# 555 timer astable multivibrator mode – Flashing LEDs

Astable mode 555 timers keep changing the output between high and low in an endless cycle as long as power is applied. The speed of which it does so depends on the value of the timing capacitor and 2 resistors.

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Assortment of commonly used integrated circuits (IC). I made a lot of videos with them.

## Schematic diagram

Process described here is illustrated below.

• When power is first applied. The output is high and the capacitor charges through one resistor (Vcc to pin 7) and either a diode or another resistor. The LED is on when the output is high because the output is at as close to the positive supply voltage as the output can provide while the other side of the LED in the circuit shown above is at 0V.
• When the capacitor charges to 2/3 of the supply voltage as detected by pin 6 (threshold), then the output goes low (the LED turns off due to both sides of it being at 0 volts) and the capacitor starts discharging through the resistor and then through pin 7 (discharge) to ground. Any current from the supply voltage through the resistor to pin 7 also goes to ground through the IC. So, it doesn’t affect the capacitor discharging at all.
• After the capacitor discharges to 1/3 of the supply voltage as detected by pin 2 (trigger), the output goes high and the capacitor starts charging again.
• Cycle keeps repeating.

In summary…

• Threshold pin (6) senses when the capacitor reaches 2/3 supply voltage
• Trigger pin (2) senses when the capacitor reaches 1/3 supply voltage.
• Discharge pin (7) discharges the capacitor through the 6-7 resistor, while also sending the Vcc to pin 7 current to ground.
• Output pin (3) can source current as shown in the diagram above, or sink current, where the load comes from Vcc and current sinks into the output when it is low (0V)

White version of the oscilloscope that I use in many of my recent videos.

### Video

Power supply I use in my videos.

###### Related topics:

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