INTRODUCTION TO TIMER IC 555 & IT'S APPLICATIONS
INTRODUCTION
In this practical we observed about the basic functions and their performance of 555 timer IC. We used NE555 IC to investigate its performances as Monostable Multivibrator and Astable Multivibrator. For this The 555 timer which gets its name from the three 5kΩ resistors it uses to generate the two comparators reference voltage, is a very cheap, popular and useful precision timing device that can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing stabilized waveform of varying duty cycles from 50 to 100%.
The 555 timer chip is extremely robust and stable 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of applications such as one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies and converters etc. the internal circuit of the IC consists of over more than 16 resistors, 20 transistors, 2 diodes, a flip-flop and many other circuit components.
A simplified “block diagram” representing the internal circuitry of the 555 timer is given below with a brief explanation of each of its connecting pins to help provide a clearer understanding of how it works.
Pin 1 – Ground, the ground pin connects the 555 timer to the negative (0v) supply rail.
Pin 2 – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state.
Pin 3 – Output, the output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately V cc – 1.5V.
Pin 4 – Reset, this pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output.
Pin 5 – Control Voltage, this pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing circuit
Pin 6 – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
Pin 7 – Discharge, the discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
Pin 8 – Supply +V cc, this is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.
Monostable Multivibrators have only ONE stable state (hence their name: “Mono”), and produce a single output pulse when it is triggered externally. Monostable Multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit.
In electronic circuits, astable multivibrators are also known as Free-running Multivibrator as they do not require any additional inputs or external assistance to oscillate. Astable oscillators produce a continuous square wave from its output. In the 555 Oscillator circuit pin 2 and pin 6 are connected together allowing the circuit to re-trigger itself on each and every cycle allowing it to operate as a free running oscillator. During each cycle capacitor, C charges up through both timing resistors but discharges itself only through one resistor, as the other side of resistor is connected to the discharge terminal, pin 7.
Next tutorial will explain method of this practical.
Laahiru C. Fernando.
INTRODUCTION
In this practical we observed about the basic functions and their performance of 555 timer IC. We used NE555 IC to investigate its performances as Monostable Multivibrator and Astable Multivibrator. For this The 555 timer which gets its name from the three 5kΩ resistors it uses to generate the two comparators reference voltage, is a very cheap, popular and useful precision timing device that can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing stabilized waveform of varying duty cycles from 50 to 100%.
The 555 timer chip is extremely robust and stable 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of applications such as one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies and converters etc. the internal circuit of the IC consists of over more than 16 resistors, 20 transistors, 2 diodes, a flip-flop and many other circuit components.
A simplified “block diagram” representing the internal circuitry of the 555 timer is given below with a brief explanation of each of its connecting pins to help provide a clearer understanding of how it works.
Pin 1 – Ground, the ground pin connects the 555 timer to the negative (0v) supply rail.
Pin 2 – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state.
Pin 3 – Output, the output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately V cc – 1.5V.
Pin 4 – Reset, this pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output.
Pin 5 – Control Voltage, this pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing circuit
Pin 6 – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
Pin 7 – Discharge, the discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
Pin 8 – Supply +V cc, this is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.
Monostable Multivibrators have only ONE stable state (hence their name: “Mono”), and produce a single output pulse when it is triggered externally. Monostable Multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit.
In electronic circuits, astable multivibrators are also known as Free-running Multivibrator as they do not require any additional inputs or external assistance to oscillate. Astable oscillators produce a continuous square wave from its output. In the 555 Oscillator circuit pin 2 and pin 6 are connected together allowing the circuit to re-trigger itself on each and every cycle allowing it to operate as a free running oscillator. During each cycle capacitor, C charges up through both timing resistors but discharges itself only through one resistor, as the other side of resistor is connected to the discharge terminal, pin 7.
Next tutorial will explain method of this practical.
Laahiru C. Fernando.
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