The altitude measurement is an invaluable reading for a pilot. The pilot must know at all times what the altitude of their aircraft is. We felt that we could not build a telemetry system for an airplane without this sensor reading.


This was not an easy circuit to design and build. The Omron sensor which we used provided an 86mV output, this was not much to work with. The output of the sensor goes to an instrumentation amplifier with a gain of approximately 40 V/V. The output from the instrumentation amplifier then goes to a difference amplifier. The second voltage on the difference amplifier is a reference voltage which is set sithin 150 mV of the instrumentation amplifier output. The output from the difference amplifer is then sent to a final amplifier stage, where the output is set to 5 volts at sea level. As the altitude goes up, the voltage output from the circuit will decrease.
The instrumentation amplifier is the first stage of the altitude sensor. The output from the pressure sensor varies from 86 to 83 mV for a 100 ft change in height. The instrumentation amplifier provides an initial gain for the pressure sensor output. The gain of the instrumentation amplifier was computed using the equation:
A1 = (1 + (2*R3)/R4)*(R1/R3).

In this case it was a gain of 45. This voltage was then sent to the second stage, the difference amplifier.

The second stage let the voltage from the first stage to be shifted to almost zero. A voltage reference was created using the LM334. It provides a constant current to the resistor R5. Rref then sets the voltage at the node Vref. This voltage was set to 150 mV. As the voltage from the instrumentation amplifier falls, the voltage from the difference amplifier also falls.
The third and final stage of the altitude sensor board is a non-inverting amlifier, denoted A2. This takes the 150mV output from the difference amplifier, D1, and amplifies it to 5 Volts. As the output from the sensor falls, the voltage from the third stage will fall to zero. The resistors for the third stage were selected to give a gain big enough to boost 0.15V to 5 volts. The gain of the a non-inverting amlifier is:
A2 = (R0 + R2)/R2.

This gain was tuned using the trimpot Ro.

Circuit Schematic:

Sensor Calibration:

We used a water column to calibrate this sensor. We hooked up an apparatus which would lower the pressure at the port of the sensor. The sensor was calibrated for ground level first, the pressure of the water column was set zero. Then the output from the difference amplifier was set to 150mV, and the output from the third stage was set to 5V. The pressure that the sensor saw was then set to a water column height of 13 inches, this approximately corresponded to 1000ft. The voltage was recorded and then the water level was lowered and the voltage was measured again. The relationship of the water heigh to voltage can be seen in the plot below.
When all was complete, the resolution of the sensor was fine. The sensor output changed by 39mV per 10ft.

Calibration Plot:

Design Considerations and Problems:

The biggest problem and consideration of this sensor was the small change in the output of the pressure sensor. The output only changed 3 mV for 100ft. This possed a large problem. The multistaged amplifier and voltage reference circuit worked as designed. By using the instrumentation amplifier the initial output voltage was scaled so that it could be used. Then by using the reference voltage and difference amplifer, fed to the final gain, the resolution was large enough to be read.
The only problem was faced when the gain of the first stage needed to be large enough to sense a change at the output. The orchestration of the three stages also took some time to iron out. If this were to be reproduced, the only advice that can be given, is to take your time.



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Bill Glenn 
Pat Malloy 
Greg Norton 
AFTS Project Group