Calculate output characteristics for 555 timer circuits in both Astable and Monostable modes. Enter your R1, R2, and C1 values to get frequency, period, duty cycle, time high, and time low — or find your monostable pulse duration from a single resistor and capacitor pair.
Frequency
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Period (T)
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Duty Cycle
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Time High (T1)
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Time Low (T0)
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Pulse Duration (Monostable)
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In Astable (Free Running) mode, the 555 timer continuously switches between high and low output, generating a square wave with no external trigger needed. In Monostable (One Shot) mode, the output produces a single pulse of a fixed duration each time it receives a trigger signal, then returns to its stable low state.
For Astable mode: Time High (Th) = 0.693 × (R1 + R2) × C1, Time Low (Tl) = 0.693 × R2 × C1, Frequency f = 1.44 / ((R1 + 2×R2) × C1), and Duty Cycle = (R1 + R2) / (R1 + 2×R2) × 100%. For Monostable mode: Pulse Duration T = 1.1 × R1 × C1.
No. In a standard astable 555 timer configuration, the duty cycle is always above 50% because R1 charges the capacitor in addition to R2, but only R2 discharges it. To achieve a 50% or lower duty cycle, you need to add a diode across R2 to bypass it during discharge.
Use the formula f = 1.44 / ((R1 + 2×R2) × C1) and rearrange it for your target frequency. Typical starting values are R1 = 10kΩ, R2 = 10kΩ, and C1 = 100nF, which gives roughly 480 Hz. Adjust R2 primarily to tune frequency while keeping R1 small relative to R2 for a duty cycle closer to 50%.
The mark-to-space ratio is the ratio of Time High (T1) to Time Low (T0), expressed as T1/T0. It tells you how long the output is on compared to how long it is off within each cycle. A ratio of 1:1 would mean a 50% duty cycle, which is not achievable with a basic astable 555 configuration.
Resistor values typically range from a few hundred ohms up to several megohms. Capacitors typically range from a few picofarads up to hundreds of microfarads. Very low resistances can damage the timer IC, while very high capacitances increase timing errors due to leakage current. Practical designs stay between 1kΩ–1MΩ for resistors and 1nF–100µF for capacitors.
In monostable mode, R2 is not part of the timing circuit — only R1 and C1 set the pulse duration via T = 1.1 × R1 × C1. When a trigger pulse is applied, C1 charges through R1 until it reaches 2/3 of VCC, at which point the output returns low. The single resistor-capacitor pair fully determines the output pulse width.
The 555 timer is relatively accurate for low-frequency applications, but timing can drift with temperature changes, supply voltage variations, and component tolerances. Typical timing accuracy is within 1–5% under normal conditions. For precision timing applications, use a crystal oscillator or a dedicated timer IC instead.