chapter 4 : timer - dunia ilmiah · chapter 4 : timer lecturer : muhammad muizz bin mohd nawawi...

CHAPTER 4 :TIMER

Lecturer : Muhammad Muizz Bin Mohd Nawawi

Electrical Engineering Department

Politeknik Kota Kinabalu, Sabah.

EE301 ELECTRONIC CIRCUITS

• 555 timer is an integrated circuit that very stable.

• Use as monostable multivibrator, astable multivibrator, analog square wave signal generator, achometer frequency meter and others.

• Basically, 555 timer operate in following mode:

– Monostable mode

– Astable mode

– Bistable mode

– Buffer-schmitt trigger

4.1 INTRODUCTION

4.1.1 PIN ASSIGNMENT &

CONFIGURATION

PIN 1 : GROUND

- All the measure voltage must refer to this pin

PIN 2 : TRIGGER

- Timer output refer to trigger amplitude pulse supply to this pin.

- Output is LOW if voltage at pin > 2/3 Vcc

- When negative trigger pulse is 1/3 Vcc, output at comparator 2 cause

the timer output HIGH.


CONFIGURATION (cont..)

PIN 3 : OUTPUT

- Output can connect at 2 output pin, pin 3 and pin 1 or pin 3 and

pin 8.

- When output LOW, current will flow through load that connected

between pin 3 and pin 8 to output terminal, know as SINK CURRENT.

- Current flow through load that connected between pin 3 and pin 1,

known as SOURCE CURRENT.

- Maximum current for source current and sink current

is 200mA.



PIN 4 : RESET

- Reset with negative pulse

- When the reset pin is not used, the pin is connected to +Vs to avoid

false trigger.

PIN 5 : CONTROL VOLTAGE

- Normally is connected to earth through 0.01µF capacitor

- If output voltage is connected to pin 5, the output

waveform bandwidth can be changed.

- 0.01µF capacitor can avoid from noise problem.



PIN 6 : THRESHOLD VOLTAGE

- Input for inverting pin at comparator 1

- When voltage at this pin ≥2/3 Vcc, output at comparator 1 is HIGH,

output timer LOW.

PIN 7 : DISCHARGE

- This pin is connected internally to collector at transistor Q1.

- When output HIGH, Q1 OFF, circuit open

- When output LOW, Q1 saturated, capacitor C

discharge through Q1.

PIN 8 : SUPPLY VOLTAGE, Vcc

- +5 V to 18V



4.1.2 555 TIMER INTERNAL CIRCUIT

4.1.2 555 TIMER INTERNAL CIRCUIT

INTERACTIVE LEARNING

Internal Elements of a 555 Timer

http://www.wisc-online.com/Objects/ViewObject.aspx?ID=SSE7806

internal elements.swf




internal elements.swf

Monostable mode:

• Functions as a "one-shot" pulse generator.

• Applications include timers, missing pulse detection, bounce free

switches, touch switches, frequency divider, capacitance measurement, pulse width modulation (PWM) and so on.

Bistable mode or Schmitt trigger:

• The 555 can operate as a flip-flop.

• If the DISCHARGE pin is not connected and no capacitor is used.

• Uses include bounce free latched switches.

4.1.3 MODES OF 555 TIMER

Astable mode:

• Free running mode: the 555 can operate as an oscillator.

• Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on.

• Selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor.

• The period of the output pulse is determined by the temperature.

• The use of a microprocessor based circuit can then convert

the pulse period to temperature, linearize it and even

provide calibration means.

4.1.3 MODES OF 555 TIMER (cont..)


Internal Elements of a 555 Timer

http://www.wisc-online.com/Objects/ViewObject.aspx?ID=DIG5303

multivibrators.swf

4.1.3 MODES OF 555 TIMER (cont..)




multivibrators.swf

4.2 MONOSTABLE MULTIVIBRATOR

• Know as shoot multivibrator.

• Pulse generator circuit which the period is calculated from RC network and connected to external of 555 timer.

• Monostable multivibrator is stable when the output logic LOW

(logic = 0).

• When a pulse is trigger at pin 2 (normally negative trigger pulse), timer output will change to HIGH (+Vs) for a while and change

to LOW (stable condition). The condition will continue

LOW until pulse is trigger again.

4.2 MONOSTABLE MULTIVIBRATOR (cont…)

• The timing period is triggered (started) when trigger input (pin 2) is less than 1/3 Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored.

• The threshold input (pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the time period over and the output becomes LOW.

• At the same time discharge (pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.


CRT )(1.1 1

• A monostable circuit produces a single output pulse when triggered.

• It is called a monostable because it is stable in just one state: 'output low'.

• The 'output high' state is temporary.

• The duration of the pulse is called the time period (T) and this is

determined by resistor R1 and capacitor C1:


Operation for monostable multivibrator:

i. Assume initial output is LOW, circuit at stable condition, transistor Q1 ON, capacitor is connected to ground.

ii. When negative pulse is triggered to PIN 2 , transistor Q1 OFF (Q1 open circuit), capacitor C start charging through resistor R1 and output is HIGH.

iii. When voltage at capacitor, C reach 2/3 Vs, output will

change to LOW through flip flop. At the same time,

flip flop output will make the Q1 ON. Capacitor will

discharge through transistor Q1.

iv. Monostable output will remain LOW until another

trigger pulse is triggered to pin 2.


Example 1

Determine the period of the

output waveform for the circuit

when triggered by a negative

pulse

Answer

Time high = 0.825 ms



The 555 Monostable Multivibrator


monostable multivibrator.swf





monostable multivibrator.swf

4.3 ASTABLE MULTIVIBRATOR

• Known as free running multivibrator.

• This type of timer do not have stable condition, the condition always change.

• Astable do not need trigger pulse for external to change the output.

• The period for LOW and HIGH can be calculated based on resistor and capacitor value that connected at outside of timer.

4.3 ASTABLE MULTIVIBRATOR (cont…)

Operation for astable multivibrator:

i. Assume the initial is HIGH. Transistor Q1 OFF and capacitor is charging through resistor RA and RB.

ii. When capacitor voltage reach 2/3 Vee, Comparator 1 will trigger flip flop and output change from change from HIGH to LOW. Resistor RB and transistor Q1.

iii. When the capacitor voltage reach 1/3 Vee, comparator

output 2 will trigger flip flop so the timer output is

HIGH. The cycle is repeated.


• Period for capacitor charging from 1/3 Vcc to 2/3 Vcc same as period for HIGH output at timer.

• Period for capacitor discharging from 2/3Vcc to 1/3 Vcc same as period for LOW output at timer.

• With a stable operation, the frequency and duty cycle are accurately controlled with two external resistors and one capacitor.


CRT

CRRT

L

H

)(693.0

)(693.0

2

21

CRRTTTPeriod LH )2(693.0, 21

CRRTTTfFrequency

LH )2(

44.111,

21

21

21

2RR

RR

TT

TDutyCycle

LH

H


Example 2

Timer 555 is connected as astable mode.

The value given are Ra = 6.8 kΩ, Rb = 3.3 kΩ, C1 = C2 = 0.1 µF.

Calculate: a) Time high

b) Time low

c) Frequency

d) Duty cycle

Answer

a) 0.6969 msec c) 1.0815 kHz

b) 0.2277 msec d) 75.4%



Example 3

Determine the frequency and

draw output waveform for the

circuit.

Answer

Time high = 1.05 ms

Time low = 0.525 ms

Time, T = 1.575 ms

Frequency, f = 635 Hz


The 555 Astable Multivibrator


astable multivibrator.swf





astable multivibrator.swf

• In bistable mode, the 555 timer acts as a basic flip-flop.

• The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via Pull-up resistors while the threshold input (pin 6) is simply grounded.

• Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to Vcc (high state).

• Pulling the reset input to ground acts as a 'reset' and

transitions the output pin to ground (low state).

• No capacitors are required in a bistable configuration.

• Pin 5 (control) is connected to ground via a small-value

(usually 0.01 to 0.1 uF); pin 7 (discharge) is left floating.

4.4 BISTABLE MULTIVIBRATOR

• In some cases the pin 6 can also be used as a trigering as like pin 4 which works as a reset.

• The circuit is called a bistable because it is stable in two states: output high and output low.

• It is also known as a 'flip-flop'.

• It has two inputs:

- Trigger (555 pin 2) makes the output high.

Trigger is 'active low', it functions when < 1/3 Vs.

- Reset (555 pin 4) makes the output low.

Reset is 'active low', it resets when < 0.7V.

• The power-on reset, power-on trigger and edge-triggering

circuits can all be used as described above for the monostable.

4.4 BISTABLE MULTIVIBRATOR (cont…)

4.4 BISTABLE MULTIVIBRATOR (cont…)

• Inverting Buffer (Schmitt trigger) also known as NOT gate.

• The buffer circuit's input has a very high impedance (about 1M ) so it requires only a few μA, but the output can sink or source up to 200mA.

• This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).

• It is an inverting buffer or NOT gate because the

output logic state (low/high) is the inverse of

the input state:

- Input low (< 1/3 Vs) makes output high, +Vs

- Input high (> 2/3 Vs) makes output low, 0V

4.5 BUFFER – SCHMITT TRIGGER

• When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state.

• This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage.

• This type of circuit is called a Schmitt trigger.

• If high sensitivity is required the hysteresis is a problem,

but in many circuits it is a helpful property.

• It gives the input a high immunity to noise because

once the circuit output has switched high or low the

input must change back by at least 1/3 Vs to make the

output switch back.

4.5 BUFFER – SCHMITT TRIGGER (cont…)

Operation for monostable multivibrator:

• The defining characteristic of any Schmitt Trigger is its hysteresis.

• In this case, it is 1/3 and 2/3 of the power supply voltage, defined by the built in resistor voltage divider on the 555.

• The built in comparators C1 and C2 compare the input voltage to the references provided by the voltage divider and use the comparison to trip the built in flip flop, which drives the

output driver, another nice feature of the 555.


Operation for monostable multivibrator: (cont…)

• The 555 can drive up to 200 mA off either side of the power supply rail, the output driver creates a very low conduction path to either side of the power supply connections.

• The 5KΩ resistors are not very accurate.

• It is interesting to note that IC fabrication doesn't generally allow precision resistors, but the resistors compared to each

other are extremely close in value, which is critical to

the circuits operation.


Functional Schematic



The Schmitt Trigger


schmitt trigger.swf





schmitt trigger.swf


The Duty Cycle of a Multivibrator


multivibrators.swf

4.6 DUTY CYCLE MULTIVIBRATOR




multivibrators.swf

4.7 556 TIMER

• The 556 consists of a pair of 555 timers in one package.

• The two timers work independently and only share common power supply connections.

• The circuit drives a piezo sounder to provide a pulsed bleeping sound (similar to the reversing warning on many vehicles).

• One half of the 556 is running at the resonant frequency of the piezo sounder, around 3-4KHz, while the other half is

running at a very low frequency of about 0.5Hz.

• The output from the low frequency oscillator is

connected to the reset pin of the high frequency

oscillator to modulate the output.

4.7 556 TIMER (cont…)

Example application circuit

Schematic diagram

1) Sketch the circuit of a 555 timer connected as an astablemultivibrator for operation at 350 kHz. Determine the value of capacitor, C, needed using RA = RB = 7.5 kΩ.

2) Draw the circuit of a one-shot using a 555 timer to provide one time period of 20 µs. If RA = 7.5 k Ω, what value of C is needed?

3) Sketch the input and output waveforms for a one-shot

using a 555 timer triggered by a 10 kHz clock for

RA = 5.1 k Ω and C = 5 nF.

EXERCISES

chapter 4 : timer - dunia ilmiah · chapter 4 : timer lecturer : muhammad muizz bin mohd nawawi...

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