The 555 astable oscillator
Make a timer blink by itself
A 555 is a timer integrated circuit: many components in one chip. Astable means it continually switches without a stable resting output. An oscillator repeats a cycle. Duty cycle is the fraction spent high; period is one cycle’s duration.
EXPERIMENT BENCH
Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.
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Why it works
Ideal standard 555 astable timing with factor 0.693. No load, startup or capacitor leakage. The diagram illustrates calculated timing, not a physically tested circuit.
The capacitor charges through Ra + Rb and discharges through Rb. Period = 0.693 × (Ra + 2Rb) × C; frequency = 1/period. Duty = (Ra + Rb)/(Ra + 2Rb). µF means microfarad, one millionth of a farad. Here C is capacitance; Ra and Rb are resistance values.
Difficulty
Build · Build it
Power on with Ra = Rb = 10 kΩ and C = 10 µF. Observe about 4.81 Hz and 66.67% duty.
Need a hint?
Ra = Rb = 10 kΩ; C = 10 µF → 4.81 Hz
Explore · Change one thing
Obtain 0.9–1.1 Hz with duty no greater than 60%.
Need a hint?
Ra = 10 kΩ; Rb = 50 kΩ; C = 13 µF → 1.01 Hz
Solve it · Solve the challenge
Obtain 8–12 Hz with duty between 50% and 55%.
Need a hint?
Ra = 1 kΩ; Rb = 10 kΩ; C = 7 µF → 9.82 Hz
Extra reading and circuit recipes
Make a timer blink by itself
A useful pattern
Doubling the capacitor doubles both timing intervals and halves frequency.
Beyond this model
The standard connection has duty greater than 50%. Other arrangements can alter its charge and discharge paths.
A useful trap to avoid
A blinking output cannot directly power any load: real 555 output and supply limits still apply.
Only C doubles. The ideal frequency…
- halves
- doubles
- stays unchanged
Read more
Both charge and discharge take twice as long, so the cycle repeats half as often.