Function of the discharge transistor in a 555 timer application In timing modes, what role does the discharge transistor play with the external timing capacitor?

Introduction / Context:
The 555 timer contains a discharge transistor tied to the timing capacitor node. Understanding when this transistor turns on and why it discharges the capacitor is essential for both astable and monostable configurations.

Given Data / Assumptions:

• Standard 555 internal block: two comparators, R-S latch, output stage, and discharge transistor.
• Timing capacitor connected to the threshold/trigger network and discharge pin.
• Operation considered in astable/monostable timing modes.

Concept / Approach:

In astable mode, the discharge transistor periodically conducts to rapidly pull the capacitor toward ground, initiating the next charge cycle. In monostable mode, after the output pulse ends, it turns on to reset the capacitor so the circuit is ready for the next trigger.

Step-by-Step Solution:

Verification / Alternative check:

Typical waveforms show a fast discharge edge at the end of each period in astable mode; the sharp drop corresponds to the discharge transistor conduction.

Why Other Options Are Wrong:

Charging roles (a, b) are incorrect; the transistor connects to ground, not the supply, so it does not charge the capacitor. “discharge … to stop the timing” (c) misstates the functional purpose; the discharge prepares for the next cycle rather than merely stopping activity.

Common Pitfalls:

Confusing the output stage with the discharge transistor; assuming the transistor sources current; overlooking that discharge action defines the low-time in astable mode.

Final Answer:

discharge the external capacitor to start the timing over again