Home

rOm
Quest
Glossary

Random
Page
Search
Site
Lush
Sim
Class
Subject
Images

Help
FAQ
Sign
Up
Log
In
 GCSE     Timing     555     >555 Astable<     555 Monostable     555 Motor Speed     Mark Space

# Timing 555 Astable

Site for Eduqas/WJEC - Go to the AQA site.

GCSE    RC Timing    Questions 0 to 20   -->  View All

The 555 Astable output is a stream of accurately timed pulses. There are zero stable states.

## 555 Astable Key Facts

This is an oscillator circuit providing a square wave output.

The astable has zero stable states. Whichever state it is in (high or low), it will soon switch to the other state.

The 555 chip CAN NOT drive a normal 8 Ohm loudspeaker. Piezoelectric sounders can be plugged straight in.

• The astable frequency depends on C1.
• The circuit below uses a piezoelectric sounder and a coupling capacitor is not needed.
• The output LED indicator is optional.

## How it Works - Exam Answer

• If the output is high, the discharge transistor is off.
• C1 charges towards Vs through Ra and Rb until the threshold rises above 2/3 of the supply voltage.
• The output goes low and the discharge transistor turns on.
• C1 discharges towards 0 Volts through Rb until the threshold drops below 1/3 of the supply voltage.
• The output goes high again and the steps above repeat.

## Frequency

The astable will oscillate at this frequency ...

F = 1.44 / ((Ra + 2Rb) C1)

## Low Time

The square wave output is low for the following period ...

tL = 0.7 RbC1

## High Time

The square wave output is high for the following period ...

tH = 0.7 (Ra + Rb) C1

## Symmetrical Square Wave

• To get a fairly symmetrical square wave, Ra should be a lot smaller then Rb.
• For adjustable frequencies, Rb can be a variable resistor.
• Try using an LDR for Rb. The frequency will depend on the light level.

## Things to Try

Use a potentiometer to adjust the frequency.

Replace the Rb timing resistor with a light dependent resistor (LDR).

## Wailing Alarm Sound

Subject Name     Level     Topic Name     Question Heading     First Name Last Name Class ID     User ID

 CloseEscapeX Q: qNum of last_q     Q ID: Question ID         Score: num correct/num attempts         Date Done

Question Text

image url

Help Text
Debug

• You can attempt a question as many times as you like.
• If you are logged in, your first attempt, each day, is logged.
• To improve your scores, come back on future days, log in and re-do the questions that caused you problems.
• In the grade book, you can delete your answers for a topic before re-doing the questions. Avoid deleting unless you intend re-doing the questions very soon.