What is the function of the comparators in the 555 timer circuit?
A. to compare the output voltages to the internal voltage divider
B. to compare the input voltages to the internal voltage divider
C. to compare the output voltages to the external voltage divider
D. to compare the input voltages to the external voltage divider
The Correct Answer and Explanation is:
Correct Answer: B. to compare the input voltages to the internal voltage divider
Explanation:
The 555 timer is a versatile integrated circuit used in various timing, pulse generation, and oscillator applications. At the heart of the 555 timer are two voltage comparators, which play a critical role in controlling the flip-flop and thus the output behavior of the timer.
Structure of the 555 Timer:
The internal architecture of a 555 timer includes:
- Two voltage comparators
- A bistable flip-flop
- An output stage
- A discharge transistor
- An internal voltage divider made of three equal-value resistors
This voltage divider creates reference voltages of 1/3 Vcc and 2/3 Vcc, which are fed to the non-inverting input of Comparator 1 and the inverting input of Comparator 2, respectively.
Function of Comparators:
The comparators are responsible for comparing external input voltages (usually at the threshold and trigger pins) to these internally generated reference voltages.
- Comparator 1 compares the threshold voltage (external input) to 2/3 Vcc (from the internal voltage divider).
- If the threshold voltage rises above 2/3 Vcc, Comparator 1 sets the flip-flop, driving the output low.
- Comparator 2 compares the trigger voltage (external input) to 1/3 Vcc (from the internal voltage divider).
- If the trigger voltage falls below 1/3 Vcc, Comparator 2 resets the flip-flop, driving the output high.
Why Option B is Correct:
The comparators do not compare output voltages nor do they reference external voltage dividers. They always work by comparing input voltages (trigger and threshold) against internally set reference voltages (1/3 Vcc and 2/3 Vcc). This makes Option B the correct and accurate choice.
Understanding the role of these comparators is key to analyzing how the 555 timer operates in both monostable and astable modes.