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555 ASTABLE CIRCUIT - BREAD BOARD

V. Ryan © 2002

 

Components:

One 555 IC
270 ohm resistor from +9v to the collector of the NPN transistor.
Two 1K resistors
One NPN transistor (try any alternative).
One 100K preset resistor.
One 47uf capacitor
Black and red wire.
One LED

   

Using a breadboard and components listed above, put together the astable 555 timer circuit. Test the circuit to check that it works. Usually any faults are due to wires, components or pins/legs of components in the wrong slots. The information below explains in detail how the timer works.

   

 

   

Astable means that the 555 can operate repeatedly, it will switch on, then off, then on, then off, continually. The 555 is sometimes called an oscillator.
This is a typical 555 astable circuit that drives an LED. It is known as a LED flasher as the LED flashes on and off. The number of flashes per minute can be altered by turning the preset resistor.
Remember the 555 is activated by current at pin two and the output is through pin three. Altering the preset resistor alters the time between ‘pulses’ at pin three. The pulse at pin three switches the transistor which allows the LED to come on.
The LED flashes on and off because with this astable circuit the pulses from pin three are repeated until the power is switched off completely.

   
 
   

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