PURPOSE: To regulate intensity of sound by connecting a sounder in series with the 1st and the 2nd transistor, to control electric current running in the sounder, when a sounder is used for raising alarms.

CONSTITUTION: One terminal of a speaker 1 is connected with the collector of a pnp-transistor Tr3 while the other terminal is connected with the collector of an npn-Tr5. The emitter of the Tr3 and that of the Tr5 are connected respectively with the positive side and the negative one of a power source VD. When a time signal is to be given, for instance, two oscillators 7, 9 with frequencies 440Hz, 880Hz are provided, their outputs are connected with an electronic selective switch 11, and its output is connected with the base of the Tr3. On the other hand, the base of the Tr5 is connected with a computer 19 through a resistor 13, a diode 17 and an output L is sent at a time t shown in the figure from the terminals aWc of the computer 19.