【正文】 se voltage, the auxiliary control circuit can receive external signals and transmit them to CPU after conversion, or convert the output signals from CPU to control the external ponents. The specific microprocessor control chip MAX813L produced by MAXIM Corporation is used as the core of watch dog circuit, which can reset system while power is on or there is something wrong with the system, advancing its credibility and veracity. Hardware Structure Diagram(1) Circuit of DS1820 DS1820 digital temperature sensor uses onboard patent technology and all of the ponents such as the temperature sensor and conversion circuit are integrated and encapsulated like a triode, with a measuring range of 55 to +125℃ in ℃ increments. Temperature is represented in the DS1820 in terms of a ℃ LSB, with MSB as the sign bit. The measuring result will be transmitted to port of 89S51 serially over the 1–wire interface through a 120Ω resistor, with a 2KΩ pullup resistor connecting to it. shows how the three pins of DS1820 are connected. The memory of DS1820 consists of a scratchpad RAM and a nonvolatile, electrically erasable (E2) RAM, which stores the high and low temperature triggers TH and TL. Data is first written to the scratchpad where it can be read back. After the data has been verified, a copy scratchpad mand will transfer the data to the E2RAM. The scratchpad is posed of eight bytes memory information with the third byte TH and the fourth one TL. (2) Voltage stabilization module The double 7805 method is shown in 。 power of the system is supplied by a UA7805C and a KA78L05Z encapsulated like triodes is applied to supply power for external sensors such as the temperature sensor DS1820 and precision adjustable vibration sensor. The voltage of cars’ battery is not stable, for example, while the engine is start up, the voltage is about 10V, 2V less than that of usual, and the output voltage from 7805 will then change a lot correspondingly. In order to avoid the sharp change at the output port of 7805, it is necessary to use LC filter circuit, including a large polarity capacitor of 1000uF and an inductor。 a capacitor is also used to avoid high frequency interference. The voltage output ports of 7805 connect to the ground through some 10uF capacitors to keep the voltage more stable. The ctrl1 and ctrl2 signals connecting to HJR410212V relay are used to realize the protecting ability of negative burst by the door。 if the door is opened, the power input port will be disconnected to the diode, so power supply of the system is cut off, unless the door is shut up again. Voltage stabilization module(3) Control circuit of running lamp While the engine is running to keep its temperature higher than TL, the running lamp will be on and off alternately. The running lamp control circuit diagram is shown in . When the engine is running, the CONTROL port will be set to high level。 the 555 integrated timer will be electrified and begin to work. At first, C1 is charged through HR2, HR3 and D12, and when the voltage of C1 is higher than 2/3Vcc, Q port will be set to high level, making the external relay electrify and the running lamp be on. At the same time, the internal discharging transistor is on。 capacitor C1 will discharge through D1 and HR4. When the voltage value of C1 declines to lower than 1/3Vcc, Q port will be reset and the running lamp will be off. The internal discharging transistor is cut off and capacitor C1 will be charged again. Repeating like this, a rectangular wave is obtained at port Q, and the running lamp will be on and off alt