Low Cost Air Pressure Sensor

For our experiments we tried to use a low cost air pressure sensor US9116 (UniSense) taken form personal blood pressure meter. It measures the air pressure in the connected pipe as a difference to atmospheric pressure. It is connected as a standard tensometer bridge, so it will need a power supply and an instrumentation amplifier connected to its output. The output voltage around 43.5 mV will be produced by the air pressure 40 kPa.

The low cost air pressure sensor

There was a difference found between datasheet and real sensor, pin 5 is not unconnected, but must be connected to power supply together with pin 3. It feeds one of the bridge branches.

The disadvantage is in very subtle construction, so you cannot expect precise measurement. Also, the tensometers are based on semiconductors, which brings better sensitivity, but error performance is very poor. For now, we will use it while we must look for better sensor.

RS485 Implementation

Standard communication library for the UART (RS232 or virtual COM port) PC connection was modded for the RS485 support. The difference is in electrical layer, where the line is half-duplex. The transmitter must be switched on an off when transmitting the message. Because I use an old library for UART communication of AVR microcontrollers using circular buffer, which is writen in different style than my, it was a tricky task to make all working well :-)

The example of message transmitted over RS485

Now, we have a fast and reliable communication with PC over thin wire pair. The USB/RS485 interface must be used on the PC side. This solution allows us to use longer communication distances (in comparison to USB) and possibility to connect another units on the same line. We will not use any wireless connection, because we need a power cables too.

The PC side USB/RS485 interface with the FTDI chip

The serial terminal receiving data messages

We are ready to communicate now :-)

Microcontroller Unit

For all our experiments, a simple bread-boarded microcontroller unit was built. It contains the ATmega32 16 MHz AVR RISC microcontroller, which have enough Flash and RAM for our tasks. Also its internal peripherals are suitable and I/O pin count is good. The board also contains voltage regulators (+5V for unit, +6V regulated for air compressor and original valve), 4 N-FET switches, 3 LEDs (red, yellow, green for standard indication), 2 programmable buttons and RESET button, ISP programming interface, UART RS485 interface (for communication with PC over a longer distance), 4 DIP switches for operation mode (application) selection and instrumentation amplifier for air pressure sensor. There are also reserved pin groups for I2C bus connected sensors and for the standard 2 x 16 characters LCD display.

Bread-boarded microcontrolelr unit

The metal shield in the right bottom corner contains the instrumentation amplifier, which is very EMC sensitive. The pressure sensor is placed on the bottom side of the board, where access will be better to it. All mechanical parts will be placed on second stage under this board.

In the first experiments, the unit will remain on bread-board. The PCB will be made after the configuration will be verified in praxis. Several changes will be made in the actual form before.

U-Tube Manometer

Because of an electronic sensor output is influenced by many factors, we need some precise instrument to calibrate it. My friend added an experienced guess, that we need about 3 kPa to measure on the vinyl inflatable toys. So we built a U-tube manometer, which will give us value of differential pressure, influenced only by the density of water and the gravitational acceleration.

http://en.wikipedia.org/wiki/Pressure_measurement

For building the U-tube manometer we use a piece of hardboard, hard plastic pipe for use in electro-pneumatic mechanisms and few cable holders. It will be good to add some ruler in the middle for easier results reading. On the back side there is an iron bracket which holds the instrument in the correct and stable position when attached to the table board.

The simple U-tube manometer

The instrument was verified and works very well. The maximal difference in water columns is up to 500 mm.