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555 Astable

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Data Sheet


Key Facts

This is an oscillator circuit providing a square wave output.

The astable has zero stable states. Whichever state it is in (high or low), it will soon switch to the other state.

The 555 chip CAN NOT drive a normal 8 Ohm loudspeaker. Piezoelectric sounders can be plugged straight in.


How it Works - Exam Answer

555 Astable Circuit and Layout Diagram



The astable will oscillate at this frequency ...

F = 1.44 / ((Ra + 2Rb) C1)


Low Time

The square wave output is low for the following period ...

tL = 0.7 RbC1


High Time

The square wave output is high for the following period ...

tH = 0.7 (Ra + Rb) C1


Symmetrical Square Wave


Mark Space Ratio


Things to Try

Use a potentiometer to adjust the frequency.

Adjustable Frequency Astable

Replace the Rb timing resistor with a light dependent resistor (LDR).

555-Astable with an LDR


Falstad Simulation

For the Falstad Circuit Simulation, CTRL+Click 555 Astable
In options, check European Resistors and uncheck Conventional Current.

Alternatively view 555astable.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.


Beep Beep Alarm Sound

555 Beep Beep Alarm 555 Beep Beep Alarm


Wailing Alarm Sound

555 Wailing Alarm Sound 555 Wailing Alarm Sound




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