555 Data Sheet from Texas Instruments
The 555 timer is used to provide accurately timed single pulses or streams of pulses.
There are several versions of this diagram. This one uses subsystems included in the exam specification. The internal modules include ...
The three 5k resistors used as voltage dividers
Two Op' Amp' Comparators
The RS Flip Flop / Bistable Latch
The Output Buffer / Driver
NPN Transistor used as a switch.
Inside the 555 timer you will find ...
Voltage Divider consisting of three 5kΩ resistors.
These three 5k resistors may explain the name of this device.
The threshold comparator goes high if the threshold input goes above 2/3 of the supply voltage.
The trigger comparator goes high if the trigger input voltage drops below 1/3 of the supply voltage.
Threshold Comparator. If the threshold input is greater than 2/3 Vs, the latch resets and the output goes low. The discharge transistor turns on and this pulls pin 7 (discharge) low too.
Trigger comparator. If the trigger input is less than 1/3 Vs, the latch is set and the output goes high. The discharge transistor turns off and pin 7 (discharge) is controlled by components external to the 555 chip.
Data Latch. This RS Flip Flop is set if the trigger input is less than 1/3 Vs. It is reset if the threshold input is greater than 2/3 Vs.
Output Buffer. This is able to source or sink up to 200mA to the output.
Reset (pin 4). If pin 4 is low, this resets or disables the timer and the other inputs have no effect.
Discharge. This pin is held low by an NPN transistor used as a switch. This happens when the 555 output is low. If the 555 output is high, this transistor turns off and it has no effect on the rest of the circuit. (This arrangement is called "open collector").
This diagram is worth understanding because each subsystem might be included in the exam.
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