Page 1129 - Basic Electrical Engineering

Figure 17.9 Block diagram representation of an IC 555 timer

Fig. 17.10 shows the functional block diagram of the 555 timer again.
Comp A and Comp B are the two voltage comparators. In the voltage

comparator a reference voltage is connected to the V -(inverting input).
in
When the signal voltage at V in+ rises above the reference, or goes below the
reference voltage, the output voltage goes high or low. Three 5 kΩ resistors

provide the reference voltages for the voltage comparators at V for
CC

comparator A and V CC for comparator B. When the threshold input at