What is the function of the discharge transistor in a 555 timer circuit?

Question

Question

This question was previously asked in

RRB ALP Electronics Mechanic 23 Jan 2019 Official Paper (Shift 2)

Answer 1

The correct answer is option 4.

IC 555 timer

The 555 timer IC is used for pulse generation.

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications.

qImage12169

Description of pins:

1.) Pin 1

It is the ground pin directly connected to the negative rail.
It should not be connected using a resistor, because all the semiconductors inside the IC will heat up due to stray voltage accumulating in it.

2.) Pin 2

It is the Trigger pin to activate the IC’s timing cycle.
It is generally a low signal pin and the timer is triggered when the voltage on this pin is below one-third of the supply voltage.
The trigger pin is connected to the Inverting input of the comparator inside the IC and accepts negative signals.

3.) Pin 3

It is the output pin. When the IC triggers via pin 2, the output pin goes high depending on the duration of the timing cycle.
For logic zero output, it is a sinking current with a voltage slightly greater than zero. For logic high output, it is sourcing current with the output voltage slightly lesser than V_CC

4.) Pin 4

It is the reset pin. It should be connected to the positive rail to work the IC properly.
When this pin is grounded, the IC will stop working.

5.) Pin 5

It is the control pin. It requires being connected to an external DC signal to modify the timing cycle.
When not in use, it should be connected to the ground through a 0.01uF capacitor; otherwise, the IC will show erratic responses.

6.) Pin 6

It is the Threshold pin. The timing cycle is completed when the voltage on this pin is equal to or greater than two-thirds of V_CC.
It is connected to the non-inverting input of the upper comparator so that it accepts the positive-going pulse to complete the timing cycle.

7.) Pin 7

It is the discharge pin.
It provides a discharge path for the timing capacitor through the collector of the NPN transistor, to which it is connected.
The maximum allowable discharging current should be less than 50 mA otherwise the transistor may damage. It can be also used as an open collector output.

8.) Pin 8

It is a positive rail-connected pin that is connected to the positive terminal of the power supply. It is also known as V_CC