Hardware development

Project description by Sinuss

The ultra thin wiring causes limitations as regards connection possibilities, such as data signal loss and a limited maximum power supply.

The sensors are connected to the wiring via a 12V boost converter. Installing a step-down converter near the sensor means a power supply can be created of 3.3V and approximately 80mA. This is sufficient to power the sensor and a low power processor.

Data from the sensor is read out by a low power processor: the Attiny86l. When operating this consumes 750uA at 3.3V and 1 MHz. The sensor data can be read out via, for example, SPI or I2C. The way the data is processed depends on the sensor.

The sensors are connected individually to the data ports of the main circuit board. This provides the 12V power supply. The data is increased to 5V via level shifters in order to compensate the signal loss in the cable. The two data wires are used as follows:

  • one wire is used as clock signal of the master circuit board;
  • the other wire is used as a bidirectional data bus.

Using an individual master clock for each sensor means that the data bus can vary in clock speed. As a result, other processes can run simultaneously on the main circuit board. The main circuit board starts the clock with start date, as a result of which the slave sensor replies with a sensor ID and the relevant data. The sensor ID enables each sensor to function independently of each bus.

The basis of the main circuit board is an ATMEGA2560 processor. The main circuit board periodically reads out the data of each sensor, processes the data and sends it as a single package via a wireless connection (433 MHz, 100mW). Because this is data hardware specific, the data cannot be used by third parties. Despite this an encryption is still generated via a key. The data is collected by a receiver with a specified range of 1 to 1.5 km in open field. It is processed in the receiver circuit board and sent to a server via a GPRS-SSL connection with JSON protocol. Such posting can take place approximately every 10 seconds. The posts are each provided with a timestamp with a resolution of the single second. This means that a single post from a single sensor can contain several measurements.

The portable main unit is powered using a LiPo battery. If the circuit board is connected to a 9-50VDC power supply, it will be charged fully automatically. The battery status is included in the data sent to the receiver circuit board. The main circuit board is fitted with a GPS/GNNS receiver. This data is also forwarded periodically. The sensors used are:

  • air quality sensor
  • outside temperature sensor
  • GPS
  • body temperature sensor (still has to be researched)
  • stress sensor (still has to be researched)
  • heart rate sensor (rejected: it is not wearable)
  • alternative: motion sensor?