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18 March 2009
Author: Giorgos Lazaridis
555 Theory

555 Tips and tricks
How to achieve duty cycle less than 50%

As shown above, the duty cycle from a 555 connected as an astable multivibrator is calculated from the following formula:

 D = R1 + R2 R1 + 2 x R2

Even if R1=0, the duty cycle cannot be lower than 50%.

To achieve duty cycle less than 50%, a diode should be connected in parallel with R2:

During charging time of the capacitor, R2 will not be taken into account as the current will pass through the bypass diode. During discharge time on the other hand, the current will pass through R2.

With this trick, a duty cycle of about 0 to 100% can be achieved. Of course absolute 0 and 100 can never be achieved with a 555, but the real duty cycle shall be very close to those numbers.

For your calculations, take into account that:

THIGH = 0.67 x R1 x C (result in Seconds)

and

TLOW = 0.67 x R2 x C (result in Seconds)

Quick monostable mode table component pickup

Quick astable mode table component pickup

Relative pages
• 555 timer basic circuits
• The transistor theory of operation
• Dr.Calculus: 555 Astable multivibrator calculator
• Dr.Calculus: 555 Monostable calculator
• International unit converter