Why can't I get a duty cycle below 50%?

In the classic circuit t_high always includes R2, so duty is above 50%. Add a diode across R2 to get below 50%.

Which capacitor type should I use?

Film or C0G ceramics are stable. Electrolytics work for slow timings but drift and have wide tolerance.

555 Timer Calculator – Astable Frequency & Monostable Pulse

The 555 is still the quickest way to blink an LED or make a one-shot delay. In astable mode it oscillates using R1, R2 and C; in monostable mode it outputs a single pulse set by R and C.

Mode

= 1 kΩ

= 10 kΩ

= 10 µF

Results

Frequency: 6.87 Hz
Period: 145.6 ms
Duty cycle: 52.38 %
High time: 76.25 ms
Low time: 69.31 ms

Pro

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Frequency: 6.87 Hz

Formula

t_high = ln2·(R1+R2)·C
t_low = ln2·R2·C
f = 1 / (t_high + t_low)
t_pulse = 1.1·R·C (monostable)

FAQ

Why can't I get a duty cycle below 50%?: In the classic circuit t_high always includes R2, so duty is above 50%. Add a diode across R2 to get below 50%.
Which capacitor type should I use?: Film or C0G ceramics are stable. Electrolytics work for slow timings but drift and have wide tolerance.

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