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555 timer

555 timer: the circuit drawn with the example's values555DISTHRTRIGVCCOUTGND1kΩ10kΩ100nF
687 Hz at 52 % with 1 kΩ, 10 kΩ, 100 nF

The 555 fills a capacitor from a third to two thirds of the supply, then flips.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Given

Mode
Astable (oscillator)
R1 (to the supply)
1 kΩ
R2 (to the capacitor)
10 kΩ
Timing capacitor C
100 nF

Working

  1. charging through both resistors
  2. discharging through R2 alone
  3. always above 50 % high in this wiring

Results

Frequency687 Hz
Duty cycle (high)52.4 %
High time762 µs
Low time693 µs
Period1.46 ms

The formulas

  1. f = 1.44 / ((R1 + 2 R2) C)astable
  2. D = (R1 + R2) / (R1 + 2 R2)always above 50 % in this wiring
  3. t = 1.1 R Cthe monostable pulse

The usual traps

  • The duty cannot go under 50 % without a diode across R2.
  • Ceramic capacitors drift: use film for a timing you care about.

Questions

How do I set the frequency of a 555 timer?

f = 1.44 / ((R1 + 2 R2) C): astable. D = (R1 + R2) / (R1 + 2 R2): always above 50 % in this wiring. t = 1.1 R C: the monostable pulse.

What does it give for typical values?

687 Hz at 52 % with 1 kΩ, 10 kΩ, 100 nF. Frequency: 687 Hz; Duty cycle (high): 52.4 %.

What are the usual mistakes?

The duty cannot go under 50 % without a diode across R2. Ceramic capacitors drift: use film for a timing you care about.

In the app

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Opens in the app on 1 December, with Circuit Notebook 1.0

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