3.1.3. Sensor node operation
The algorithm of the wireless sensor node operation is presented in Fig. 4. It is implemented in C language in Integrated Development Environment (IDE) “Keil Vision”. The sensor node starts its operation with the initialization of MCU and wireless transceiver. Then sensor node periodically measures the gas concentration in the environment. To do so, the sensors have to be heated up to a predefined temperature (450◦C for methane). The heating signal is generated by MCU DD4 and operational amplifier DD2 (see Section 3.1.1). During the sensing mode the voltage on the sensor sensitive layer, also known as ‘sensor response’, is measured by a built-in ADC of MCU. The measured value is processed and compared with a preset threshold specified in the memory of the sensor node. If the measured methane concentration exceeds 1% vol. threshold, the node checks the status of the valve stored in its memory and if the valve is opened the node activates ZigBee transceiver and sends a request to wireless actuator to close the gas valve. When the valve is closed and the acknowledgement is received by the sensor node it goes to Sleep mode. In the alternative situation, i.e. the concentration threshold is not exceeded and gas valve has been already closed, the sensor node goes in Sleep mode as well.
It is worth noting that the algorithm presented in Fig. 4 can be easily extended: upon detection of dangerous concentration of gas in the environment the sensor node can also notify a WSN operator by sending an alert message as it is shown in the algorithm. This situation, however, is out of scope of this work.
As specified in Section 4.1, we conduct the experiments on methane detection. For methane, the lower limit of its explosion is5% vol. [6]. The dangerous situation occurs when methane
concentration is up to 1% vol. and the system closes the gas valve. When the concentration is equal or below 0.15% vol. an extra measurement is typically conducted [17]. This is done in order to monitor the status of environment and capture in time a potentially dangerous situation