Astable multivibrator mode 555 timer keeps alternating the output high and low.
The output is high while the timing capacitor charges. Usually it charges through either 2 resistors or a resistor and a diode. Once pin 6 (threshold) senses that the capacitor is charged to 2/3 supply voltage, then pin 3 (output) goes low while pin 7 (discharge) also goes low.
The capacitor then discharges through one resistor into pin 7 until pin 2 (trigger) senses that the capacitor has discharged to 1/3 or less of the supply voltage. That sets the output high again and disconnects pin 7 from doing anything so that the capacitor can charge again.
Cycle keeps repeating. If there’s LEDs connected to the output, then they will flash accordingly.
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