| 摘要 |
PROBLEM TO BE SOLVED: To simplify the circuit constitution and to relax the fitting precision of an infrared pyroelectric sensor by detecting temperature variation accompanying the starting operation of a precedent vehicle by the sensor. SOLUTION: A Fresnel lens 4 restricts a direction where infrared rays can be made incident on the pyroelectric sensor 3 from the front by the lens shape which spreads laterally and an installation angle and also adjusts an incidence state of the infrared rays so that a detection area is sectional in plan view and multi-layered facing obliquely down to the front by specific angles in side elevation view. The infrared pyroelectric sensor 3 detects temperature variation of exhaust gas, tail lamps or the like, accompanying the starting operation of a precedent vehicle 7 from variation in the incidence quantity of the infrared rays, converts the temperature variation into an electric signal, and outputs it to an ECU 6 through a filer circuit 8. When the output value from the infrared pyroelectric sensor 3 is equal to or less than 1 Hz and varies above a specific threshold, it is judged that the precedent vehicle 7 starts and a notification buzzer 5 is made to sound.
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