013 555 timer astable mode basics

Astable mode 555 timer is when the 555 timer integrated circuit output keeps alternating between high and low.

That means that the output keeps changing between 0V, and as close to the supply voltage as the output can go. This happens as long as power is applied. It never stops. It has no stable state of either high or low. That’s why it is referred to as astable.

Basic circuit example:

555 timer astable multivibrator oscillator circuit diagram by electronzap electronzapdotcom
555 timer astable multivibrator oscillator circuit diagram by electronzap electronzapdotcom
  • The timing capacitor charges from the positive supply voltage through one resistor and a diode when wired as above. The output is high while it does so. Therefore the LED in this circuit is lit (on). Usually the output voltage is about 1.5V less than the positive supply voltage while it is high.
  • When pin 6 (threshold) senses that the capacitor has charged to 2/3 or more supply voltage, then the output switches to low (connects to ground). That turns off the LED in this circuit. Pin 7 (discharge) also goes from being off (no conduction) to low. Being low means that they both connect to 0V ground really well. The capacitor starts discharging through the resistor that is between the connected together pins 6 (threshold) and 2 (trigger), and pin 7 (discharge).  Any current through the + supply voltage and pin 7 resistor goes right to ground at this time and does not affect the capacitor discharge time at all.
  • When pin 2 (trigger) senses that the capacitor has discharged to 1/3 or less of the supply voltage, then the output goes high. That turns the LED back on, and pin 7 (discharge) basically turns off. It stops conducting current through it, and the capacitor starts charging again.

High side load and good voltage measurement points:

Astable mode 555 timer multivibrator flashing LEDs with oscilloscope measurement locations schematic diagram by electronzap
Astable mode 555 timer multivibrator flashing LEDs with oscilloscope measurement locations schematic diagram by electronzap

This diagram  has and LED and protective resistor on the high side of the output, as well as the low side.

The high side (connected to +5V) LED lights up when the output is low.

The low side (connected to ground) LED lights up when the output is high.

I added “scope” bubbles to show good spots to take oscilloscope measurements. That way you can see the voltage changing in real time.

You touch the measurement probe/red alligator clip to where the bubble is pointing to and attach the ground/black clip to 0V ground.

Video:

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