the internal capacitance, low level in signal in the energy consumed during the capacitance on working until high-level coming again to parasitic power (battery). Parasitic power mode has three advantages: 1) distance measuring temperature, without the local power supply. 2) no conventional power in the condition reads the ROM. 3) circuit, with only one more concise root I/O realize temperature measurement. Want to make precise temperature conversion chip DS18B20, I/O line must ensure that the temperature conversion period, due to provide enough energy conversion in temperature during each DS18B20, when the current 1mA to work a few temperature sensor in the same root hanging on the I/O multi-point temperature measurement, only by 4.7 K and resistance will not be able to provide enough energy, which cannot be switchover temperature or errors.
The external power source supply way: in the external power supply modes, DS18B20 work power by VDD pin, I/O access line does not need strong pull up, there is no shortage of electricity power, can ensure accuracy and conversion in the bus theory can be articulated multiple sensor DS18B20, multipoint temperature measuring system.
This system USES the external power source. Connection method is one foot grounding and DS18B20 2 feet (DQ) and AT89S52 foot an I/O port, 3 feet line up + 5V. A89S52 in the I/O port and + 5V connection between a 4.7 K pull-up resistors, to ensure the normal operation of the data collection. If you want to test system, composed multi-point temperature in the same root chip I/O port in the same line, and the method of connecting the parallel more pieces of DS18B20 chip.
（3）LCD1602 LCD
1602 LCD module can and monolithic integrated circuit AT89S52 directly interface.
（4）Buzzer driver circuit
Due to the working current buzzer is compared commonly big, so I/O microcontroller is not directly driven by amplifying circuit, so it is generally used to drive, to enlarge current transistor. When the temperature is below six degrees Celsius, alarm.
（5）Fan circuit
When the temperature is higher than 80 ° c, start fan circuit. Because the job is great, so use current amplifier circuit to drive, to enlarge current transistor. When the temperature is above 80 ° c, give a command, P2.6 single-chip microcontroller foot output level, triode conduction, fans, electric circuit, which began to turn it down.
4、The design of the software system
This system USES AT89S52 devices as the core, with DSl8B20 after the collected data, and the temperature inside the data storage AT89S52 deposit, LCD display, and compared with the prior value, and then the output signal by MCU control circuit and alarm circuit fan. Multi-functional temperature testing system