Question

Threshold voltage at the comparator of 555 timer is approximately

• A. 1/3 Vcc
• B. 2/3 Vcc
• C. 1/4 Vcc
• D. Vcc

Hint:

The 555 timer has two internal comparators with reference voltages derived from a voltage divider.
Upper comparator (Threshold comparator): Reference voltage is \(2/3 V_{CC}\). Input from Threshold pin (6).
Lower comparator (Trigger comparator): Reference voltage is \(1/3 V_{CC}\). Input from Trigger pin (2).

Answer 1

The Correct Option is B

The 555 timer IC has an internal voltage divider network consisting of three equal resistors (typically 5k\(\Omega\) each) connected between Vcc and ground. This divider creates two reference voltages:
The voltage at the junction of the top two resistors is \( \frac{2}{3} V_{CC} \). This voltage is connected to the non-inverting input of the upper comparator (often called the threshold comparator). The threshold pin (pin 6) of the 555 timer is connected to the inverting input of this comparator.
The voltage at the junction of the bottom two resistors is \( \frac{1}{3} V_{CC} \). This voltage is connected to the inverting input of the lower comparator (often called the trigger comparator). The trigger pin (pin 2) is connected to the non-inverting input of this comparator. The question asks for the "Threshold voltage at the comparator". This refers to the reference voltage for the threshold comparator. The threshold pin (pin 6) is compared against an internal reference voltage of \( \frac{2}{3} V_{CC} \). When the voltage at the threshold pin exceeds \( \frac{2}{3} V_{CC} \), the output of this comparator changes state, which typically resets the flip-flop inside the 555 timer. So, the reference voltage for the threshold comparator is \( \frac{2}{3} V_{CC} \). \[ \boxed{2/3 \text{ Vcc}} \]