555 timer circuit

*The 555 timer internal circuit is basically composed by a voltage divider, 2 comparators, a flip-flop SR and a NPN transistor; *The voltage divider has 3 resistors with the same value (R1, R2, R3); *Due to it, the upper comparator (U1) has a switching point: UTP = (2/3)*VCC, and the lower comparator (U2) has a switching point: LTP = (1/3)*VCC; *The threshold pin (THR) is connected to the upper comparator; *When the threshold voltage is higher than UTP value, the upper comparator will have a high level output; *This output is connected to the S input of the flip-flop SR and will set it; *In a similar way, the trigger pin (TRIG) is connected to the lower comparator; *When the trigger voltage is lower than LTP value, the lower comparator will have a high level output; *This output is connected to the R input of the flip-flop SR and will reset it; *The discharge pin (DIS) is connected to the collector of the NPN transistor; *This transistor connects the discharge pin to the ground when it receives a high level voltage on its base; *The reset pin (NRESET) is connected to the base of a PNP transistor in this circuit; *This transistor connects the R input of the flip-flop SR to VCC when it receives a low level voltage on its base, forcing reset independently of the lower comparator output; *The output pin (OUT) is connected to NQ of flip-flop circuit; *The control pin (CTRL) can be used to change the UTP value (and consequently change the LTP value as well); *This example shows the 555 circuit in astable mode.