| 摘要 |
PURPOSE:To improve reliability in detecting insertion of a foreign matter by securing pressure sensitive sensitivity of a pressure sensitive sensor sufficiently even in a low temperature condition, and making pressurization sensitivity of a pressure detecting circuit almost constant. CONSTITUTION:A feedback resistor 11 is connected between input and output terminals of a reversing amplifier 12, and a pressure sensitive sensor 13 whose resistance value Rs changes according to pressurizing force is connected between an input terminal of the amplifier 12 and negative voltage (-Vc). An NPN transistor 30 is connected in parallel to the pressure sensitive sensor 13. Output voltage Eo is outputted from an output terminal of the amplifier 12, and the output terminal is connected to a (-) input terminal of a comparator 14. A reference voltage source 16 is connected to a (+) input terminal of the comparator 14, and the reference voltage Er is compared with output voltage, and a compared result is supplied to a base of the transistor 30 through a resistor 17. The resistor 17 and an earthing capacitor 18 constitute a primary low pass filter 19. An MOS type or joining type N or P channel electric field effect transistor or a PNP transistor is used instead of the NPN transistor 30. |
