As solar LED street lights are widely used in urban lighting system, how to save energy and improve the street light energy utilization has become a problem urgently needs to solve. Solar LED street light involves the photovoltaic battery, LED lamp, battery, and street lamp control system, can maximum efficiency use of solar energy and prolong the service life of LED lamp holder, is now an urgent need to solve the problem. ZigBee technology with its low power consumption, reliable communications and network characteristics of large capacity for the street light automatic control domain provides a more appropriate solution [1-3].
Source the ZigBee technology and JN5139 mixed signal microcontroller, starting from the basic unit of the wireless sensor network (WSN), adopt the illuminance sensor, temperature sensor, dc voltage sensor and current sensor sampling current photovoltaic battery voltage, battery voltage, current, the LED lamp holder, such as temperature and illuminance data, design with full function based on JN5139 module equipment (FFD) of a flexible, superior performance of solar LED street light sensor nodes, for building high performance wireless sensor network to do the basic work. Combines ZigBee technology of network sensor technology, solve the problems existing in the other control methods: choose brightness sensor real-time acquisition LED lamp illumination, reduce the mistake by special environment and special time by chance, get rid of the manual intervention.
1 solar LED street lamp status, the structure of the sensor node
Sensor nodes structure as shown in figure 1, mainly including sensors, signal conditioning circuit, A/D converter, microprocessor, radio frequency communication module, positioning module and power supply module, etc. Sensor module is responsible for the monitoring area information acquisition and data transfer; Processor module is responsible for the control of the whole action of sensor nodes, data storage and processing itself and data from other nodes; Wireless communication module is responsible with other sensor nodes in wireless communication, exchange of sending and receiving data and control information; Power supply module provides the energy needed for sensor nodes.
2 the function of the sensor node
Generally the ZigBee network is composed of three kinds of nodes: coordinator, routers and end devices. The coordinator is the center of the network nodes, responsible for the network of organizations and maintenance; Router is responsible for network data frame routing; The terminal equipment is implementation specific functional unit. This design for a fully functional nodes (FFD) equipment, have the effect of routing, and is responsible for the local solar LED street lamp state parameters, such as data acquisition, which can realize the following functions:
(1) sensor nodes can regularly send monitoring sub-center of solar LED street lamp status measurement data;
(2) the sensor nodes to responding to the requirements of monitoring information, the real-time collection of solar LED street light condition data;
(3) when sensor nodes detected data more than the threshold or their energy is low, send alarm messages;
(4) can be automatic storage of solar LED street lamp status data in accordance with the time, at the same time can query of solar LED street light at a certain moment condition data;
(5) the miniaturization, low power consumption, low cost, high reliability, stability and security.
3 sensor node hardware design
Sensor node is composed of a fully functional device (FFD), its structure diagram as shown in figure 1.
3.1 the microprocessor module
As a node in a ZigBee network, low power consumption design is particularly important. After a detailed device power dissipation compared, selected JN5139 mixed signal microcontroller as the core of the processor module. JN5139 is uFl antenna is integrated in the high power module, can in the shortest possible time inner implement IEEE802.15.4 under the lowest cost or ZigBee compatible systems. The table put Jennic JN5139 wireless micro controller module used to provide a complete solution of RF and RF devices. Module provides a developing wireless sensor networks need rich peripheral devices. The module features: integration of uFl antenna slot; Compatible with 2.4 GHz, IEEE802.15.4, and ZigBee agreement; 2.7 V ~ 3.6 V voltage operation; Sleep current (including sleep timer is active) 2.8 mu A; Receiving sensitivity – 100 dBm. MCU features: 16 MHz 32 bit RISC CPU; 96 KB of RAM, 192 KB ROM; 4 input port, the 12 bit ADC, 2 11 bit DAC, two comparator, two application level timer/counter, two serial ports (one for the system on-line debugging), a SPI interface, support for 5 pieces. Able to form a robust, safe and low power wireless network applications.
3.2 sensor and regulate circuit module
Battery current and voltage detection circuit design principle diagram is shown in figure 2. Electric current detection circuit composed of hall current sensor TBC10SY and sampling resistor, level adjustment circuit, follower circuit, filter circuit, etc; Voltage detection circuit is composed of sampling circuit, follower circuit, filter circuit, etc. Need to be aware of is the charging current and discharge current of the detection circuit of current in the opposite direction, need through the boost voltage circuit converts load voltage is positive, and dealt with in the program.
Photovoltaic cell current and voltage detection circuit design principle diagram as shown in figure 3 . The photovoltaic battery power series precision small resistance of the circuit of signal as a current detecting signal, USES the integrated operational amplifier ICL7650 differential amplifier circuit, so that we can minimize the impact on the circuit being measured. On a big resistance voltage divider will be folded into a photovoltaic cell to obtain small signal as a voltage signal, also adopts integrated op-amp ICL7650 make differential amplifier circuit. In order to eliminate interference, using two equivalent resistance respectively in two amplifier input and in between, at the same time the amplifier output increase filter circuit, after filtering, the current and voltage signal output to the A/D conversion interface controller JN5139.
LED lamp illumination detection circuit as shown in figure 4. Intensity of illumination detection using On9658 integrated sensor, sensor to obtain the signal after amplifier amplification and filtering the output to the controller after the A/D conversion interface JN5139.
LED lamp holder temperature detection circuit as shown in figure 5. Battery temperature adopts the integrated temperature sensor SHT11.
4 sensor node software design
4.1 the overall design of the software system
The main functions of the software system includes the sensor data acquisition and processing, wireless transceiver and the node positioning, using modular design. Sensor data acquisition and processing module mainly set the battery status signal sampling parameters and control; Wireless transceiver module by setting registers, control of command or a data receiving and sending; Node positioning module for real-time positioning nodes. Sensor node design for a fully functional devices (FFD), has a routing functions at the same time, the program flow chart is shown in figure 6. In the task are added into the task queue for data acquisition, alarm, detection and energy detection and call the ZigBee send task; Produce JN5139 pin is interrupted, the CPU turns to execute the ZigBee receiving interrupt service routine. If collect command is executed immediately data acquisition and sending; Immediate execution, if it is a routing packet routing updates.
4.2 design of node localization algorithm 
Node indicator based on the received signal strength positioning algorithm to realize accurate positioning. Known to launch the node’s signal strength, the receiving node according to the strength of the received signal to calculate the signal propagation loss, then according to the signal propagation model formula will propagation loss into distance, reuse the trilateral measurement method to calculate the unknown node location. In actual positioning, to ensure that the unknown node is in three or more emission signal intensity and position coordinates of known reference node of communication within the scope of unknown node according to the received signal strength to calculate the signal propagation loss, and then calculate the node position.
Based on wireless sensor network are introduced in this state of high precision solar LED street lamp design of sensor nodes, in the actual testing process, the system runs stably, the measured results accord with actual, completely achieved the collection and wireless transmission of signal with high precision, good monitoring effect is obtained. The system combined with wireless sensor network with low power consumption, low cost and node many advantages, such as in remote wireless communication technology, high reliability, the application will be more and more widely in the process of problem solving.