with Motorola MPXS4100 A and
Technical Data of Wireless Transmitter and Receiver
Initial Test Setup
PCB Evaluation Board
Schematics of Evaluation Board
Available PIC Assembler Code
Sensor Ordering Information
Other Circuits and Utilities
The current concept incorporates a wireless transmitter and
receiver and is thought to be used for remote controlled
airplanes or appliances with two seperate parts. In other words,
we have one dedicated transmitter (acquisition, filtering) and
one dedicated receiver part (user-interface, look-up table,
calibration, storage), capable of being connected together with
any physical layer, e.g. wired, wireless, infrared. If you want
to build a standalone altimeter/variometer just for hiking or
mountaineering, this setup can obviously be simplified by
omitting the wireless components.
So far, only the transmitter part with all its analog circuitry has been completed entirely - the digital receiver part has still to be done, but does not necessarily have to be a PIC microcontroller. For instance, it could also be a personal computer, connected through the standardized RS232 protocol and a wired/wireless interface to the transmitter part.
The (pending) challenge of the receiver implementation is the
field evaluation of the most suitable and accurate temperature
and non-linear pressure-altitude correction algorithms. There is
maybe need for adding a temperature sensor to the transmitter
I reached 56 cm per unit, so the whole range is :
Increase the range by taking a better A/D converter or decrease the Amplifier-Gain (reduction of resolution).
In practice, I'll set the resolution to 1 meter, so a total range of 4096 meters will be available. This is enough precise for my R/C models, but allows to use this altimeter also for hiking and mountaineering.
The microcontroller builds an average value of 64 A/D samples to reduce any noise to a minimum.
I've developed the following routines in PIC Assembler
See related stuff at the document section below.
The following components have been used: Analog to digital
converter NSC ADC12130, a Microchip PIC16F84 controller, a quad
op-amp NSC LMC660, a Maxim MAX232 RS232 level shifter, and the
Motorola MPXS4100A absolute pressure sensor.
Removable module containing Motorola absolute pressure sensor MPXS 4100A, 10nF and 100nF ceramic and 10uF tantalum capacitors.
Circuit Design CDP-RX-01 wireless receiver, 434 MHz, up to 7.5 kb/s, uni-directional
Top view of removable pressure sensor module.
Circuit design CDP-TX-01 wireless transmitter, 434 MHz, up to 7.5 kb/s, uni-directional
Additional information is available at Circuit Design Inc. -> Products
Data sheets of wireless transmitter and receiver:
|Frequency:||433.920 MHz, 434.075 MHz
458.650 MHz (United Kingdom)
|Frequency stability:||+/- 2.5 kHz (-10° up to +55° C)|
|HF channels:||single (fixed channel)|
|Range:||up to 1000 m (free distance, line of sight)|
|Baud rate (specified):||300 - 4800 baud|
|Bit rate (measured at short distance):||up to 7500 bits/s|
|Operating conditions:||-10° up to +60° C|
|Type approval:||I-ETS 300 220 / Germany, France, Switzerland, Sweden, UK, Holland, Austria, EMC|
|HF power output:||10 mW +/- 3 dB @ 50 Ohm|
|Modulation:||FM narrow band|
|Start-up time:||30 ms|
|Input signal type:||digital, 5 Volt|
|Supply voltage:||5.5 - 10 Volt|
|Power consumption:||18 mA typ.|
|Dimensions:||36 x 26 x 10 mm|
|Type:||double superheterodyne, crystal oscillator|
|Sensitivity:||-120 dBm (12 dB/SINAD, CCITT filter)|
|Selectivity:||+/- 5 kHz @ -6 dB|
|Demodulation:||FM narrow band|
|Distortion:||< 5 % @ 1 kHz|
|Output signal type:||digital, open collector|
|Other outputs:||RSSI and AF|
|Supply voltage:||4.5 - 14 Volt|
|Power consumption:||10 mA typ.|
|Dimensions:||50 x 30 x 7.5 mm|
A lot of people have asked me where I got the Motorola absolute pressure sensor from.
Please see the section Sensor Ordering Information below.
By the way, Motorola distinguishes the sensor characteristics, feature set and package type by the sensor name, so you can also get 4100 sensor types similar to mine with different naming, e.g. MPXT 4100A or PPXA 4100A:
See Motorola Sensor Selector Guide at the section Documents below.
The Circuit Design wireless transmitter and receiver are not necessary to use the evaluation board, since there exists a direct RS232 link to the PC.
Test setup to check the initial concept
An initial test setup for checking the mixed signal design: A
significant problem was the digital noise in the analog circuitry
supply voltages. Finally the noise could be minimized by
splitting up the power supplies to three independent sources: one
digital power supply voltage, one for the sensor and A/D
converter, and one supply with slightly higher voltage for the
operational amplifiers of the filter stages. The above test setup
contains no wireless transmitter, the data is directly
transmitted to the computer using the RS232 protocol and a MAX232
In this setup, I have used the LMC660 / LMC662 low-power rail-to-rail quad operational amplifiers for the fourth order Chebyshev filter stages.
Moving from the test board to the first PCB, I have only made slight adaptations in the analog part of my design: I have altered the filter characteristics from Chebyshev to Butterworth - but as a consequence, I had to replace the LMC660 operational amplifier by a LM324 type, due to oscillating filters. Conclusion: In the analog world, nothing runs properly if it has not been tested.
If someone knows a good single supply, low-power, rail-to-rail operational amplifier with clean and linear output characteristics in the entire input range, please let me know! The LMC660 is exactly specified this way, but showed up a really bad non-linear characteristic in the upper input range. Bob Krech suggested the LMC6064 precision quad OP amplifier with pin-for-pin replacement for the LM324.
PCB-based evaluation board suitable for first field measurements
Description of the above PCB layout
The PCB-based evaluation board consists of analog circuitry at the left side and digital components at the right side of the board. In the upper left corner are the three independent power supplies (5 V analog, 6.8 V analog, 5 V digital), all served from one battery (8 - 10 V). The evaluation board contains further the active Butterworth filter stages built of one LM324 (left side), the NSC ADC12130 A/D converter (center), the PIC 16F84 microcontroller (at right from A/D converter), a dot LCD display and a PORTB connector (lower right corner), a direct RS232 interface with MAX232 level shifter (upper right corner), and an interface for the wireless transmitter allowing for first field measurements (bottom center). The system owns two oscillators. A 4 MHz crystal oscillator provides the conversion clock for the A/D converter, and a separate 4 MHz crystal for the microcontroller allows to increase processor performance easily if necessary. This setup provides two A/D converter input channels: Channel 0 is already used for the pressure sensor, but channel 1 can be used freely, e.g. for voltage surveillance of the R/C receiver battery. If two channels are not sufficient, this system can easily be upgraded to eight channels by integrating the NSC ADC12138 A/D converter. The LCD connector and the RS232 interface serve only for evaluation and debugging purposes in this setup. Finally, the noise characteristics of my approach are very promising!
Although this board is now ready, a suitable R/C plane - my
Piper Cherokee - has to be finished first for
Latest version 1.03 from April 23rd, 2001:
Top.pdf (8.5 kB)
Sensor.pdf (4.0 kB)
Filter1.pdf (6.5 kB) (replaced LMC660 with LM324, due to oscillating filters)
Filter2.pdf (6.3 kB) (replaced LMC660 with LM324, due to oscillating filters)
Digital.pdf (12.6 kB)
LCD.pdf (4.6 kB) (corrected some errors)
RS232.pdf (7.7 kB)
PowerSupply.pdf (8.9 kB)
|Main File||HEX Files|
|View assembler source code:
Download assembler source code: alti_tx.asm (16.7 kB)
|The above program needs additional include files (modules) to get successfully assembled: m_bank.asm, m_wait.asm, m_rs096.asm|
|For those, who are not familiar with interfacing a PIC to the RS232 using a MAX232: RS232-Interface.pdf (9.7 kB)|
An Excel worksheet has been used to visualize the captured
With the NSC ADC12130 A/D converter, there are two channels available.
Download Excel worksheet and drivers: alti_2channel.zip (203 kB)
I've done some measurements to see whether LSB toggling is sufficient low using the Excel RS232 data capture interface. All measurements have been carried out at room temperature (without any temperature compensation) and without moving the test board. The test period has been one hour with adequate warm-up time for the sensitive analog circuitry (pressure sensor, A/D converter and op-amp).
Note: Because the entire design is laid out very
sensitive in order to get a high resolution (in the range of one
meter), the accumulated drift could also originate from natural
Test setup: Drift of 2 bits during one hour (with a single spike).
Test setup: Drift of 4 bits during one hour.
Test setup: Drift of 3 bits during half an hour. The strange curve comes possibly from temperature variations.
PCB-based evaluation board: The yellow curve represents the pressure sensor, the blue curve is only a test voltage for comparision purposes. The pressure sensor behaves quite stable in conjunction with the PCB setup, only ½ LSB toggling during one hour. The comparision voltage - originating from a simple voltage divider - is even more toggling. This toggling may result from a voltage just reaching the A/D converter LSB threshold.
Data sheet of Motorola Absolute Pressure Sensor Series
MPX4100A: MPX4100A (PDF,
Motorola Sensor Selector Guide (Acceleration, Pressure, and Smoke): SensorSelectorGuide (PDF, 166 kB)
Noise considerations for integrated pressure sensors: AN1646 (PDF, 153 kB)
Data sheets of wireless transmitter and receiver:
A lot of people have asked me, where they could get the Motorola absolut pressure sensor from. I bought it from www.conrad.com years ago, but now, these sensors may occassionally not be available there. Don't ask me where you can get them elsewhere, I don't know...
As of 08.04.2005, there was one pressure sensor model from Motorola available at www.conrad.com:
Item: SMD-DRUCKSENSOR MPXA4100A6U
Ordering#: 150110 - 13
Price: about 25.00 Euro/pc.
ADC12130 Test Interface
Dot-matrix LCD Interface
Excel Data Capture
Automatic Table Generator for MPLAB Assembler
RC data logger V2.5 with altitude capture
German page about a self-made RC data logger using PIC 16F84 and MPXS 4100A absolute pressure sensor.
Previous version with nice Palm interface pictures: RC data logger V1.x
This work is currently on hold due to limited time.
Last updated: 2005/11/09