Here is another update on my "Wearable Sudden Impact and Health Monitor" project.
In this update I will show how I added a measurement feature for body temperature.
The body temperature is measured using the Analog Devices ADT7320, which is very small (4mm x 4mm) and it is mounted on the front inside the helmet, so that it touches the forehead. To ensure good contact with the skin, the ADT7320 sensor is placed on a layer of elastic sponge-type material. This implementation can be seen in the figure below:
The ADT7320 temperature sensor is controlled by an Arduino microcontroller through the SPI data bus, which is shared with ADXL375 and ADXL362 sensors as it is shown in the schematic diagram of the main unit, as I am showing in the following picture.
Because Arduino operates at 5V and ADXL375 at 3.3V the interface needs level shifters. I have purchased two BD-LLC inexpensive level shifters, but since the package delivery has been delayed and I didn’t have the patience to wait, I have implemented a temporary “work-around” by adding 1kOhms series resistors on the SPI bus. The current flowing from 5V output pins of Arduino module and into the ADXL375 tends to increase the voltage up to the point where the ADXL375 input power clamp diodes start to conduct current, at about 0.6V above 3.3V supply. This voltage is acceptable for the gate oxide break-down of input MOS transistors, but the current flowing through the internal clamp diodes may damage the ADXL375 chip. The 1kOhm series resistor limits the current flowing from 5V supply into the ADXL375 pin to about (5V – 3.9V) /1kOhm = 1.1mA, which is handled without any problem by the internal clamp diodes. Well, this is the temporary work-around, and I will replace the resistors with the level shifters after I receive the order.
That is it for now; I will come back with new updates as I get more work done.