| 摘要 |
PURPOSE:To cnsiderably reduce the influence of noise, to easily change sensing range, and to simplify the constitution of a device by providing a preamplifier on a transmitter and receiver side by utilizing a level switch and a capacitor. CONSTITUTION:An amplifier amplifies and outputs a received signal from an oscillator to a connection part for an external device. Then, a level switch prevents the received signal from being applied directly to the connection part and also prevents the output signal of the amplifier applied to the oscillator. Further, a rectifying and smoothing circuit rectifies and smooths part of a transmitted signal inputted from the external device to the connection part to power up the amplifier. The level switch 14 consists of, for example, two diodes 21 and 22 which turn on when a signal exceeding an about 0.7 threshold level is applied, and level switches 15 and 16 consist of diodes 23 and 24, and 25 and 26 of the same kind respectively. Further, the rectifying and smoothing circuit consists of a capacitor 17, a diode 18 and a resistance 19 and is used for power of the preamplifier 13. |
