555 Timer Configuration Recognition Which operating mode is used when a 555 timer has two external resistors and one external capacitor connected for timing?

Introduction / Context:
Identifying the physical RC network around a 555 quickly tells you its operating mode. Two resistors and one capacitor in the standard arrangement indicate the oscillator setup.

Given Data / Assumptions:

• Two timing resistors labeled RA and RB.
• One timing capacitor C from threshold and trigger node to ground.
• Discharge transistor connected to the RA–RB junction to discharge through RB.

Concept / Approach:
In the astable 555, the capacitor charges through RA + RB and discharges through RB only. That requires two resistors. In monostable, only one resistor and one capacitor set the single shot pulse width.

Step-by-Step Solution:
1) Observe two resistors plus one capacitor: classic astable network.2) Charging path: Vcc → RA → RB → C.3) Discharging path: C → RB → discharge transistor → ground.4) Result is free running oscillation with period T = 0.693 * (RA + 2 * RB) * C and duty cycle set by RA and RB.

Verification / Alternative check:
Monostable uses single R and C and needs an external trigger; Schmitt trigger references input conditioning, not standard 555 timing network.

Why Other Options Are Wrong:

• monostable: Requires one resistor and one capacitor.
• pulse stretching: A function, not a canonical 555 mode.
• Schmitt triggering: Refers to hysteresis behavior, not the 555 astable network.

Common Pitfalls:
Assuming any two resistor network is monostable; forgetting the discharge path through RB only.

Final Answer:
astable