The 555 astable
In astable mode, the capacitor charges through R1 and R2, then discharges through R2 alone, over and over. The output is high while it charges and low while it discharges, so the high time is 0.693 × (R1 + R2) × C, the low time is 0.693 × R2 × C, and the frequency is 1.44 ÷ ((R1 + 2 × R2) × C). Because charging goes through both resistors, the duty cycle is always above 50%. For a duty cycle near or below 50%, put a diode across R2 so it's bypassed while charging.
The 555 monostable
In monostable mode, a trigger pulse makes the output go high for a fixed time, 1.1 × R × C, then it returns low and waits for the next trigger. It's used for delays, debouncing switches and stretching short pulses.
Practical tips
- Keep R1 at 1 kΩ or more, and the timing resistors below about 1 MΩ.
- Electrolytic capacitors can be 20% or more off their marked value, so expect the timing to differ by as much. Use film capacitors for accuracy.
- Put a 10 nF capacitor from pin 5 to ground and a 100 nF across the supply pins for steady timing.
- The CMOS versions, such as the 7555 and TLC555, use far less current and work down to 2 to 3 V.
Radio amateurs use 555s as CW sidetone oscillators, keyer timers and simple test-signal sources. The Morse code trainer is a good place to practise once your oscillator is built.
Questions
What is the 555 astable frequency formula?
f = 1.44 ÷ ((R1 + 2 × R2) × C).
How do I get a 50% duty cycle from a 555?
Put a diode across R2 so the capacitor charges through R1 only, and make R1 equal to R2. Or use a much larger R2 than R1, which gets close.
What is the 555 monostable formula?
The pulse length is 1.1 × R × C.
What's the highest frequency a 555 can produce?
About 100 kHz to 500 kHz for the bipolar NE555, and up to a few MHz for CMOS versions.