digital electronics

1. University polytechnic
JAMIAMILLIAISLAMIA
NEWDELHI
SESSION 2022-2023
DIGITAL ELECTRONICS ANDMICROPROCESSOR (DEL 305)
TOPIC:- IC 555
SUBMITTED BY-VIKAS KANNAUJIYA
SUBMITTED TO-Mr. SHABBIR AHMAD

2. IC 555
INTRODUCTION (www.electronicsforu.com)
The 555 timer IC is an integral part of electronics projects. Be it a simple project involving a single 8-bit
micro-controller and some peripherals or a complex one involving system on chips (SoCs), 555 timer
working is involved. These provide time delays, as an oscillator and as a flip-flop element among other
applications.
Introduced in 1971 by the American company Signetics, the 555 is still in widespread use due to its low price,
ease of use and stability. It is made by many companies in the original bipolar and low-power CMOS types.
According to an estimate, a billion units were manufactured back in the year 2003 alone.
Depending on the manufacturer, the standard 555 timer package includes 25 transistors, 2 diodes and 15
resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). Variants consist of
combining multiple chips on one board. However, 555 is still the most popular. Let us look at the pin diagram
to have an idea about the timer Integrated Circuit (IC) before we talk about 555 timer working.

3. IC555
555 Timer IC:Block Diagram 8 Pin Configuration

4. Pin 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 1/2 of CTRL voltage (which is typically 1/3 Vcc, CTRL being 2/3 Vcc by default if CTRL is left open). In other
words, OUT is high as long as the trigger low. Output of the timer totally depends upon the amplitude of the external trigger voltage applied to this pin.
3 OUT This output is driven to approximately 1.7 V below +Vcc, or to GND.
4 RESET A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides threshold.
5 CTRL Provides “control” access to the internal voltage divider (by default, 2/3 Vcc).
6 THR The timing (OUT high) interval ends when the voltage at threshold is greater than that at CTRL (2/3 Vcc if CTRL is open).
7 DIS Open collector output which may discharge a capacitor between intervals. In phase with output.
8 Vcc Positive supply voltage, which is usually between 3 and 15 V depending on the variation.
Pin diagram and description (www.electronicsforu.com)

5. Some important features of the 555 timer:
555 timer is used in almost every electronic circuit today. For a 555 timer working as a flip flop or as a multi-
vibrator, it has a particular set of configurations. Some of the major features of the 555 timer would be,
 It operates from a wide range of power ranging from +5 Volts to +18 Volts supply voltage.
 Sinking or sourcing 200 mA of load current.
 The external components should be selected properly so that the timing intervals can be made into several
minutes along with the frequencies exceeding several hundred kilohertz.
 The output pin of a 555 timer can drive a transistor-transistor logic (TTL) due to its high current output.
 It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature which is
equivalent to 0.005 %/ °C.
 The duty cycle of the timer is adjustable.
 Also, the maximum power dissipation per package is 600 mW and its trigger pulse and reset inputs has logic
compatibility.

6. NAME (www.wikipedia.com)
Several books report the name "555" derived from the three 5 kΩ resistors inside the chip. However, in a
recorded interview with an online transistor museum curator, Hans Camenzind said "It was just arbitrarily
chosen. It was Art Fury (marketing manager) who thought the circuit was gonna sell big who picked the name
'555'.
555 Timer working
The 555 generally operates in 3 modes:
A-stable.
Mono-stable.
Bi-stable modes.

7. Astable mode (www.wikipedia.com)
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. The 555 can be used as a
simple ADC, converting an analog value to a pulse length (e.g., selecting a thermistor as timing resistor allows
the use of the 555 in a temperature sensor with the period of the output pulse 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.
In the astable configuration, the 555 timer puts out a continuous stream of rectangular pulses having a specific
frequency. The astable configuration is implemented using two resistors R1 and R2. and one capacitor C. In this
configuration, the control pin is not used, thus it is connected to ground through a 10 nF decoupling shunt
electrical noise. The threshold and trigger pin are connected to the capacitor C; thus they have the same
voltage.
Initially, the capacitor C is not charged, thus the trigger pin receives zero voltage, which is less than 1/3 of the
supply voltage. Consequently, the trigger pin causes the output to go high and the internal discharge transistor
to go to cut-off mode. Since the discharge pin is no longer short-circuited to ground, the current flows through
the resistors R1 and R2 to the capacitor, charging it. The capacitor C starts charging until the voltage become
2/3 of the supply voltage.

8. At that time, the threshold pin causes the output to go low and the internal discharge transistor to go into
saturation mode. Consequently, the capacitor starts discharging through R2 until it becomes less than 1/3 of the
supply voltage, at which point the trigger pin causes the output to go high and the internal discharge transistor to
go to cut-off mode once again. And the cycle repeats.

9. Monostable (www.wikipedia.com)
In this mode, the 555 functions as a "one-shot" pulse generator. Applications include timers, missing pulse
detection, bounce-free switches, touch switches, frequency dividers, capacitance measurement, pulse-width
modulation (PWM), and so on.
In monostable mode, the output pulse ends when the voltage on the capacitor equals 2/3of the supply voltage. The
output pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of
R and C

10. Bistable (www.wikipedia.com)
In bistable mode, the 555 timer acts as an SR flip-flop. The trigger and reset inputs are held high via pull-up
resistors while the threshold input is grounded. Thus configured, pulling the trigger momentarily to ground acts as
a "set" and transitions the output pin 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 are required in a bistable configuration. The
discharge pin is left unconnected or may be used as an open-collector output.

11. Schmitt trigger (www.wikipedia.com)
A 555 timer can be used to create a Schmitt trigger inverter gate which converts a noisy input into a clean digital
output. The input signal should be connected through a series capacitor, which then connects to the trigger and
threshold pins. A resistor divider, from VCC to GND, is connected to the previous tied pins. The reset pin is tied
to VCC.