# 50% duty cycle

1. May 24, 2006

### electronic engineer

When Operating NE555 as astable vibrator

In how many ways can we make the ON pulse exactly equal to OFF pulse?

2. May 24, 2006

### Staff: Mentor

Can you post a link to a datasheet for the 555? What specs for duty cycle are listed? BTW, is this a homework problem?

3. May 24, 2006

### Averagesupernova

If I recall correctly, it is NOT possible to obtain a perfect 50% duty cycle output with a 555.

4. May 27, 2006

### electronic engineer

let's look into the internal structure of 555, we see pin 5 (voltage control pin) plays its role as voltage alternator for threshold voltage of upper comparator so charging time of external capacitor C changes thus the frequency of osscillation changes , but i don't know how to get exactly 50% duty cycle (Mark=Space), how to get charging time equal to discharging time??
does the threshold changing of upper comparator result in corresponding threshold change of lower comparator?!

i need help , thanks

5. May 27, 2006

### electronic engineer

let's look into the internal structure of 555, we see pin 5 (voltage control pin) plays its role as voltage alternator for threshold voltage of above comparator so charging time of external capacitor C changes thus the frequency of osscillation changes , but i don't know how to get exactly 50% duty cycle (Mark=Space), how to get charging time equal to discharging time??

what's the relation between control voltage and discharging time?

6. May 28, 2006

### Staff: Mentor

If the 50% duty cycle is important to you, just run the 555 at 2x frequency, and add a divide-by-2 flipflop after the 555. Or use a better oscillator than the 555.

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