Abstract: The ultrasonic wave generator of the present invention always drives a transducer with the resonant frequency f.sub.o of the transducer. Although the resonant frequency f.sub.o changes due to the temperature change etc., the driving frequency automatically follows the change of the resonant frequency f.sub.o. The control of the driving frequency is performed by means of a feed-back loop including a voltage controlled oscillator and means for applying a control signal to said voltage controlled oscillator according to the amplitude and/or phase of the driving signal of the transducer.

Abstract: A conductive protection board for protecting an aperture board from dust is mounted in an ink mist type high speed printer. The ink mist type printer operates on the principle that an ion stream modulated by an aperture board according to the pattern of the character to be printed, charges the ink mist, which is then attracted by an electric field to the surface of the paper. If an aperture of said aperture board is obstructed, a portion of the printed character will not be evident. In order to avoid the obstruction of an aperture the conductive protection board which has a plurality of apertures, each of which corresponds to each aperture of the aperture board, is placed very close to said aperture board.

Abstract: A direct current supply circuit for supplying a direct current to a communication terminal device as well as for repeating information from said communication terminal device to an electronic exchange has been disclosed. Said direct current supply circuit comprises a transformer, which has four primary windings. By arranging the direction of the direct current in said primary windings, saturation of magnetic flux in said transformer can be prevented and thus, the size of the transformer can be reduced.

Abstract: An exposure indicating means comprising a plurality of light emitting elements which are selectively energized to indicate exposure information in a digital form is connected with an exposure measuring circuit. The light emitting elements are connected with a warning circuit. The warning circuit comprises a warning signal generator which generates a warning signal when a warning is required, a square wave generator which constantly generates a square wave and an AND circuit which transmits the square wave to the light emitting elements when the warning signal is transmitted thereto. The light emitting elements are intermittently turned off by the square wave coming from the AND circuit. The frequency of the square wave is selected so that the intermittent turning off of the light emitting elements is sensed by the photographer viewing through the viewfinder of the camera.

Abstract: Light emitting elements such as light emitting diodes provided in the viewfinder of a camera are connected with a light measuring circuit including a photodetector through an A-D converter, a decoder and a gate circuit so that the light emitting elements may indicate exposure information in a digital form in accordance with the output of the light measuring circuit. An oscillator is connected with the A-D converter and the gate circuit for giving sampling pulses to the A-D converter and clock pulses to the gate circuit. Between the oscillator and the gate circuit is provided a duty modulator to change the duty cycle of the square wave transmitted from the oscillator to the gate circuit. The duty modulator is connected with the light measuring circuit or the A-D converter so as to control the duty cycle of the square wave in accordance with the output from the light measuring circuit.

Abstract: An accurate, simple and compact analog-to-digital converter has been found. The converter comprisesA voltage divider circuit including a plurality of stepped voltage dividing points separated by a plurality of resistors for dividing the input voltage received from the analog input terminal;A group of switches for receiving said divided voltages; andA switching decision circuit for determining the amplitude of the voltage from a decision point through said switch group, the output of said decision circuit being applied to a logic circuit to provide digital signal output.