555 oscillator

The capacitor fills through RA + RB and empties through RB. Make the bucket bigger and both parts of the rhythm get longer.

f = 1 / [ln(2) × (RA + 2RB) × C]

Classic NE555 astable topology, not a diode-modified circuit. Ideal thresholds are ⅓ and ⅔ of supply; high duty is above 50%. Use a 4.5–16 V supply for NE555. Check RA/RB limits, leakage, tolerance and decoupling in the datasheet. Supply voltage is not an input to this ideal timing model.