes by spherical probe head mounted on the outer end of a shaft that is flexibly mounted in silicone rubber at the top of a housing so as to enable motion imparted to the head upon impact to be transmitted to a grounded electrode secured to the shaft within the housing. Excitable electrodes are mounted in the housing in a fixed position, spaced apart from the ground electrode and forming, with the ground electrode, capacitor pairs. Movement of the ground electrode results in changes in capacitance, and these difference in capacitance are used for measurement or derivation of momentum vectors along each of the three axes. In one embodiment, the ground electrode is mounted at the base of the shaft and is secured to a silicone rubber layer at the top of the housing, providing for cantilevered movement.

Abstract: An electrode system (ES) according to the present invention provides greater flexibility in choosing the shape of the sample to be investigated, and reliable measurements when oriented in either a horizontal or vertical position. Moreover, means are provided for easily maintaining and calibrating the parallelism of the electrodes.

Abstract: A method of semiconductor surface measurement for measuring electrical characteristics of a surface of a semiconductor body is disclosed, by which an electrode, whose surface, which is opposite to the surface of a semiconductor substrate, is flat and the gap between the electrode and the surface is smaller than 0.5 .mu.m.

Abstract: Apparatus for detecting the amount or presence of material in a detection zone comprising a first electrode, input signal means for supplying an electric input signal to the first electrode; a second, smaller electrode parallel to the first electrode; the detection zone being located between the first and second electrodes; a third electrode which is larger than the second electrode and substantially surrounds the second electrode in a plane parallel to the first electrode, and receiving means for detecting signals on the second electrode, the signal on the second electrode being responsive to material in the zone. The second and third electrodes are not electrically connected but are maintained at near the same electric potential whereby the electric field between the first and second electrodes is, in use, substantially linear and fringing fields, if any, occur only between first and third electrodes at their outer edges.

Abstract: A fuel mix ratio sensor system comprises a cylindrical housing having a small diameter portion on one end at which an electrode is supported. A sealing means, such as an O-ring, directly contacts an inner periphery of the small diameter portion in order to secure the sensor against leakage of the detected fluid.

Abstract: A capacitive sensor senses the wall thickness of an object. The sensor has an electrode covered by an adhesive tape including a non-transparent glass cloth tape layer which engages the object and a layer of silicon or acrylic adhesive for securing the tape layer to the electrode. The adhesive and tape have a dielectric constant that is constant throughout the temperature range in which the sensor is to operate.

Abstract: An electrical capacitance clearanceometer system adapted clearanceometer system to measure the distance between a housing and a moving member within and closely adjacent the housing as a function of electrical capacitance therebetween, utilizes a high temperature probe which contains a high temperature electrical inductor potted in one end and an electrical capacitor electrode in a braze sealed tip at the other end and electrically connected to the inductor. The probe is inserted in a housing to have its capacitor electrode positioned adjacent the moving member. The electrode is braze attached to a ceramic insulator at the end of the probe and the ceramic insulator is brazed to a transition metal sleeve which is then brazed to the probe to fix the insulator in the probe. An air passage is formed between the insulator and the probe body to provide cooling air to the insulator and capacitor electrode.

Abstract: A micropower electronic motion detector including a sensing element and a source element for measuring the magnitude and direction of the relative motion of the sensing element with respect to the source element. Electronic apparatus for nulling the output signal of the detector and of other electronic sensing apparatus in its initial or static state is also disclosed. The detector may be used in a safety ski binding to measure the forces being exerted on a skier's leg so that a binding release or non-release decision may be made.

Abstract: In a hot gas turbine engine a capacitive probe clearanceometer is employed to measure the clearance gap or distance between the continuous surface of a compressor or turbine wheel such as, for example, a shroud ring turbine and its confining casing under operating conditions. The probe positions a capacitor electrode at the inner surface of the casing adjacent the continuous surface path. Electrical power is supplied to the probe through an oscillator having a tuned circuit in which the probe is a component. A high frequency signal of the oscillator is amplitude modulated by changes in electrical capacitance between the probe capacitor electrode and the passing continuous surface. The modulated oscillator signal is processed to provide an electrical signal which is proportional to the distance between the casing and the continuous surface. A preferred combination of high temperature and contamination resistant metal and ceramic materials are combined in an improved gas tight braze sealed probe tip.

Abstract: An on-line capacitance sensing system, including a capacitance sensor, for measuring changes in thickness of a plastic film which passes over the sensor in contacting relationship; the sensor is mounted to a carriage which may be reciprocated transversely across the width of the film, and a partial vacuum is applied to the sensor to assist in holding the moving plastic film in contacting relationship against the sensor.

Abstract: A system rapidly and accurately assesses the lachrymal fluid content in a person's eye which minimizes patient discomfort. The preferred system includes a lachrymal fluid sample device and an assessment device. The preferred sampling device includes a lachrymal fluid absorbent sample pad and a non-absorbent handle. The preferred assessment device includes a receiver for receiving a lachrymal fluid sample pad and includes a control circuit coupled with the receiver for determining the electrical capacitance of the pad which capacitance varies in accordance with variations in lachrymal fluid content, and for producing an output correlated with the lachrymal fluid quantity in the pad.

Abstract: Apparatus for inspecting the thickness of a container wall, such as a glass or plastic container wall including a plurality of elongated sensors disposed in a linear array in spaced end to end relationship. The containers are urged into intimate contact with the sensor elements and are rotated thereover. The containers are moved from sensing element to sensing element with each element inspecting a portion of the circumference. Oscillators convert the change in capacitance to a corresponding voltage which, in turn, in a processor is converted to an actual thickness reading which is compared with desired predetermined thickness levels. By segmenting the sensors in this manner, a plurality of containers may be inspected simultaneously with a processor combining the individual segments of sensor readings so as to reconstruct the complete thickness evaluation of the container. A corresponding method is provided.

Abstract: An electrical device for measuring the dry film thickness of a paint film applied to a conductive coating adhered to a plastic substrate without destroying the film; the device contains the followintg components: a. an electric sensing device of two separate electrodes isolated from each other and positioned in a material capable of conforming to the surface of the film being measured where the sensing device is placed in contact with the surface of the film and measured under a uniform and reproducible pressure; b. a gain and phase measuring device electrically connected to the sensing device for measuring the gain and phase of an alternating current electrical signal passed from one electrode through the film and conductive coating to the second electrode; c.

Abstract: In a fuel composition sensor in which a fast step voltage is applied to a capacitive voltage divider comprising a sensor capacitance and a reference capacitance in series, a padding capacitor is connected in parallel with the sensor capacitor to produce a more linear output across the full range of fuel composition. The value of the padding capacitor is sufficient to produce a capacitance ratio of the voltage divider substantially less than one and, in particular, less than two thirds.

Abstract: A device and method for the determination of the dielectric characteristics of a material sample. The device includes a probe (3) connected to a coaxial line (2) by a connector (30). The probe comprises a conductive tubular member (78) having a conductive rod (8) coaxial to the tubular member and centered on the latter. An annular member is made of a dielectric material is housed in the tubular member, and is integral with the rod. The annular member/rod assembly is slidably carried in the tubular part (7). A resistive force urges the probe in contact with the material sample (4) so that good contact is ensured with the material sample.

Abstract: A monitoring system for testing the integrity of seal structures, filled with an electrically non-conductive fluid and for validating measurements of a measurement device. The monitoring system includes electrodes disposed in the fluid for measuring an electrical property of the fluid. The system indicates when the measured electrical property changes, which indicates contamination of the fluid.

Abstract: A capacitance probe (10) includes readily removable outer layers (64,66) which facilitate its use in the field to measure the width of slots or clearances (12) of different widths; an optionally perforated guard electrode (64,66,76,82) which permits width measurements over wider side wall bands with minimal effect on sensitivity; convergent or tapered side edges (102) to ease insertion and movement of the probe and a carrier block (124) which ensures proper positioning of the probe within such slots.

Abstract: Safety apparatus for indicating a live alternating current voltage condition in an insulated electrical conductor having a central conductor wire comprises: an electrically conductive external sheath extending along a predetermined length of the insulation forming with the central conductor wire a capacitive alternating current path across the insulation; and a signal generating circuit including a full-wave rectifier connected directly between the sheath and ground. A capacitor and a neon or other gas discharge bulb characterized by avalanche breakdown at a predetermined voltage are connected in parallel with the rectifier. Optionally, a piezo electric or other sound-generating device may be in series with the neon bulb. When the central conductor wire is energized at 1000 volts AC or above, the bulb flashes, and the optional sound generator activates, at a frequency dependent on the length of the external sheath along the insulation.

Abstract: A planar interdigitated dielectric sensor useful for measuring the surface properties of a material is disclosed. The sensor is formed on an insulating substrate. Attached to the surface of the substrate is an excitation and response electrode disposed in an interdigitated pectinate configuration. Filling the space between the electrodes is an insulating material of known dielectric properties. The upper surface of the electrodes are generally coplanar with respect to the insulating material between the electrodes thus forming a flat upper surface on the sensor. This flat upper surface serves to eliminate air gaps between the sensor surface and sample when analyzing relatively viscous materials.

Abstract: A capacitance gauge system is described for determining the thickness of thin-film non-conducting material (i.e. polyethylene, vinyl, etc.). An elongated capacitance sensing head is applied to one side of the material being measured thereby disturbing the electric field produced by the head. Internal circuitry monitors the effect of the material under test and produces an output proportional to the variation in the material's thickness. A new method of resolving capacitance changes results in the ability to resolve capacitance changes on the order of 5.times.10.sup.-17 Farads in a linear manner with the benefits of simplicity, excellent stability, low power requirement, and small physical size. These factors allow said apparatus to be incorporated into a small hand-held unit. Because of this advancement in portability, accurate measurements can be made more quickly and conveniently than previously possible.

Abstract: A rotation-sensing device for providing output signals that correspond to rotary hand movement made about the device by an operator is disclosed. The device comprises a cover member including an outer surface having a generally cylindrical, longitudinally elongate, non-rotatable protusion carried thereon. The protrusion projects outwardly from the outer surface and is provided with an outer periphery thereon. The device further includes a plurality of generally longitudinally elongate, circumferentially spaced, sensing means which are positioned adjacent the outer periphery of the protrusion and are responsive to the rotary hand movement of the operator for providing output signals corresponding to such movement. In addition, the device includes means coupled to the sensing means for leading the signals generated in the sensing means away from the switch device.

Abstract: A light-weight density sensor has a rigid open structure that does not impede fluid flow and provides the ability to accurately and reproducably measure dielectric constant and densities over long distances without interference from connecting coaxial cables during periods of vibration and thermal shock. A novel immersion cell and an oscillator circuit provide a variable frequency output related to the dielectric constant of the cryogen within the immersion cell and from which the density of the cryogen can be determined. The immersion cell comprises an elongated electrode located centrally within an open outer electrode formed by a plurality of elongated conductors spaced around and from the central electrode conductor. The open outer electrode does not impede the flow of cryogen into the immersion cell and by its structure permits accurate density determinations dependent upon the dielectric constant of cryogen present within the cell.

Abstract: A device for measurement of moisture in a body of particulate material, such as a grain in a pile or in a silo, is in the form of a spear with a set of conductive plates 2 carried at one end of a shaft 1 for immersion in the body of particulate material, and a read out meter 4 and carrying handle 3 at the other end of the shaft. Normally the set of conductive plates measure capacitance of the material via a phase locked loop oscillatory circuit. The plates are disposed and shaped to facilitate entry into and removal from the body of particulate material.

Abstract: Capacitive-type measuring apparatus for making absolute measurements comprises first and second support members which are relatively displaceable with respect to a measurement axis; an array of first electrodes disposed on the first support member; an array of second electrodes disposed on the second support member such that differing portions of the second electrode array are capacitively coupled with the first electrode array in dependence on the relative positions of the support members; an array of third electrodes disposed on the second support member so as to be electrically connected to corresponding second electrodes and spatially offset therefrom by an amount which varies according to the position of the third electrodes from a reference position; and a fourth electrode arrangement disposed on the first support member for providing in combination with the other arrays a plurality of discrete signal transmission paths each having a capacitive transfer function with two components, one of which varies

Abstract: Electronic devices such as hybrid integrated circuits such as those having test points spaced less than 1250 .mu.m are advantageously evaluated utilizing a two-probe process. In this process the probes are moved between test points in a pattern that reduces movement distance without concern for any ordering imposed by the nets themselves or the test to be made. Additionally, the test is made so that the movement time is the limiting factor.

Abstract: A capacitance sensor arrangement is used to determine the distance of an element such as a shearing handpiece from a surface such as the skin of a sheep. The arrangement processes data from one or more sensors which include a variable capacitor dependent on the distance, and provides an output voltage consistent with the distance, the output voltage being dependent on the voltage drop across a reference capacitor in the sensor circuit.

Abstract: A probe (20) that inspects a countersunk hole (21) in a conductive body, such as an aircraft part (22), is provided. A plurality of nested cylinders form an electrode cartridge (54) with a countersink portion (74) having a frustoconical shape and a hole portion (76) having a cylindrical shape. A plurality of lower hole electrode strips (72) are mounted on a lower hole cylinder (110) and a plurality of upper hole electrode strips (70) are mounted on an upper hole cylinder (116). A plurality of lower countersink electrode strips (68) are coupled to a lower countersink cylinder (124) and a plurality of upper countersink electrode strips (66) are coupled to an upper countersink cylinder (132). A dielectric layer (64) covers a plurality of measuring electrode tips (67, 69, 71 and 73) and an outer surface (150) of the hole and countersink portions (76 and 74). A grounding electrode (158) is mounted on the upper hole cylinder ( 116).

Abstract: A system rapidly and accurately assesses the lachrymal fluid content in a person's eye which minimizes patient discomfort. The preferred system includes a fluid sampling device and an assessment device. The preferred sampling device includes a lachrymal fluid absorbent sample pad and a non-absorbent handle. The preferred assessment device includes a receiver for receiving a lachrymal fluid sample pad and includes a control circuit coupled with the receiver for determining the electrical capacitance of the pad which capacitance varies in accordance with variations in lachrymal fluid content, and for producing an output correlated with the lachrymal fluid quantity in the pad.

Abstract: A device for detecting objects behind a wall or like surface has a multiplicity of capacitor plates at spaced intervals and a circuit for sensing alterations in the dielectric constant of a region of the surface being inspected and adjacent to each of the capacitor plates. A multiplicity of quantitative display elements are arranged at intervals in a visual display, and each display element provides a quantitative value representative of the strength of the signal generated by the associated capacitor plate. Thus, the images in the display elements will increase and decrease to reflect the quantitative signals and effectively image the object being detected.

Abstract: Wood grain angle information is detected electronically by use of a sensor having a common electrode and a plurality of electrodes spaced from the common electrode and arranged angularly about a sensor axis. The common electrode can serve as a sense electrode or as a drive electrode for the remaining angularly spaced electrodes. By modulating applied drive signals and demodulating the sensed signals that result from capacitive coupling of an adjacent wood specimen, values that are a function of wood grain angle are identified.

Abstract: A linear capacitance displacement transducer for measuring absolute displacement is formed from a coaxial variable capacitor and a precision capacitance measuring electronic circuit. The plates of the coaxial capacitor are attached to the members of an assembly whose displacement relative to one another is to be determined. Linear displacement of the members causes a linear displacement between the capacitor plates which is reflected in a linear capacitance change. A capacitor controlled oscillator utilizes the coaxial variable capacitor at its input. The capacitor controlled oscillator whose period of oscillation is been determined by the capacitance and locked in phase with changes of capacitance is utilized as a precision measure of capacitance. The period of the square wave output of the oscillator is a linear function of the capacitance of the variable capacitor at any time. The transducer may be adapted to determine the absolute position of a piston rod in a hydraulic cylinder.

Abstract: A capacitance-type measuring apparatus includes first and second support members which are movable with respect to one another along a measurement axis; a first electrode array and a second electrode disposed on the first support member; and a third electrode array disposed on the second support member and capacitively coupling the first electrode array and second electrode for transmission of a signal between the first electrode array and second electrode through the third electrode array. One of the first electrode array or the second electrode is formed to have end portions configured such that the degree of coupling between the first electrode array and second electrode through the third electrode array decreases linearly along the extent of each end portion relative to the measurement axis. This configuration permits improved measurement accuracies by reducing the effects of tilt between the first and second support members which changes the spacing between such members along the axis of measurement.

Abstract: An edge detection system for measuring lateral position in the presence of height variations such as found in gauging the edge position of a wafer uses first and second capacitive probes formed of passive elements with the active components kept outside the probe and at a distance that permits a smaller, stabler and more economical probe to be located at the location of the wafer. The probes are instrumented in an electronic circuit having plural operational amplifiers that drive the probes to compensate the edge detection for wafer height effects.

Abstract: A method for measuring the width of structures in a semiconductor wafer comprises the step of constructing test structures on the wafer shaped to function as moats for confining electrically conductive liquid. The moats have an elongated shape. By measuring the electrical resistance exhibited by the liquid within the moat, the dimensions of the moat and, thus, the other structures on the wafer can be measured.

Abstract: A capacitance sensor for measuring changes in thickness of a dielectric film, such as plastic film, passed through the sensor, having an upper and lower housing projecting from a back plate, the lower housing having an upper electrode surface and the upper housing having an insulator aligned with the electrode surface; the insulator having an electrode conneced to an electrical conductor; the materials for constructing the two housings, the electrode and the insulator all having a coefficient of linear temperature expansion of less than 1.times.10.sup.-6 per degree Centigrade.

Abstract: A moisture sensing device for an irrigation system. A pair of concentrically disposed cylindrical conductors are separated by a layer of beads, such that a capacitor is formed whose resistance will vary in response to the level of moisture present in the fibrous material. An electrical circuit is connected across the cylindrical conductors for measuring the variable resistance caused by the level of moisture present in the layer of beads. If the soil is dry enough to require watering, the sensor circuit will produce an output signal to the appropriate solenoid in the irrigation system to permit watering.

Abstract: A fuel sensor for sensing a mixing ratio of a given liquid in a mixed fuel comprises a housing; a structure housed in the housing, the structure having a fuel passage formed therethrough; and an electrode assembly housed in the housing and held by the structure, the electrode assembly including at least one pair of probe portions which are exposed to the fuel passage. The full sensor also includes an output control circuit housed in the housing, the output control circuit issuing an amplified voltage output which varies in accordance with a small information signal applied to an input portion of the output control circuit; and an electrically conducting member housed in the housing and electrically connecting the electrode assembly to the input portion of the output control circuit, so that the output control circuit issues an amplified voltage output which varies in accordance with a capacitance established between the paired probe portions.

Abstract: To increase the sealing effect with increasing fluid pressure in a capacitive fluid sensor for sensing a dielectric constant of fluid, the sensor comprises a sensor housing, a fluid outlet pipe connected midway in the sensor housing, an internal electrode, a first cylindrical insulating internal electrode support member disposed within the sensor housing between the fluid outlet pipe and a sensor housing end, and a sealing member disposed between the first cylindrical support member and sensor housing end. It is preferable to dispose a second flat insulating internal electrode support member formed with a central annular portion and plural arms extending from the annular portion so as to be fitted to a swollen-out portion of the sensor housing to reduce fluid resistance.

Abstract: A device and method for capacitive measurement and control of the fly height of a recording slider. A recording slider is vertically spaced from a rotating disk surface by an "air bearing". The disk has a first electrically conductive pattern at its surface. The first pattern has an edge with a length extending, for example, radially. The recording slider has one or more electrically conductive rails extending transverse to the radial pattern on the rotating disk. As the disk rotates, the edge of the pattern on the disk scans across the rail of the slider. By measuring the electrical capacitance between the slider and the pattern on the disk, and by processing the electrical capacitance measurement signal, a vertical spacing signal proportional to the vertical spacing between the disk and the slider can be obtained.

Abstract: A method of measuring the moisture content of soil wherein a tapered probe is inserted into a tapered hole formed in the ground. The walls of the hole are tapered substantially in correspondence with the taper of the probe and such that the soil adjacent the walls is not compacted or otherwise affected. In this manner, good contact between the surface of the probe and the soil is obtained. The probe is a capacitance sensing probe with a pair of spaced electrodes along the tapered surface with a dielectric material in between. The electrodes serve as structural members of the probe body and the hollow internal portion is backfilled to prevent moisture from permeating the probe.

Abstract: A three-dimensional capacitance density imaging of a gasified bed or the like in a containment vessel is achieved using a plurality of electrodes provided circumferentially about the bed in levels and along the bed in channels. The electrodes are individually and selectively excited electrically at each level to produce a plurality of current flux field patterns generated in the bed at each level. The current flux field patterns are suitably sensed and a density pattern of the bed at each level determined. By combining the determined density patterns at each level, a three-dimensional density image of the bed is achieved.

Abstract: A capacitive sensor is constructed to eliminate electrical conduction through a liquid being measured. The electrical conduction in the liquid occurs between the plates of the reference and measuring capacitors. The capacitive plates which are spaced from the common capacitive plate to form the reference and measuring capacitors are of unitary construction and formed from anodized aluminum. The anodization forms an insulated surface which prevents conduction between the electrodes through the liquid. A ratio taken between the measuring capacitor and reference capacitor is therefore indicative of the level of the liquid within the gap of the measuring capacitor and is independent of the conductance of the liquid and its dielectric constant. Such a probe is particularly useful in applications where the liquid undergoes indeterminable changes in conductivity such as a lubricant which becomes contaminated by metallic particulates.

Abstract: This invention relates to a capacitance standard for use with capacitance sensitive probes. The standard comprises a capacitor having a pair of spaced plates and a dielectric positioned therebetween. A shield attached to one of the plates forms an enclosure around the other plate to essentially shield the capacitor. The capacitance standard is therefore not affected by outside electrical fields and an accurate, reproducible capacitance value is provided.

Abstract: A novel thin, three-terminal capacitive transducer which is positioned in a clearance gap to be measured is presented. This transducer comprises an insulated assembly having three parallel conductive planes with the first and second plane each containing a single electrode and the third plane containing a pair of spaced electrodes. Preferably, each electrode is composed of a thin layer of copper mounted on an insulated substrate such as an epoxy-glass composite. An air space is provided between each of the pair of electrodes in the third plane and the single electrode in the second plane. Another important feature of the present invention is a novel electronic circuit for use in conjunction with the novel three terminal capacitive transducer. This circuit provides a means of "synthetic resonance" whereby a small capacitance (such as generated by the three-terminal capacitive sensor of this invention) functions as if it were at or near series resonance with a synthesized large inductance.

Abstract: Apparatus for monitoring the dielectric composition of articles, particularly articles such as blisterpacks and laminated boards. The apparatus comprises an amplifying circuit including a capacitor between the electrodes (1, 3) of which an article (22) is moved in use. The gain of the amplifying circuit varies with the capacitance of the capacitor so that the amplifying circuit provides an output signal, the peak value of the output signal varying with the dielectric constant of the article (22) between the capacitor electrodes (1, 3). Typically the amplifying circuit includes an operational amplifier (9) the non-inverting input (10) of which is connected to a ground potential (6) and the inverting input (8) is connected to one electrode (3) of the capacitor. A feedback loop (11) is applied between the output (14) of the operational amplifier (9) and the inverting input (8) of the operational amplifier to force the inverting input to the same potential as the non-inverting input.

Abstract: A compensated capacitive probe system in which the probe and a guard ring are placed in the feedback path of an operational amplifier. An input signal is applied to the operational amplifier in combination with a portion of the guard signal. The resulting configuration compensates for stray capacitance at a measuring electrode of the probe and for the fact that the operational amplifier is other than mathematically ideal.

Abstract: A capacitance probe extends into a container for material which increases the capacitance between the probe and the container when the material contacts the probe. A fixed RF voltage is supplied to the base of a transistor in an electronic translator having a capacitor between the probe and the transistor's emitter. A first diode conducts between the base and RF ground. A second diode conducts between the emitter and the base. A small-value capacitor is connected between an ungrounded DC supply terminal and the probe. A third diode conducts variable DC to the transistor's collector from a variable DC signal line supplied with DC for the DC terminal through a resistor. A comparator has a first input connected to the line and a second input connected to a reference voltage. The comparator's output is adapted to energize an output signal and a relay when the DC signal on the line drops below the reference voltage.

Abstract: A record detector includes a member having a capacitance that changes in response to capacitive coupling with a metallic layer of the record. The variable capacitance member controls an electrical oscillator that produces an electrical signal having a frequency that varies in response to the value of the capacitance. Changes in the frequency of the electrical signal are measured in analog or digital form. The magnitude of the frequency change is compared to one or more reference values to determine whether an optical disc record is present and to determine other characteristics, such as size and orientation, of an optical disc record. The variable capacitance member is constructed by applying one or more metallic plates to an optical disc record support, or to the periphery of an optical disc record insertion slot, that is part of an optical disc record playback unit.