Pressure Sensor-Based Altimeter Circuit

NEW: Updated links to schematic - even though you CAN NOT obtain the pressure-sensor itself anymore.


[Document Version: 1.01] [Last Updated: May_12_1995 (minor 4/4/07)]

1. About the Author

Altimeter Construction and Tuning Notes

Author: Paolo Zini
Date posted: 12 May 1995

My signature:

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Paolo Zini                TTTTTTTTT
CNUCE institute of CNR    | | | | |
Pisa ITALY               ===========
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2. Introduction

The circuit uses an absolute pressure sensor to measure barometric pressure, it depends on the altitude but not linearly, remember this when using the circuit. The pressure sensor can be absolute or differential, this means that the output depends on absolute pressure or on the pressure difference between two ports of the device. It is a four terminal device: if a current flows between terminals 1 and 3 the DDP found between pins 2 and 4 is (almost) proportional to pressure (absolute or differential). The output depends on the current and on temperature, therefore, this must compensated. The output is very small so we need amplification.

3. What the circuit does

The device has been built for towed gliders contests, in those contests gliders are towed by a big rc model to a fixed altitude and released, after the release the flight time is measured, and so on. To do this is required a device which measures the altitude and signals when the release altitude has been reached, the release is left up to glider's pilot. The circuit measures the air pressure and compare it with a given reference (trigger altitude) when the pressures drops under the reference the command pulses sent from the receiver to the throttle servo are cut off and replaced by a locally generated pulses which throttles the engine to a preset speed (usually a fast idle); when the altitude drops under the reference, the throttle control is switched back to the receiver.

4. The circuit

The circuit is powered by the receiver's pack, the current drain is low, specially if compared to drain of servos.

U1a VZ2 and Q1 are the CURRENT source of the sensor VZ2 is temperature compensated, use the listed component except for Q1 there you can use any pnp silicon transistor, resistor must be metal oxide 2%(better) or 5%.

R4 must be adjusted to obtain about 2 Volt between pin 3 and 1 of Pressure Sensor, do not replace it with a trimmer, replace the resistor, the DDP is not important, its stability IS VERY IMPORTANT.

PS1 is the pressure sensor, R12 (METAL!) is there to compensate its thermal drift (See later). U1b is the first amplifier, adjust R23 to compensate its thermal offset drift; resistors R5 R13 R14 R22 MUST BE METAL 1%.

U1c give more amplification and add a voltage proportional to the trigger altitude, to be set using P2. Use metal resistor also here, p2 must be a GOOD quality miniature 10 turn trimmer, do not use cheap part here. The gain can be adjusted replacing R19, the value given set the output to be 1mV = 1 meter if you want 1 mv = 1 foot you must set R19 =~ 60k.

U1d is the comparator, it compares the output of U1c to a reference voltage provided by the group VZ1 R9 R10 R11 R16 R20.

The four NOR of 4001 are the digital switch, it switch the pulse output (U3d pin 11) from receiver to the output of NE555; the specs are more relaxed here you can use standard part. P1 adjust the idle throttle, you can use a single turn trimmer, multiple turn trimmer can be used but requires a longer time to adjust on the field.

CN1 is the servo in and out, it carries also the power for the altimeter. Get a servo extender (wire with a male conn. in one side and female in the other), split it in the middle, use the two half as input and output cable, obviously connect the wires according to the disposition of your rx and servo. CN2 is the meter connector, adjust the voltage between pin 2 and 3 to be equal to the trigger altitude required DO THIS JUST BEFORE STARTING THE ENGINE.

5. The Schematic & PCB layout

PDF Schematic: ALTIMSCH.PDF (29KB)
PostScript Schematic: ALTIMSCH.PS (69KB)

I'm sorry but I can no longer find the PCB layout file. I will keep me eye out for it - and if anyone has it please let me know!

6. Tuning and adjusting

7. Installation in the model

Place the altimeter inside the model in accessible position, avoid aerodynamic pressure change (how to explain? close has many holes has you can). The altimeter get power from the receiver and then is switched on and off from the rx switch.

8. Use

9. Part List

PS1	Pressure sensor		Motorola 	MPX100AP
VZ1,VZ2 Reference voltage diode Motorola	LM285-2.5
D1	Diode			Any		1N4148 or equiv.
Q1	PNP Transistor		Any		BC307 BC308 BC320 ....
U1	Quad Op Amp		Motorola	LM324 
U2	Timer			Any		NE555
U3	Quad Cmos Nor		Any		4001
P1	single turn miniature trimmer		15K	
P2	10 turns miniature trimmer		1.8K	
C1	Ceramic capacitor			10nf	
C2	Ceramic capacitor			120nf
Note: All resistor 1/8W metal 1% or 2%

10. More Links!

And, don't forget that google is your friend.

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