| 摘要 |
PURPOSE:To decrease the variation in the distance between a transmitter and a receiver when an alarm is raised by providing a delay time until an alarm is raised after an input level reaches a prescribedlevel and changing the time depending on the inputlevel of a signal radio wave. CONSTITUTION:An undesired signal is eliminated from a signal radio wave received by an antenna 11 by a filter 14 via a high frequency amplifier 12 and a detection circuit 13 and the result is supplied to a conversion circuit 15. A DC voltage proportional to the intensity of electric field is obtained by the circuit 15 and the result is fed to a discharge circuit 16. The circuit 16 is constituted that its discharge time constant is changed by the DC voltage and the charging time constant is selected sufficiently larger than the discharge time constant. When the capacitor 18 is charged up to a prescribed voltage, a comparator circuit 19 detects it and inverts the output voltage thereby causing a buzzer drive circuit 20 to be in operation. Thus, in varying the delay time until the terminal voltage of the capacitor 18 reaches a comparison level raising an alarm depending on the input level of the signal radio wave as shown as tl,t2 in figure (or tl', t2'), the variation of the distance between the transmitter and the receiver is reduced.
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