Some time ago I had a problem with availability of the accelerometers for my inertial module. One of key features of my on-board computer is modular construction, allowing to be independent from the sensor availability. If one module is not available, another can be used instead. Problem was, because I had not another inertial module, but had newly acquired ITG-3200 gyroscope and new, not tested yet magnetometer MMC3140.
I decided to build another inertial module based on cheap sensors with digital I2C interface. Added to it LIS302DLH accelerometer. It could be enough, but sensors occupied only small part of area on my standard module, so decided to add some more sensors, all on I2C bus. First was added pressure sensor BMP085 and for comparision with it HP03S and MS5803. Also added accelerometer BMA180. The board start to be full. Most of the sensors are new to me, so always is possibility to make an error on PCB, so this module since begining is experimental. In this version will not be manufactured in volumes higher then initial 10 pcs. Next versions will be reduced to necessary amount of sensors.
Pneumatic module in version 2.0 contain set of sensors for measurement parameters of the air in flight like: pressure, altitude, speed, vertical speed (variometer). Additionally for optimal filling 8 channels of ADC module has 2 inputs for thermocouple and 2 channels for user sensors working in range 0-4V.
Communication with module is done via SPI bus, by reading data from 16-bit A/D converter. Module need 3,3V supply for digital interface and 5,0V for sensors.
From software point of view actual version of firmware measure all parameters, make temperature calibration and individual calibration offset and gain for every sensor.
I mounted first 25 pcs of rotating speed sensors. It has 2 IR LEDs lightening the rotating surface and photoelement connected with operational amplifier working in comparator circuit. The op-amp compare actual light level with moving average. In that way it can adapt itself to wide range of lighting conditions.
Sensor can works with own source of the light, for example lightning black and white sticker on electric outruner engine or with existing ambient light looking on it through propeler. In case of work with own reflected light the distance between sensor and rotating element should be not bigger then 5cm. In factory settings all optical components are set paralelly to each other. The LEDs emit a beam with an angle of aperture 50°. Photo element collects th light from the angle 24°
ADCIO module (means ADC + IO) consist 8 input 16-bit Analog to Digital Converter and 8 digital IO. First 4 digital pins configured as output can drive 4 MOSFET transistors and switch 4A constant current in Open Drain configuration. Next 4 pins can works as universal inputs or outputs in 3.3V CMOS standard.
Inputs of the analog to digital converter are connected to 4 conectors. Every connector have 5V supply, ground and 2 inputs working in range 0-4V.
GPS external module is a source of information about latitude, speed above ground and alternative source of information about altitude and course. Module was made as external to avoid interferences from other electronics. Module can be mounted far from computer outside fuselage. To protect electronics from environmental factors the PCB was resized to fit into popular Z-47U enclosure. Of course module can works without a box.
Module consist 10Hz, 66-chanel GPS receiver with internalantena based on MTK chipset. Depending mounting place: inside or outside fuselage, the LED can be mounted in SMD or THT version. Also backup capacitor can be mounted on the top or bottom side of PCB.