Page "Relaxation oscillator" Paragraph 7

A similar relaxation oscillator can be built with a 555 timer IC ( acting in astable mode ) that takes the place of the neon bulb above.

That is, when the capacitor reaches a certain value, comparators within the 555 timer cause the latch to activate a transistor switch that discharges the capacitor through a resistor to ground.

At the instant the capacitor falls to a sufficiently low value, the switch deactivates to let the capacitor charge again.