1.Introduction 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. 2.Pin Configuration 3.Working of Pin 4.555 Integral circuit 5.Operating modes of IC 6. Bistable Mode 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 μF). Pin 7 (discharge) is left floating 7.Monostable 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. 8.Astable Mode Astable 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. 9.Applications Schmitt trigger PPM PWM Linear Ramp generator Precision Timing Pulse Generation Time Delay Generation Sequential Timing Used as a quad timer 10. Conclusion 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.

1. By
-Abhijit H Jadhav (Roll no 02)
-(Roll no 58)
*IC 555

2.  Introduction
 Pin Configuration
 Purpose of pin
 555 Integral circuit
 Operating modes
 Bistable
 Monostable
 Astable
 Application
 Conclusion
 Biblography
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3.  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.
*

4. •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
*

5. *
Pin 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.

6. *

7.  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.
*

8.  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 μF). Pin 7 (discharge) is left floating
*

9.  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.
*

10. 1
3
+VCC
7
6
2
8
One-shot operation
R
C
1/3 VCC
R
S
input trigger
Discharge
transistor
Q
Inverter
The input trigger resets the flip-flop and C then charges
until the upper comparator trips and sets the flip-flop.

11. *
 Astable 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.

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

13. 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.
*

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