Call signal generating circuit

A call signal generating circuit comprising:

• a step-up transformer having a primary winding and secondary windings;
• first switching means, coupled between the primary winding of said step-up transformer and a D.C. power source, for turning ON/OFF at a frequency higher than a frequency of a call signal which is to be generated by the call signal generating circuit;
• rectifying means, coupled to the secondary windings of said step-up transformer, for rectifying voltages induced at the secondary windings into a positive polarity voltage and a negative polarity voltage;
• second switching means, coupled to said rectifying means and responsive to the positive and negative polarity voltages, for alternately outputting a positive polarity square wave voltage and a negative polarity square wave voltage with a quiescent time in which both the positive and negative polarity square wave voltages are not output;
• a capacitor, having first and second terminals coupled to said second switching means, for receiving the positive and negative polarity square wave voltages output from said second switching means;
• a resister coupled to the first terminal of said capacitor;
• third switching means, coupled to said resistor, for discharging said capacitor during the quiescent time, said resistor and said third switching means being coupled in series to form a series circuit which is coupled in parallel to the first and second terminals of said capacitor;
• and
• a pair of output terminals, respectively coupled to the first and second terminals of said capacitor, for outputting the call signal.