Astable multivibrator mode 555 timer keeps alternating the output high and low.

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Diagram

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.

Video

Quick 555 timer astable multivibrator mode circuit schematic to breadboard build by electronzap



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