Inside the 555 Timer — Role of the Comparators What do the two internal comparators in a 555 timer compare relative to the internal resistor divider?

Introduction / Context:
The 555 timer contains two comparators and an SR latch. A three resistor divider sets two reference levels at approximately 1/3 Vcc and 2/3 Vcc. Understanding what the comparators sense is crucial to predicting operation.

Given Data / Assumptions:

• Upper comparator monitors the threshold pin.
• Lower comparator monitors the trigger pin.
• Reference levels come from the internal divider.

Concept / Approach:
The comparators compare the voltages on the trigger and threshold inputs to the internal reference levels. When threshold exceeds 2/3 Vcc, the upper comparator resets the latch. When trigger falls below 1/3 Vcc, the lower comparator sets the latch.

Step-by-Step Solution:
1) Internal divider produces two reference voltages: Vref_low = 1/3 Vcc and Vref_high = 2/3 Vcc.2) Lower comparator: compares trigger input against Vref_low. Trigger < Vref_low causes latch set.3) Upper comparator: compares threshold input against Vref_high. Threshold > Vref_high causes latch reset.4) Latch output drives the discharge transistor and output buffer to form timing behavior.

Verification / Alternative check:
Standard 555 block diagrams show comparators referenced to the internal divider, not to an external divider for basic operation.

Why Other Options Are Wrong:

• compare the output voltages to the internal divider: The output is controlled by the latch, not compared.
• compare the output voltages to the external divider: There is no such comparison in basic 555 operation.
• compare the input voltages to the external divider: References are internal in the standard configuration.

Common Pitfalls:
Confusing trigger/threshold with the output pin; assuming external resistor networks set references instead of the internal divider.

Final Answer:
to compare the input voltages to the internal voltage divider