|Pressure sensor||BME280||I²C||1 hpa||Pressure Sensor BMP280|
|Differential pressure sensor||SDP3x-Digital||I²C||0.1Pa||Datasheet SDP3x Digital|
The main pressure sensor used is a Bosch BME280.
Secondary pressure sensor, used for redundancy, is an Xtrinsic MPL3115A2.
Both sensors are sourcable in large quantities.
No sourcing difficulties are expected with these components.
The datasheet specifies an absolute presssure accuracy of 1hPa (= 1cmH20) and a relative accuracy of 0.12hPa.
The sensor is used in 1X oversampling setting (resolution +-3Pa) at a sampling rate of 100Hz.
For both types of pressure sensors the Adafruit libraries are used.
The system uses 3 pressure sensors:
-2 in the sensor module near the patient. (one BME280 + one MPL31115A2)
These measure pressure delivered to the patient at 100Hz.
These sensors are read over I2C.
-1 BME280 sensor on the motherboard, to reference the P Peak against atmospheric pressure, when the machine operates over longer duration without needing to reset the machine.
This sensor is read over SPI.
Flow sensor is a differential pressure sensor, SDP31 from Sensirion
Availability with short lead times has become questionable since mid April
We are investigating alternatives.
This sensor is read over I2C.
SENSOR BOARD V2
As all sensors in the module near the patient use I2C, they can be implemented on a single sensor board, and connected through the machine with a single 4-wire cable.
A single micro-USB cable is being used for this purpose.
SENSOR HOUSING V2
PRESSURE SENSOR BOARD V1
This board has 2 mistakes on it – do not use this version.
The pressure sensor consists of 2 redundant MEMS sensor devices, respectively a BME280 and a MPL3115A2. Both are wired to the same I2C bus with 3.3V signaling levels, which are then level shifted using a discrete MOSFET level shifter to 5V signaling levels.
The USB connector is a USB micro type specifically chosen for its ability to be made airtight using a rubber seal: https://www.digikey.com/product-detail/en/cui-devices/UJ2W-MIBH-4-SMT-TR/102-4009-1-ND/6187931
Both pressure sensors run at 3.3V, which is provided on-board by a linear regulator of type MIC5504-3.3.
FLOW SENSOR BOARD V1
SENSOR MODULE HOUSING V1
The rotary encoder has its own PCB and consists of a Hall sensor of type AS5601 and an I2C buffer of type P82B715D which acts as a line driver.
The connector is a standard USB mini connector: https://www.digikey.com/product-detail/en/amphenol-icc-fci/10118193-0001LF/609-4616-1-ND/2785380
Communication interface is exclusively through the I2C bus. The read address of the AS5601 is 0x36.
Power supply of the board is strictly 4.5V – 5.5V DC. When power is applied correctly, a green LED lights up.