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

Threshold Voltage at the Comparator of the 555 Timer 

The 555 timer is a popular integrated circuit (IC) used in various timer, delay, and pulse generation applications. It contains two voltage comparators, a flip-flop, and a discharge transistor, among other components.

The comparator's role in the 555 timer is to compare the input voltage with a reference voltage to decide the state of the output. Specifically, the output of the timer depends on the voltage at the **threshold pin (pin 6)**.

The 555 timer's internal comparators use two voltage thresholds to trigger the flip-flop and change the output state:

• Upper threshold voltage: The output switches from high to low when the voltage at the threshold pin exceeds this value.
• Lower threshold voltage: The output switches from low to high when the voltage at the threshold pin falls below this value.

The threshold voltage for the **upper comparator** (which determines the high-to-low output transition) is approximately \( \frac{2}{3} V_{CC} \), where \( V_{CC} \) is the supply voltage.

Mathematical Expression: \[ V_{T} = \frac{2}{3} V_{CC} \] Here:

• \( V_{T} \) is the threshold voltage at the comparator.
• \( V_{CC} \) is the supply voltage applied to the 555 timer.

This means that when the voltage at pin 6 (threshold pin) rises above \( \frac{2}{3} V_{CC} \), the output of the 555 timer switches from high to low. This point is crucial for defining the timing behavior in various applications, such as generating PWM signals or controlling frequency.

Example: If the 555 timer is powered with \( V_{CC} = 9 \, V \), the threshold voltage will be: \[ V_{T} = \frac{2}{3} \times 9 \, \text{V} = 6 \, \text{V} \] So, when the voltage at pin 6 exceeds 6 V, the output will switch from high to low.

Additional Information: The **lower threshold** for the 555 timer comparator (which controls the low-to-high transition) is approximately \( \frac{1}{3} V_{CC} \). This means that the output switches from low to high when the voltage at pin 6 drops below \( \frac{1}{3} V_{CC} \).

The 555 timer's output voltage, therefore, oscillates between low and high based on the voltage at the threshold pin and the capacitor charging/discharging in a typical astable multivibrator configuration.

Conclusion:

The threshold voltage at the comparator of a 555 timer is approximately \( \frac{2}{3} V_{CC} \). \[ \boxed{ \frac{2}{3} V_{CC} } \]