How does a 555 monostable time the length of its pulse?

The capacitor charges through the timing resistor until its voltage reaches two thirds of the supply. The 555 then ends the pulse and discharges the capacitor.

State the equation for the pulse length of a 555 monostable.

The pulse length is T = 1.1 × C × R. T is in seconds when C is in farads and R is in ohms.

Calculate the pulse length of a 555 monostable with R = 100 kΩ and C = 100 μF.

The pulse lasts 11 s. T = 1.1 × (100 × 10⁻⁶) × (100 × 10³) = 11 s.

CCEA A-Level Technology and Design 8900

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Electronic Control Systems

555 monostable and astable timers

3 things to know
AS 1 Paper 2

Overview

The 555 monostable and astable, pulse length and frequency equations with worked values, period and frequency, mark/space ratio and typical applications.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

A 555 monostable must give a 10 s pulse using a 100 μF capacitor. Calculate R.

Key term

What does a 555 monostable circuit do?

Key term

How is a 555 monostable triggered?

Questions from this subtopic

Each one with the model answer underneath.

1
What does a 555 monostable circuit do?
Model answer
A 555 monostable gives a single output pulse of fixed length each time it is triggered. The output then returns to its stable low state.
2
How is a 555 monostable triggered?
Model answer
The trigger input, pin 2, is briefly taken below a third of the supply voltage. A push-to-make switch to 0 V usually does this, against a pull-up resistor holding the pin high.
3
How does a 555 monostable time the length of its pulse?
Model answer
The capacitor charges through the timing resistor until its voltage reaches two thirds of the supply. The 555 then ends the pulse and discharges the capacitor.
4
State the equation for the pulse length of a 555 monostable.
Model answer
The pulse length is T = 1.1 × C × R. T is in seconds when C is in farads and R is in ohms.
5
Calculate the pulse length of a 555 monostable with R = 100 kΩ and C = 100 μF.
Model answer
The pulse lasts 11 s. T = 1.1 × (100 × 10⁻⁶) × (100 × 10³) = 11 s.
6
A 555 monostable must give a 10 s pulse using a 100 μF capacitor. Calculate R.
Model answer
R must be about 91 kΩ. R = T ÷ (1.1 × C) = 10 ÷ (1.1 × 100 × 10⁻⁶) = 90.9 kΩ. A 100 kΩ variable resistor can be set to this value.

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