Components - The 555 TimerMany electronic systems require timing operations, synchronizing pulses for

digital systems or flashing lamp indicators, both can be supplied by the 555 timer. Formed as an 8 pin d.i.l. integrated circuit it can operate from

any dc supply from 3 to 15 volts and is not static sensitive.

The 555 timer can be used as a monostable producing a single pulse or as an astable continuous free running oscillator.

Example

Circuit Symbol

Vcc

Gnd

output

threshold

discharge

trigger

controlvoltage

reset

6

7

2

1

8 4

3

5

Manufactures use an alphanumeric code printed on the device as identification e.g. LM555, µA555, NE555.

It is necessary to refer to manufacturers data sheets to obtain lead information and ratings.

Single and dual (556) timers are available in 8 and 16 pin dual-in-line (DIL) packages. In each case pin 1 is always on the top left.

555 Markings and Packaging

output 3

NE5

55reset 4

7 discharge

Pin 1 marker

6 threshold

trigger 2

ground 1

5 control voltage

8 VCC

Pin layout for 555 timer

555 timer Connections

Vcc

Gnd

output

threshold

discharge

trigger

controlvoltage

reset

6

7

2

1

8 4

3

5

Pin Function Pin Function

1 0 volt supply 5 Frequency Modulation input

2 Active low trigger input 6 2/3 VCC sense input

3 Output 7 Capacitor discharge output

4 Active low reset input 8 + supply

Monostable Mode

In this mode a single pulse is produced at the output when the trigger (2) input is taken low.

The duration of the pulse is determined by timing components C1 and R1.

~~ 1.1R1C1Pulse width T

VCC

R1

C1

C2

Vcc

Gnd

output

threshold

discharge

trigger

controlvoltage

reset

6

7

2

1

8 4

3

5

T

Gnd

Astable Mode

In this mode a continuous stream of pulses appear at the output (3) as the trigger input is taken low each time the capacitor is discharged.

The frequency is determined by timing components R1, R2 and C1.

VCC

R1

C1

C2

Vcc

Gnd

output

threshold

discharge

trigger controlvoltage

reset6

7

2

1

8 4

3

5

Gnd

R2

ton ~~ 0.7(R1 + R2)C1

toff ~~ 0.7R2C1

toff

ton

operating frequency f = 1.44

(R1 + 2R2 )C1

Monostable (one-shot)(practical activity)

This task is to build and test a monostable using a 555 timer integrated circuit to illuminate an led for a set period of time.

On completing the circuit compare your results with others in the class. you may also wish to investigate further by observing the effect of changing C1 for a value of your choice.

circuit diagram

The activity also provides support to your final achievement.

Gnd (0v)

VCC +12V

R1

100k

C1

10μF

Vcc

Gnd

output

threshold

discharge

trigger

controlvoltage

reset

6

7

2

1

8 4

3

C2

10nF

5

R2

1k

LED

R2

10k

Astable Oscillator (practical activity)

This task is to build and test an astable oscillator using a 555 timer integrated circuit to cause an led to flash at a 2:1 duty cycle.

On completing the circuit compare your results with others in the class. you may also wish to investigate further by observing the effect of changing R1 for a value of your choice.

circuit diagram

The activity also provides support to your final achievement.

Gnd (0v)

VCC +12V

R1

150k

C1

10μF C2

10nF

Vcc

Gnd

output

threshold

discharge

trigger controlvoltage

reset6

7

2

1

8 4

3

5

R2150k R3

1k

LED

duty cycle = ‘on’ time ‘off’ time