Mobile Incoming Call Indicator (1)

The circuit is a variable duty cycle, constant frequency, astable multivibrator using the 555 timer. Diodes Dchg and Ddchg separate the charge and discharge paths respectively. They also create a practically symmetrical voltage loss along the two paths. Potentiometer Rdcadj adjusts the duty cycle with slight effect on frequency. When the potentiometer is adjusted the resistance subtracted from one path is added to the other path. Thus the mark and space time of the output can be varied while maintaining their sum almost constant. This results in variable duty cycle and constant frequency output signal.