Abstract:

For the last several years, I have been building a small two place airplane. Much of the work to date has been construction oriented, but as the cockpit area nears completion, I have been looking into instruments as needed to complete the project. This list of instruments is rather lengthy and includes required flight instruments as well as many smaller informational instruments that display volts, amps, fuel levels, temperatures, etc (i.e. non-flight critical supplemental information). As I began to shop for the instruments and panel gauges, I noticed that the total expense was quite high per gauge depending on the function. Furthermore, each sensor is usually connected to a single 2 1/4" diameter gauge that tends to fill the available panel space in an already crowded area.

The majority of the modern instruments that I ran across were electronic LCD based, but still setup to mount into the 2-1/4" panel hole. I began to wonder if using an 8051 based processor coupled with readily available A/D, display interface, and electronic sensors could replace the bulk of the instruments normally found in a small aircraft. This research led to the realization that the world of sensors and A/D converters has been exploding in the last few years (probably as a result of the auto-makers and other industry utilization). At this point, I decided to plan a prototype instrument that not only read 0-5 Vdc on a single display, but would also interface with a couple of the more interesting sensors that I had run across. This would realize the goal of building an integrated display device that had the following features.

• Display multiple sensor data to save panel space, weigh less, and conserve power.
• Provide calibrated information in the desired units.
• Experiment with newly available Motorola pressure sensors.
• Portable so that the instrument could be tested in flying aircraft as well as installed into a fixed panel.

In order to see what might be accomplished with a fairly simplistic approach (i.e. not too much time or money invested), I decided to look for a small microprocessor based platform that could be mated with the following hardware:

• 4x20 character LCD module - The LCD interface allows the user to see all display data and is used for program debug as well.
• 2 pole, 16 detent position quadrature switch with a center push button switch - This switch allows the user to scroll up or down through a list of data and to select data by pushing the center push button while the selected data is highlighted.
• LTC1298 12 bit A/D - The A/D contains a two-channel input and SPI access to the data.
• Serial RS-232 port for debug information.

After some time on the Internet, the SimmStick platform using the Atmel 89C2051, or possibly a PIC processor, seemed like the best choice. This platform coupled with two of the Motorola pressure sensors would yield a prototype instrument that could be used for barometric pressure measurement, altitude measurement, and airspeed measurement, or expanded to display more sensor information as needed.

The major hardware pieces of the project became:

• Motorola MPX5010DP - 0-1.6 PSID differential pressure sensor for airspeed.
• Motorola MPX4115AP - 0-16.6 PSIA absolute pressure sensor for barometric pressure.
• Dontronics DT004 - four slot SimmStick motherboard w/ power supply.
• Dontronics DT104 - AT89C2051 SimmStick CPU.
• Dontronics DT201 - prototype board for the LTC1298 A/D interface.
• Dontronics DT206 - programmer board.
• 4 x 20 LCD Module - 4 lines by 20 character display.
• 2 pole quadrature switch with center push button.

Block Diagram:

A block diagram of the major functions and pieces of the project is listed below.

Note that as the project planning developed, and the pressure sensor/instrument took shape, the following additional hardware was obtained to see how it might be integrated with the original pressure sensor instruments. Although part of the hardware developed, the software to access these devices is not part of this project.

• LM34-DZ - 0-225 F calibrated temperature sensor.
• MAX-186 - Eight channel A/D for additional inputs to the microprocessor.

Prototype Construction:

SimmStick Motherboard:

The SimmStick motherboard is assembled as suggested in the Dontronics web based instructions with the following notes:

• Only two SimmStick slots are installed.
• The programmer connector is installed underneath the PCB so that the bus signal names can be read from the top of the board.
• The power supply connector mates to a 12 Vdc pigtail so that the electronics can be powered from a fixed power source or a portable nine-volt battery.
• The reset button is installed.

SimmStick CPU:

The SimmStick CPU is assembled as described on the Dontronic's web page for use with the AT89C2051 processor with the following notes and additions:

• The RS-232 port is installed and connected to a DB-9 pin connector just to the left of the Max-232 chip (the wire is shown, but not the DB-9).
• The EEProm is installed but not currently used.
• The crystal is 11.0592 MHz in order to implement a 9600 baud RS-232 link for program debug.
• The 14 pin connector on the right side of the board is added to the prototype area in order to connect the 4x20 LCD display and rotary encoder switch cable to the appropriate CPU digital I/O pins.
• The suggested electronics for "brown out" reset of the processor are installed.

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SimmStick Prototype Board - A/D interface:

The SimmStick prototype board contains the 12 bit A/D interface. Both the two channel LTC1298 ADC and the eight channel 12 bit MAX-186 ADC are shown, however, only the two channel LTC1298 is used in this project. Note that the chip orientation is setup so that the analog signals into the chip are along the upper edge of the prototype board, while the digital inputs and outputs are along the lower, bus edge.

• Connection to the SimmStick bus was derived from the LTC1298 and MAX-186 data sheets for normal SPI data transfers to/from the ADC chips.
• The POT at the upper left corner is used to provide static test analog signals to the MAX-186 (debug use only).
• The analog signal connection along the upper edge consists of the three rows of PCB holes. The top row is bussed power to the sensor (usually 5 Vdc). The middle row is bussed analog ground to/from the sensors. And the bottom row (nearest to the ADC chips) is the analog signal input to the ADC chips.
• The capacitors and diodes are all suggested by the respective vendor datasheets.

LTC1298 - 12 bit, 2 channel ADC Diagram:

MAX-186 - 12 bit, 8 channel ADC Diagram:

LCD Module:

The LCD module is a standard 4 x 20-character display setup for four bit data transfers into the 14-pin connector (shown at the top left edge). A connector on the top right hand part of the DT104 CPU board connects to the 14-pin ribbon cable that in turn, connects to the rear of the LCD module via a small VERO board adapter. This adapter/connection contains the LCD contrast POT and also wires to the quadrature switch (described later).

The display is showing a typical static reading of 29.58 "Hg and a corresponding ground elevation altitude of 661 feet (the elevation of my desk). The airspeed sensor has zero differential pressure across the transducer and is also reading zero Knots. 

2 Bit Quadrature Encoder Switch - User Interface

The 2 bit,16 detent position, quadrature, encoder switch provides the user with access to various LCD displays and adjustment of the altitude (i.e. barometric offset).

As the switch is turned through it's detents to the left, the display scrolls up a line and as the switch is turned to the right, the display scrolls down. This feature is used to put not only the sensor information on the display, but to be able to scroll the information up in order to view additional information or debug information.

• The switch is fairly inexpensive and easy to operate even in moving environments (i.e. bouncing aircraft).
• The switch can also be pushed in for momentary contact. This allows the software to scroll the altitude barometric offset value up or down as needed. Pressing the switch again returns the software to normal scrolling of the display information.
• When the software is first started the display shows the make/model of the aircraft and the pressure sensor readings in engineering units (see first LCD picture).



• When the switch is turned one detent position to the left, the display scrolls down one line, and the next line is displayed at the bottom, while the other lines are moved up one line position (see second LCD picture). Note that in the example below, the display has shifted up a line, and now a line of debug information is visible (see software for further information).

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Power Supply:

 The SimmStick motherboard can be connected to either a wall transformer converted to 8-13 Vdc or connected to a battery when portable operation is desired. The nine-volt battery (shown to the right), operates the SimmStick processor, display, and sensors for about twenty hours.

When connected to the AT89C2051 programmer board, the SimmStick motherboard must be connected to a 15 Vdc wall transformer supply.

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Altitude Sensor:

The altitude sensor is derived from a Motorola MPX4115AP absolute pressure sensor. The sensor is calibrated and temperature compensated to provide linear 0 to Vref/Vsup signal over the sensed range of 0 to 16.6 PSIA (pounds per square inch, absolute). The sensor uses a "ratio-metric" output with the supply voltage and the ground as the range and zero respectively. Of note are:

• Low power consumption.
• Sensor is linear, calibrated and temperature compensated.
• Only three wires to connect. Signal, power, and ground.
• This sensor is soldered to a small VERO board in order to connect wires to the A/D SimmStick board.
• The MPX4115AP has a single pressure sense port that measures absolute barometric pressure.

Airspeed Sensor:

The airspeed sensor is like the altitude sensor with the following exceptions:

• Differential port configuration. Measures the difference between a high pressure and low pressure source (i.e. flow measurement).
• It is connected to a DB-9 for ease of connection and disconnection.
• The part number for this device is Motorola MPX6010DP and the sensor range is 0 to 1.6 PSID (pounds per square inch, differential).

Software:

The software is written in C and comprised of a main program and a few hardware interface functions. The table below defines how the digital I/O points on the AT89C2041 are wired to the respective devices. These connections define how the software accesses the respective devices via the AT89C2041 digital I/O architecture. Of the fifteen available digital I/O pins, P1-1 is neither used for input or output and can be utilized for a chip select on another device. The rest of the I/O pins are used for hardware access both on and off the DT204 board.

Currently, the MAX-186 8 channel ADC is included for future additions of other sensors in addition to the altitude and airspeed sensors.

The C files/functions (contained in the associated zip file) that acquire the A/D data, create the altitude and airspeed measurements, and then display the data on the LCD module, are contained in:

• MAINPROG.c - C main program with hardware initialization, calculations, LCD display scrolling,
• LCD.c - LCD module display functions.
• LTC1298.c - 2 channel, 12 bit A/D function.
• IASTable.c - Lookup table to convert 12 A/D info into "indicated airspeed information".
• Switch.c - Process rotary encoder switch information

Other functions are also included that handle the serial port and other utilities. The present memory map is quite full, and additional software may require the use of the AT89C4021 with 2Kbytes more memory. In any event, all of the source software that is used to program, download, and run the Altitude/Airspeed instrument application is included in the zip file.