| 摘要 |
PURPOSE:To avoid a transistor (TR) from being malfunctioned because of noise signals, by applying a voltage with opposite polarity to the power supply voltage to a base circuit of the TR in a decoder circuit. CONSTITUTION:An oscillating output signal of an astable multivibrator 22 is applied to a wave shape circuit 41 of a circuit 35 to produce a voltage of opposite polarity and to a switching circuit 31. An output signal shaped at a wave shape circuit 41 is applied to an inverse voltage rectifying diode 42, where the signal is rectified and smoothed at a smoothing capacitor 43. The voltage of opposite polarity thus produced is applied to the bases of TRs 17, 20... as switching elements via resistors 44, 45.... When a tone signal is inputted to an input terminal 2, the switching circuit 31 passes through an output of the astable multivibrator 22 and a callout tone is produced from a speaker 33. |
