EASA / CAR Part-66 · Module 4 · 4.1.2 Transistors · Cat B1/B2
An astable oscillator flips between high and low on its own, no input needed. A resistor charges a capacitor up to a threshold, it flips, discharges, and repeats. R and C set the frequency.
For a 555-style astable, f ≈ 1.44 ÷ (R·C) (using equal timing resistors). Bigger R or C means slower charging, a lower frequency. The output is a square wave, the clock behind timers, flashers and digital circuits.