This document discusses the 555 timer integrated circuit and its use in building a digital clock circuit. It provides details on the 555 timer pins and its three operating modes: astable, monostable, and bistable. It then describes a digital clock circuit built using a 555 timer chip that displays time on 7-segment displays and can be set using buttons. Links are provided for additional information on 555 timer clocks.

1. 555 Timer & Digital Clock
Noor-Ul-Haq
Hyder Abbas
Rizwan Ahmed
Asad Aziz

2. 555 Timer
• A 555 timer is one of the most famous IC used in
various timer circuits.
• It is the single chip version of ‘multivibrator’ circuit.
• A standard 555 IC consists of 25 transistors, 2 diodes
and 15 resistors on a silicon chip

4. Pins
• There are 8 pins in a 555 timer IC.
1. Ground
2. Trigger
3. Output
4. Reset
5. Control
6. Threshold
7. Discharge
8. Supply

5. Ground (Pin 1)
Connects to the 0v power supply.
Trigger (Pin 2)
Detects 1/3 of voltage to give output HIGH, which means that it
works when voltage drops to 1/3 of the supply voltage. It has
control over threshold (pin 6). If pin 2 is LOW, and pin 6
LOW, output is HIGH. If pin 6 is HIGH, and pin 2 LOW, output is
LOW.
Output (Pin 3)
Pin 3 is the output pin. The output is either low, which is very close
to 0 V, or high, which is close to the supply voltage that’s placed on
pin 8. The exact shape of the output — that is, how long it's high
and how long it's low, depends on the connections to the remaining
five pins.

6. Reset (Pin 4)
It resets the timing of the IC by grounding it’s input and to restart the
timing a voltage greater then 0.7V must be provided through pin 2
(trigger).
Control (Pin 5)
Provides "control" access to the internal voltage divider. By applying a
voltage to this pin, it is possible to vary the timing of the device.
Threshold (Pin 6)
The purpose of this pin is to monitor the voltage across the capacitor
that's discharged by pin 7(discharge). When this voltage reaches two
thirds of the supply voltage (Vcc), the timing cycle ends, and the output on
pin 3 goes low.
The timing interval ends when the voltage at THR ("threshold") is greater
than that at CTRL (2/3 VCC if CTRL is open).

7. Discharge (Pin 7)
This pin is used to discharge an external
capacitor that works in conjunction with a
resistor to control the timing interval. In most
circuits, pin 7 is connected to the supply voltage
through a resistor and to ground through a
capacitor.
Vcc (Pin 8)
Pin 8 is connected to the positive supply voltage. This
voltage must be at least 4.5 V and no greater than 15 V.

8. Modes of 555 Timer
• There are 3 Modes of 555 timer IC
• Astable mode
• Monostable mode
• Bistable Mode (or Schmitt Trigger)

9. Astable Mode
An Astable Circuit has no stable state - hence the
name "astable". The output continually switches
state between high and low without any intervention
from the user, called a 'square' wave. This type of
circuit could be used to give a mechanism
intermittent motion by switching a motor on and off
at regular intervals. It can also be used to flash lamps
and LEDs, and is useful as a 'clock' pulse for other
digital ICs and circuits.

10. Monostable mode
A Monostable Circuit produces one pulse of a set
length in response to a trigger input such as a push
button. The output of the circuit stays in the low
state until there is a trigger input, hence the name
"monostable" meaning "one stable state". his type of
circuit is ideal for use in a "push to operate" system
for a model displayed at exhibitions. A visitor can
push a button to start a model's mechanism moving,
and the mechanism will automatically switch off
after a set time.

11. Bistable Mode
A Bistable Mode or what is sometimes called
a Schmitt Trigger, has two stable states, high and low.
Taking the Trigger input low makes the output of the
circuit go into the high state. This type of circuit is
ideal for use in an automated model railway system
where the train is required to run back and forth
over the same piece of track.

12. Digital Clock Using 555 timer IC

13. Here is a closer view of the time 10:51, from left to right. This was the only way to make the
clock fit on one proto board. The periods on the 7 segment displays blinked with the seconds,
on one second and off one second so on and so on for 60 seconds.

14. This is the part of the circuit where the 555 chip is adding a timing frequency for the clock
to keep time with.

15. These two buttons here are for setting the time, one for the minutes and one for the hours.

16. • http://www.learningaboutelectronics.com/Art
icles/555-timer-clock-circuit.php
• http://www.engineersgarage.com/contributio
n/digital-clock-without-microcontroller
• http://ehelion.net/projects/digitalclock/clock.
html