Disclosed is a unique probe and amplifier circuit for use with a digital height gauge or other measuring device. Fundamentally, the invention utilizes a probe tip operatively connected to the primary and secondary windings of an LVDT; the core of the LVDT being fixedly positioned between the windings. Deflection of the probe in either direction along the axis of the core results in an output signal from the LVDT, the amplitude of the signal being indicative of the magnitude of the deflection. The signal is translated into a varying DC level, the value of the DC level being linearly related to the deflection of the probe. The DC voltage is applied to an operational amplifier having selective feedback networks associated therewith which prescribe the gain of the amplifier and hence its subsequent effect upon a calibrated meter connected to the output thereof. The meter provides visual readings indicative of the deflection of the probe. The operational amplifier has a DC bias associated therewith to compensate for the diameter of the probe tip such that measurements may be taken in a bi-directional manner without compensation having to be made for the probe tip diameter. Also connected to the output of the operational amplifier is circuitry for producing a binary state DC signal at that time at which the meter indicates that a measurement should be taken.
A voltage enhancement device in the nature of a DC-to-DC converter is used in a centrally monitored alarm system of the type having at least one alarm detection device at a first location, an alarm monitoring panel at a second location, and telephone lines interconnecting the device in the panel. The telephone lines have an internal resistance which causes some of the voltage applied at the input of the lines at the first location to be dropped thereacross with resultant decrease in voltage at the second location. The alarm monitoring panel includes an indicator, such as a meter, which requires a minimum detectable voltage to provide a suitable indication or deflection of the meter to distinguish between an alarm, normal or trouble condition at the first location. The DC-to-DC converter only utilizes the voltage available at the first location and increases the magnitude of the voltage to compensate for the internal line resistance to assure that the voltage which is available at the second location is at least equal to the minimum detectable voltage.
A position indicator is disclosed which includes the combination of a linear voltage differential transformer (LVDT) and accuracy assurance circuitry. The LVDT includes a movable core connected to a mechanism whose position is to be indicated. The accuracy assurance circuitry permits the disclosed indicator to operate substantially independent of fluctuations in input voltage or frequency. This substantial independence is achieved in the following manner. First, the voltage across the LVDT is rectified by a first precision rectifier. Simultaneously, the LVDT tap voltage is rectified by a second precision rectifier. Each precision rectifier output is filtered and then applied to an analog divider which "cancels out" the effects of input voltage or frequency fluctuation. The divider output is an analog signal which is a true ratio of tap voltage to voltage across the LVDT. This true ratio signal accurately represents the position of the mechanism connected to the movable core.
