An air pressure monitor includes an electro-mechanical sensor comprising a flexible diaphragm whose deflection is proportional to the air pressure applied thereto, and a variable inductor including a coil and a movable core inside of the coil, the movable core being attached to the flexible diaphragm for movement in response to the deflection thereof to adjust the inductance of the coil in proportion to the air pressure applied to the diaphragm. The monitor further comprises electrical circuits including a first oscillator circuit including the coil for producing a pressure signal at a frequency proportional to the pressure in response to the inductance of the coil, and a second oscillator circuit for producing a reference signal at a frequency corresponding to the frequency produced by the first oscillator when the pressure is substantially zero. Mixer and filter circuits are provided, connected to the two oscillator circuits for providing a signal whose frequency is proportional to the difference in frequencies between the pressure and reference signals and a Schmitt trigger and divider are connected in series from the filter circuit for producing a chain of digital pulses at a frequency proportional to that of the difference signal. An adjustable gate generator is provided for producing a gate signal having a predetermined period, such that the number of digital pulses produced at the output of the divider during the predetermined period comprises a digital signal corresponding to a measurement in predetermined, fixed units of the pressure at the diaphragm.

A capacitive transducer for converting a small displacement of a mechanical input member into an electrical signal directly proportional to the quantity being measured, even if the transfer-function between input quantity and output signal is non-linear, by use of a cylinder-and-piston capacitor with suitably shaped electrodes, and a plurality of adjustable vernier-capacitors. When used in an alternating-current bridge-circuit, an a.c. voltage proportional to said mechanical input quantity results. (Claim 1). To provide a digital output signal directly proportional to a mechanical input quantity being measured, a fixed high-frequency oscillator with an attached binary counter is combined with a second, substantially identical oscillator-and-counter circuit whose frequency is varied by said capacitor assembly. A third counter generates said digital output signal by a count proportional to the frequency difference of said two oscillators, thus substantially compensating for temperature-, supply-voltage-, and ageing drifts which affect both oscillators equally. (Claim 2). The polarity of bi-directional displacements is determined by a digital priority detection circuit which generates a NEGATIVE INPUT bit whenever a negative input displacement occurs. (Claim 3).

A single pressure sensor is caused to provide a pressure measurement at two separate pressure regions through the use of a differential pressure switch whose switch over point occurs when the two regions are at a fixed pressure differential or offset. The sensor is connected to sense a first region with a sensor output signal being compared to a signal related to the count in a counter. Upon switch over the counter is actuated to make a signal equal to the sensor output signal. The counter signal, compensated for the offset, is related to the pressure at the second region.

Apparatus for providing a digital signal for an aircraft transponder indicating aircraft altitude with a unique sensor forming part of an oscillator, a counter which accumulates counts from the sensor during a predetermined gating period, a ROM which is addressed during a further gating period to apply a sequence of digital numbers to a comparator. The address indicating one thousand foot increments a second counter which receives pulses from the sensor together with the first counter after the comparator indicates the ROM number exceeds the first counter number so that the number in the second counter indicates one hundred foot and 10 foot increments when the comparator indicates the first counter number equals the ROM output to the comparator and gating for applying the outputs of the second counter and ROM address to the transponder for transmission. In one embodiment structure for providing barometric correction is included.