By

-Abhijit H Jadhav (Roll no 02)

-(Roll no 58)

*IC 555

Introduction Pin Configuration Purpose of pin 555 Integral circuit Operating modes

Bistable Monostable Astable

Application Conclusion Biblography

*Overview

The 555 IC was designed in 1971 by Hans Camenzind under contract to SigNetics Corporation.

555 timer is a highly stable circuit used to generate time delays, or Oscillations.

A single 555 timer can provide time delay ranging from microseconds to hours.

It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts supply voltage.

*Introduction

•Pin–1:  Ground•Pin–2:  Trigger•Pin–3:  Output•Pin–4:  Reset•Pin–5:  Control Voltage•Pin–6:  Threshold•Pin–7:  Discharge•Pin–8:  Vcc

*Pin Configuration

*Purpose of PinsPin no Name Purpose

1 GND Ground reference voltage, low level (0 V)

2 TRIG The OUT pin goes high and a timing interval starts when this input falls below ½ of CTRL voltage.

3 OUT Driven approx. 1.7 V below Vcc or GND

4 RESET A timing interval may be reset by driving this input to GND.

5 CTRL Provides “control” access to the internal voltage divider (by default 2/3 Vcc) at THR is greater than that at CTRL

6 THR The timing interval ends when the voltage at THR is greater than that at CTRL

7 DIS Open collector output which may dishcharge the capacitor between interval in phase with output.

8 Vcc Positive supply voltage, which is usually between 3 and 15 V depending on the variation.

*555 Integral circuit

IC 555 has three operating modes:

1. Bistable mode or Schmitt trigger – the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.

2. Monostable mode – in this mode, the 555 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.

3. Astable (free-running) mode – the 555 can operate as an electronic oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on.

*Operating modes of IC

In bistable (also called Schmitt trigger) 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 floating.

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 timing capacitors

Pin 5 (control voltage) is connected to ground via a small-value capacitor (usually 0.01 to 0.1 F). Pin 7 (discharge) is left floatingμ

*Bistable Mode

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

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.

The timing period is triggered (started) when trigger input (555 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 (555 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 (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.

*Monostable Mode

1

3

+VCC

7

6

2

8One-shot operation

R

C

1/3 VCC

RS

input trigger

Dischargetransistor

 Q

Inverter

The input trigger resets the flip-flop and C then chargesuntil the upper comparator trips and sets the flip-flop.

*Astable ModeAstable multivibrators are

also known as Free-running Multivibrator.

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.

C charges through RA+ RB

and discharges through RB.

Schmitt trigger PPM PWM Linear Ramp generator Precision Timing Pulse Generation Time Delay Generation Sequential Timing Used as a quad timer

*Applications

Hence 555 IC timer can produce very accurate and stable time delays, from microseconds to hours. It can be used with supply voltage varying from 5 to 18 V. Timer can be used in monostable mode of operation or astable mode of operation. Its various applications include waveform generator, missing pulse detector, frequency divider, pulse width modulator, burglar alarm, FSK generator, ramp generator, pulse position modulator etc.

*Conclusion

www.wikipedia.org www.falstad.com www.microelectronics.com www.stelectronics.com www.IEEEsignalprocessing/IC555 www.falstad.com/circuit

*Biblography