555 Timer & Digital Clock

Noor-Ul-HaqHyder Abbas

Rizwan AhmedAsad Aziz

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

Pins• There are 8 pins in a 555 timer IC.1. Ground2. Trigger3. Output4. Reset5. Control6. Threshold7. Discharge8. Supply

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.

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).

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.

Modes of 555 Timer

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

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.

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.

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.

Digital Clock Using 555 timer IC

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.

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

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

• http://www.learningaboutelectronics.com/Articles/555-timer-clock-circuit.php

• http://www.engineersgarage.com/contribution/digital-clock-without-microcontroller

• http://ehelion.net/projects/digitalclock/clock.html