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Wiring a 555 timer in astable mode results in its output alternating between high and low continuously. The timing is usually determined by 2 resistors, a capacitor, and often a diode.
- When the power is turned on, the timing capacitor starts charging through both the resistor closest to Vcc and the diode, until the capacitor reaches 2/3 of the supply voltage (Vcc).
- Output is high (LED on) while capacitor is charging.
- Once output gets to 2/3 Vcc or more, output and pin 7 both go low (connects to ground). That discharges the capacitor through the resistor parallel to the diode and the LED will be off because 0V ground is on both sides of it. There’s no voltage difference across it.
- After the capacitor voltage drops to 1/3 supply voltage or less, the output pin (3) goes high and the discharge pin (7) basically turns off. Pin 7 is an open collector pin. It’s either open (off), or it is low/connected to 0V ground. The capacitor starts charging again, just like it was doing at the top of the list.
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