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)]
Altimeter Construction and Tuning Notes
Author: Paolo Zini
Date posted: 12 May 1995
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Paolo Zini TTTTTTTTT
CNUCE institute of CNR | | | | |
Pisa ITALY ===========
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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
The output depends on the current and on temperature, therefore, this must
compensated. The output is very small so we need amplification.
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.
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.
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!
- Before soldering it, measure the resistance between pins 1 and 3 of PS1, use a
good multimeter, R12 must be about 1.5 times the measured resistance; the value
printed on the circuit is only orientative.
- Double check all the circuit, ask a friend to repeat the check for you.
Check also the quality of the work: soldering, isolations, fastening of part to
- Power on, look for smoke....no smoke? OK go on.
- Check the current source VZ2 U1a etc. measure the voltage between pin 3 and 1 of
PS1: it must be about 2 Volt (1.8 - 2.3 can be ok) if it near 0 or near power
check all again, else adjust (replace or mount one resistor in parallel) R4
(higher R4 higher Voltage). Connect a millivoltmeter between pin 2 and 3 of cn2,
adjust P2 to read 50-100mv, going up and down the stairs you must see the
reading change, about 1mV each meter.
- First Thermal adjust: disconnect R13 from PS1 pin 4 connect it to PS1 pin 2,
(short the input of OpAmp together) and start a thermal circle. For thermal circle
i mean: put all but battery and millivoltmeter in the refrigerator, wait 15
minutes record the reading and, put all in free air wait ... record the reading,
warm all to 50 Centigrades (please do you the conversion ...) (use a table
light for this) and repeat. Modify R23 to minimize the drift, repeat all the
circle until you are satisfied by result (the quality of the result is
proportional to your patience).
- In some cases r23 must be disconnected from pin 5 and connected to pin 6 of U1b.
- Reconnect R13 to PS1 pin 4, reconnect the millivolmeter and re-adjust P2, repeat
the thermal cycle, but more accurately (now you need a thermometer) plot the
output reading versus temperature between 0 and 40 Centigrade.
- Adjust R12 for minimum drift: this means replace R12, repeat all and compare
the plots if the second is better you are going in the correct direction etc.
A good plot look like this:
xxx xxxx xxxxx
0 20 40 C AND NOT 0 20 40 C
If you obtain a good plot you are near the end.
- Use clear plastic tube, fold it in U shape, half fill it of water,
connect one side of tube to the port of Press. Sensor: move the U to level the
water between the two half of U tube, adjust p2 to read 0.0 Mv.
- Move the U to have a difference of water level of 14.5 inch: this correspond to
about 1000 feet of altitude; correct R19 to obtain the correct reading a good
choice can be 500.0 mV (1 mV = 2 feet).
___ | __| _______ < to PS1 port
| | A |x|
| | | |x|
| | 14.5"|x|
| | | |x|
| | | |x|
| |__ V |x|
- You are ready. fix all, paint both sides of pcb with a clear paint, let it dry.
(The circuit is sensitive to humidity).
- Mount it in your model, adjust p2 to have a negative reading (this switch on
the internal pulse generator), switch on Tx and Rx, verify the throttle: it must
be controlled by P1, adjust p1 for a fast idle (fast..to be safe), adjust p2 to
have a positive reading, the throttle is now under TX control.
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.
- At field, before refueling, connect the millivoltmeter adjust P2 to required
trigger altitude, test the operation of throttle, disconnect millivoltmeter
- If weather change repeat the adjustment, (barometric pressure change with
- The altimeter is temperature sensitive (you learned it?) park the model in a
fresh place, do not use dark colors and so on.
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%
And, don't forget that google is your friend.