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electronic variometer: Developing and building a electronic variometer is a very interesting task - especially for a glider pilot who studies electronics. 

Based on the experience with the altimeter I developed a electronic variometer, which is described on this page. It wasn't possible to expand the hardware of the altimeter for a variometer. The design of the e-variometer "MR1" took this into consideration  and got enough performance for implementing even more than just a variometer.

With the hardware of the E-Vario it is possible to implement the following functions:

        1.) Variometer

        2.) Integrator

        3.) Speed command

        4.) Altimeter

        5.) Speed indicator

        6.) Final glide path calculator

        7.) GPS interface

        8.) Thermometer

        9.) Board voltage control

The picture shows the components of the variometer. Two Motorola pressure sensors are used and their analog signal is converted to digital. The analog to digital converter is a 24 bit type from Analog Devices   (AD771A). This high resolution is required, because both signals, height and speed, have to be differentiated. For the speed sensor 24 bits are not really needed, but it makes sense to use the same ADC for both pressure sensors.

A 10 bit ADC is not enough for the differential pressure sensor! I had to do this experience, then I tried to use the PIC-internal ADC for converting the differential pressure sensor signal. The one who knows about the Nyquest theorem will quickly see the reasons for this. I didn't spend the effort to calculate the needed number of bits and just used the same high resolution ADC like the one for the altitude.

The temperature measurement is done with a self calibrating technique like it is described on a Microchip application note. High resolution, robustness against capacitor changes and low cost are the advantages of this circuit. It consists just of one thermistor, two resistors and one capacitor.

The heard of the electronic variometer is a PIC17C756. This microcontroller contains a 16 Kbytes EPROM program memory, which is more than enough for implementing the needed functionality. 

There are several output devices. The LCD-display is a 4 x 16 character standard alphanumerical display. Because this low cost display features user defined characters, it is possible to program a graphical bar needle. The source code shows how to implement this. The result is a good to read digital needle. Furthermore a mechanical instrument can be connected via the 8 bit digital to analog converter. The used MAX517 is connected to a I2C bus. This bus makes it possible, to control several devices this just 2 wires. The MR1-variometer contains four  I2C  ICs (one EEPROM and three DACs). The second DAC is controlling the LCD-display contrast, which can be changed comfortably via the MR1 firmware.

Of cause there is a acoustic output as well. Especially for this a sound unit has been developed. This unit contains a PIC16F876 and and a digital sinus generator IC (ML2036). It receives via RS232 the climb value and whether the variometer or speed command mode has been set. Furthermore this module understands instructions, for setting the volume and several initialization parameters. For development purpose a Windows test program has been written.   The screen shot can be seen on the right hand side. The program can trigger all commands of the sound unit. Every thing else can bee seen by going through the firmware. The Eagle file shows the hardware.  

The sound unit works fine, but presents a quite costly solution. It may be better to connect the sinus generator chip directly to the PIC17C756. A interrupt controlled program then will control  the sound.

The human interface is a keypad and a variometer/speed command mode/auto switcher. It is connected conventionally. For details refer the schematic. 

For sensor calibration coefficients and user data a EEPROM is connected via I2C. All measurement results as well as the calculation of the sensor coefficients are summarized in a Excel file. If you want to start to develop a e-vario, then think of, that you will have to measure and calibrate your hardware! Don't underestimate the effort, it takes to build up the measuring devices!  

A battery control is not really necessary, but easy to implement. Therefore just two resistors and some program lines firmware are needed. It's a little effort and nice to have the knowledge of the battery voltage .

Because the detailed schematics are to big for showing on a web page, you should download the light version of Eagle from the Cadsoft homepage. With this program you can open the schematics and PCBs for watching and changing it for your own, non commercial, needs. 

 

Eagle is a very powerful, easy to use, PCB program. After two hours of "learning by clicking" you will be able to enter schematics, define new devices and design PCBs. 

 

 

 

 

The test set up is done bye two PCBs. One contains the analog and the other one the digital part of the circuit. Both PCBs are connected via a 24 poles connector.

On the digital PCB a pin grid is placed. Test circuits can be soldered there. This pin grid as well as the footprint for the LCD display has been placed outside of the "light" area restrictions of Eagle light. This is possible, by using a little trick: Design the footprint of the device this way, that after you placed it into the PCB design, the origin will stay inside the valid area, but the structures will be outside, there you want them.

The hardware result can be seen on the picture. A aluminum box shields the variometer from out side and the environment from the inside. 

Equipped with pressure connections and a Western connector for the electrical connection, the hardware is ready for test flights. 

Much more than the hardware effort has to be spend into developing the MR1 firmware. The "C" source code, which has been developed by me, is opened and and shown here. It contains all hardware control, measure functions and test programs. It is already possible, to test the variometer in the air.  

 

But even before the Firmware development can start, one problem has to be solved: "How to program the PIC17C756 with a PICSTART PLUS?" This has been discussed a lot within newsgroups, but I couldn't find a building instruction anywhere. Together with a friend from California, it was possible to develop a programming adapter (thanks again Darek!). You can find the building instruction on my web page.

Originally it has been planed, to develop this variometer until the state of production  Unfortunately I did become very busy at the end of my study. Now during my PhD the situation hasn't changed and so I did decide, to publish my development. You can find all circuits, firmware, programs and documents, which I developed for the variometer, on this web page. Developing the variometer meant  a lot fun to me and I hope to motivate you, to start with your own development. But I want to warn people, which do think, they could save money by building a e-variometer. If you belong to these people, I can just say "forget it". To develop a reliable, robust and really good working variometer needs a lot of effort to spend, much more, than to work and buy one for money (except you are a shoe shop assistant ;-).  

If you want to build the MR1 electronic variometer, then I will help you with advice. But because of my spare time, I can't provide you more code or circuitry. You can use all the files for your own needs. If you want to publishing it on a other place, then you have to refer the source. It is not allowed to use this development, or part of it, for commercial purposes! If you want to do so, then contact me via Email. For this case I will be willing to carry on with the work (for money of cause).

 

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