How the circuit works.....
The voltage at the output of this subsystem is 0 volts until
the switch has been pressed. When the switch has been operated, it is
said to have been made and the output voltage immediately rises to +9 volts. The
switch shown is a push to make type. The resistor in series with it is known as a pull-down
resistor. It is there to "pull" the output of the subsystem down
to 0 volts when the switch has not been pressed. This is essential if
you connect the subsystem to digital inputs of "logic" circuits or PIC's, as they need to see a high (+) or a low 0 volt (-) signal.
This module is sourcing the next subsystem
as it is providing a positive voltage. The arrow head on the module
indicates this positive voltage and its tail a negative voltage. In this example, when
you connect the buzzer, you need to take its other contact back to
the negative side of the power supply, as shown. By doing this you are
making a complete circuit.