A 555 timer can act as a bistable (flip-flop) with two stable states and no timing capacitor. The internal comparators at 1/3 VCC and 2/3 VCC drive an SR latch. Drive TRIG (pin 2) momentarily below 1/3 VCC to SET: OUT (pin 3) goes high and the discharge transistor turns off. Drive THRESH (pin 6) momentarily above 2/3 VCC—or pull RESET (pin 4) low—to RESET: OUT goes low and the discharge transistor turns on. Pin 7 and any RC network are typically unused. Use pull-up/pull-down resistors and debounced or filtered inputs for reliable switching. The two thresholds provide built-in hysteresis, giving good noise immunity and clean state changes. We have a high-quality video on CircuitBread.com that goes through these steps visually.
