This is part 3 of a series of articles on the 555 timer. You might want to start at part 1, 555 Timer Basics – Monostable Mode if you haven’t read it already.

Astable Mode of the 555 Timer

The astable mode is what most people think of when it comes to the 555 timer. Astable mode can be used to make an LED flash on and off. It has a lot of other interesting applications too though. For example, it can be used to generate sound, or as a simple analog to digital converter (ADC). When in astable mode, the 555 timer produces an accurate pulse frequency that can be adjusted depending on the values of two resistors and one capacitor in an external circuit.

The 555 timer in astable mode acts as an oscillator that generates a square wave. The period of the square wave is described by the equations:



Where t(on) is the length of the on output pulse in seconds, and t(off) is the length of the off output pulse in seconds. R1 and R2 are the resistance values in Ohms of resistors 1 and 2 respectively, and C1 is the capacitance in Farads of capacitor 1.

How Astable Mode Works

555 timer astable mode circuit

The output of the 555 timer in astable mode continuously cycles through two output states- on and off. By observing the schematic above, we can see that the trigger pin is connected to the threshold pin. At the beginning of the cycle, the voltage across the capacitor (C1) is low. Since they are connected to the capacitor, the voltages at the trigger pin and threshold pin are also low. Remember that when the trigger pin voltage is low, the output is switched on, and the discharge pin is switched off. This allows current to flow through resistors R1 and R2, charging capacitor C1 until the voltage across it reaches 2/3 Vcc. Once the voltage reaches 2/3 Vcc, the threshold pin is high, so it switches the output off. Remember also that when the output switches off, that the discharge pin switches on. This drains all of the charge accumulated on capacitor C1 to ground. Once enough charge has drained from the capacitor to make the voltage at the trigger pin 1/3 Vcc, it switches the discharge pin off, and charge begins to accumulate on the capacitor, starting another round of the cycle.

Example Astable Mode Circuits

To observe the 555 timer in astable mode, try connecting up a circuit which uses the 555 timer’s oscillating output to make an LED flash on and off:

555 Timer Flashing LED

R1: 4.7K Ohm resistor

R2: 4.7K Ohm resistor

R3: 1K Ohm resistor

C1: 100 μF capacitor

The values of R1, R2, and C1 are not that critical, so if you don’t have these exact values, just get as close as you can. Then try replacing R1 and C1 with different values to see how the resistance affects the timing of the circuit. You should be able to see that higher values of either R1 or C1 (or both) will make the LED stay on and off for a longer period of time.

Flashing LED Controlled by a Potentiometer

An easy way to observe the effect of resistance on the 555 timer is to connect a 10K Ohm variable resistor (potentiometer) in place of R2, like so:

555 Timer Potentiometer

Turn the knob to see how the flashing rate is affected by the variable resistance of the potentiometer.

Watch the video to see the 555 timer in action:

Flashing LED Controlled by a Photoresistor

Instead of using a potentiometer to control the flash rate, try connecting a photoresistor. The resistance of a photoresistor decreases as more light shines on it, so the LED should flash more quickly when exposed to more light.555 Timer Astable Mode

R1: Photoresistor

If you want to learn more about the 555 timer, the book Timer, Op Amp, and Optoelectronic Circuits and Projects Book Vol. 1 By Forrest Mims is a great resource to have on your bench. The book has lots of information about the 555 timer, OpAmps, and other IC’s too.

If you have any questions or are having trouble with this project, please leave a comment below and I will answer it as soon as possible!

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