How 555 timers Work - The Learning Circuit

Summary

In this episode of 'The Learning Circuit,' Karen explores the 555 timer, a versatile and popular IC in hobby electronics. She delves into the internal workings of the 8-pin chip, detailing its components like comparators, flip-flop, and voltage divider. Karen explains the function of each pin and how they interact to create a timer, oscillator, or flip-flop. The video promises a future episode on practical applications of the 555 timer in various circuit modes.

Takeaways

• 🌐 The 555 timer is a widely used integrated circuit (IC) in hobby electronics, known for its versatility in various applications.
• 🔌 It features 8 pins, including ground (pin 1), trigger (pin 2), output (pin 3), reset (pin 4), control voltage (pin 5), threshold (pin 6), discharge (pin 7), and VCC (pin 8).
• 🏭 Inside the 555 timer, there are two comparators, a flip-flop, an inverter, two transistors, and a voltage divider made up of three 5kΩ resistors, which might be the origin of the '555' name.
• 🔋 The voltage divider divides the supply voltage (VCC) and feeds the comparators, with typical VCC values ranging from 5 to 15 volts.
• ⚖️ Comparators compare voltages at their inputs and output a digital signal indicating which input is larger, playing a crucial role in the 555's operation.
• 🔄 The flip-flop within the 555 timer is of the SR type, with set and reset inputs that control the output state, and only the NOT Q output is utilized.
• 🔧 The output of the flip-flop determines the state of the 555 timer's output pin, which is inverted before being outputted, creating a toggling effect.
• 🔩 Pin 5, the control voltage pin, can adjust the voltage at the negative input of comparator one, allowing for some control over the 555's operation.
• 🔄 Pin 4, when connected to VCC, ensures the 555 timer functions correctly; if grounded, it triggers a reset, causing the output to go low.
• 🔌 Pin 7, the discharge pin, is used with external capacitors to control the timer's duration, discharging the capacitor when the circuit is reset.

Q & A

What is the 555 timer and what is its primary function?

The 555 timer is a widely used integrated circuit (IC) that functions as a timer, oscillator, or flip-flop. It is contained in an 8-pin IC and is popular in hobby circuits due to its versatility and ease of use.

How many pins does the 555 timer have and what are their typical functions?

The 555 timer has 8 pins, which serve various functions: pin 1 is for ground, pin 2 is the trigger, pin 3 is the output, pin 4 is for reset, pin 5 is the control voltage, pin 6 is the threshold, pin 7 is the discharge, and pin 8 connects to VCC.

What are the main components inside the 555 timer?

Inside the 555 timer, there are two comparators, a flip-flop, an inverter, two transistors, and a voltage divider made up of resistors.

How does the voltage divider inside the 555 timer work?

The voltage divider is composed of three 56 kilo-ohm resistors and divides the supply voltage, feeding two comparators. It typically divides the VCC voltage so that one comparator receives two-thirds of the supply voltage and the other receives one-third.

What is the typical supply voltage range for the 555 timer?

The typical supply voltage range for the 555 timer is from 4.5 to 16 volts, but VCC is usually between 5 and 15 volts.

How do the comparators in the 555 timer function?

The comparators compare the voltages at their inputs and output a digital signal indicating which input is larger. If the positive input is larger, the output is high; if the negative input is larger, the output is low.

How does the flip-flop within the 555 timer contribute to its timing function?

The flip-flop in the 555 timer has set and reset inputs that control its output. The flip-flop's output is used to control the timing function by setting and resetting based on the comparator outputs.

What is the purpose of the discharge pin (pin 7) on the 555 timer?

Pin 7, the discharge pin, is used with external capacitors to control the timing duration of the timer. It connects to a transistor whose base is controlled by the flip-flop output, allowing the capacitor to discharge quickly when the circuit is reset.

How can the control voltage pin (pin 5) be used to modify the 555 timer's behavior?

Pin 5, the control voltage pin, can be used to adjust the voltage at the negative input of comparator one, which typically receives two-thirds of VCC. This pin can be used to modify the timing behavior of the timer in certain applications.

What happens when both comparators in the 555 timer output high?

When both comparators output high simultaneously, it is considered an invalid state because it causes the flip-flop's outputs to behave unpredictably. This situation should be avoided in proper circuit design.

Outlines

00:00

💡 Introduction to the 555 Timer

Karen introduces the 555 timer, a popular integrated circuit (IC) used in hobby circuits for its versatility as a timer, oscillator, or flip-flop. The video aims to demystify the inner workings of the 555 timer by examining its 8 pins: 1 for ground, 2 for trigger, 3 for output, 5 for control voltage, 6 for threshold, 7 for discharge, and 8 for VCC connection. The internal components are explored, including resistors, transistors, two comparators, a flip-flop, an inverter, and a voltage divider made from 3x 5k ohm resistors, which might be the origin of the '555' name. The voltage divider's function is explained, dividing the supply voltage into two-thirds and one-third for the comparators' inputs. The comparators' operation is detailed, with comparator one outputting high when the threshold pin (6) exceeds two-thirds VCC and comparator two outputting high when the trigger pin (2) is below one-third VCC. The flip-flop's role in the circuit is also explained, with its set and reset inputs controlling the output based on the comparators' signals.

05:01

🔄 Understanding the 555 Timer's Operation

This section delves deeper into the 555 timer's operation, focusing on the flip-flop's need to be reset to produce more than one timer tick. The process involves the trigger pin (2) going above three volts to reset the flip-flop, followed by the threshold pin (6) exceeding six volts to reset the flip-flop again. The importance of avoiding invalid states where both comparators output high is emphasized, as it leads to unpredictable behavior. The signal path from input to output is detailed, with the flip-flop's output passing through an inverter before appearing at pin 3. Additional pins' functions are explained: pin 5 for control voltage, which can adjust the comparators' input voltages, and pin 4 for resetting the circuit by grounding it. Lastly, pin 7's role in discharging external capacitors to control the timer's duration is discussed, with the explanation of how the flip-flop output controls the charging and discharging of the capacitor. The video concludes with a teaser for future episodes that will cover the 555 timer's applications in various circuit types and modes.

Keywords

💡555 Timer

The 555 Timer is a widely used integrated circuit (IC) known for its versatility in various electronic applications. It's often employed in hobby circuits due to its ease of use and the availability of numerous schematics. In the video, the 555 timer is described as a switching circuit contained in an 8-pin IC that can function as a timer, oscillator, or flip-flop, showcasing its central role in the discussion.

💡Pin Configuration

Pin configuration refers to the specific arrangement and function of the pins on an IC like the 555 timer. Each pin serves a distinct purpose, such as ground, trigger, output, reset, control voltage, threshold, discharge, and VCC connection. Understanding pin configuration is crucial for correctly interfacing and utilizing the 555 timer in circuits, as the video explains in detail.

💡Comparators

Comparators are electronic components that compare two voltage levels and output a digital signal indicating which input is larger. In the context of the 555 timer, there are two comparators that play a critical role in determining the output state based on the voltage levels at their inputs. The video describes how these comparators interact with the voltage divider to set the thresholds for the timer's operation.

💡Flip-Flop

A flip-flop is a type of bistable multivibrator that holds and outputs one of two stable states. In the 555 timer, the flip-flop is part of the internal circuitry that toggles between states based on the inputs from the comparators. The video explains how the flip-flop's set and reset inputs are used to control the timer's output, which is a key aspect of its functionality.

💡Voltage Divider

A voltage divider is a passive linear circuit that produces an output voltage that is a fraction of its input voltage. In the 555 timer, the voltage divider is made up of resistors and is connected to power and ground, dividing the supply voltage to feed the comparators. The video highlights that the voltage divider is crucial for setting the reference voltages for the comparators.

💡Control Voltage (Pin 5)

Pin 5 of the 555 timer is used to adjust the control voltage, which affects the timing behavior of the circuit. The video mentions that this pin can be used to modify the voltage at the negative input of comparator one, thus providing a means to fine-tune the timer's operation.

💡Reset (Pin 4)

The reset function in the 555 timer is facilitated through pin 4, which when connected to VCC, ensures the timer operates correctly. If pin 4 is grounded, it triggers a reset, causing the output to go low. The video script uses this pin as an example to explain how external control can influence the timer's behavior.

💡Discharge (Pin 7)

Pin 7 on the 555 timer is associated with the discharge function, which is integral when using the timer in conjunction with external capacitors to control the timing duration. The video explains that this pin is connected to a transistor that, when turned on, allows the capacitor to discharge quickly, which is a critical aspect of the timer's operation in certain circuit configurations.

💡Oscillator

An oscillator is an electronic circuit that produces a periodic signal, and the 555 timer can be configured to function as one. The video touches on this capability of the 555 timer, emphasizing its adaptability to generate signals that can be used in various applications, such as generating a clock signal for digital circuits.

💡Monostable and Bistable Modes

The video script introduces the concepts of monostable and bistable modes, which refer to the operational modes of the 555 timer. In a monostable mode, the timer produces a single output pulse in response to a trigger, while in bistable mode, it maintains its state until a trigger is received to change it. These modes are fundamental to understanding the diverse applications of the 555 timer in electronic circuits.

Highlights

Introduction to the 555 timer, a popular IC used in hobby circuits.

The 555 timer can act as a timer, oscillator, or flip-flop.

Description of the 8 pins of the 555 timer IC.

Explanation of the internal components of the 555 timer, including resistors, transistors, comparators, a flip-flop, and an inverter.

The voltage divider inside the 555 timer is made of three 5 kilo-ohm resistors, contributing to the IC's name.

The supply voltage is divided by the voltage divider to feed the comparators.

Comparators compare voltages and output digital signals indicating which input is larger.

How the flip-flop within the 555 timer is set and reset by the comparators.

The importance of not having both comparators output high simultaneously to avoid unpredictable behavior.

The role of the output pin in the 555 timer and how it is affected by the flip-flop's output.

The function of pin 5, the control voltage pin, and its impact on the comparator's input.

Pin 4's role in resetting the circuit and its connection to VCC.

The use of pin 7, the discharge pin, in conjunction with external capacitors to control the timer duration.

The charging and discharging of capacitors through pin 7 and its effect on the timer's operation.

Upcoming episode teaser on how the 555 timer can be used in various circuit types and modes.

Invitation for viewers to join the element14 community for further learning and support.

Transcripts

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hi and welcome back to the learning

circuit

I'm Karen and today we're going to learn

about a very useful I see the 555 timer

[Music]

the 555 timer is one of the most popular

ICS to use in hobby circuits it's easy

to find schematics that show any one of

the numerous ways to use the 555 but to

better understand how they work in

circuits I want to talk about what's

happening inside the 5 5 5 the 5 5 5

timer is a switching circuit contained

in an 8 pin IC that can act as a timer

oscillator or flip-flop let's take a

look at those 8 pins as usual

we can find pin 1 to the left of the

notch or dot on the surface of the chip

pin 1 is for ground 2 is the trigger 3

the output for reset 5 control voltage 6

threshold 7 discharge an 8 for

connecting to VCC we can look at what's

inside the 555 to explore what all those

pins do the 555 is made up of resistors

and transistors but those can be broken

up into various main components here's a

block diagram of those components inside

the 555 there are two comparators a

flip-flop and inverter two transistors

and a voltage divider looking closer at

the voltage divider you can see that it

is made of 3 5 kilo ohm resistors some

believe this is where the 5 i5 gets its

name the voltage divider is connected to

power and ground dividing the supply

voltage and feeding the two comparators

since the resistors are all the same

value the supply voltage or VCC is

divided with the input of comparator 1

getting two thirds supply voltage and

the input of comparator two getting one

third supply voltage while the supply

voltage range is typically four point

five to 16 volts VCC is usually between

5 and 15 volts let's say our circuit is

running on a 9-volt battery therefore

VCC is 9 volts from the voltage divider

the negative input of comparator 1 would

be 2/3 of that 6 volts while the

positive input of comparator 2 would be

1/3 of that 3 volts the positive second

input of comparator 1 goes to pin 6 the

threshold and the negative second input

of comparator 2 goes to pin 2 the

trigger a comparator is a device that

compares the voltages at its inputs and

outputs a digital signal indicating

which of its inputs is larger

positive input is larger the output is

high if the negative input is larger the

output is low let's take a look at

comparator one the positive input is

connected to pin six threshold and the

negative input is connected to the

voltage divider with nine volts VCC the

negative input has six volts well the

negative input at six volts is larger

the comparator will output low but if

the voltage at pin six is larger above

six volts the comparator will output

high if pin six ever drops below six

volts making the negative input larger

the comparator output would revert back

to low now let's take a look at

comparator two on this comparator the

positive input is connected to the

voltage divider getting 1/3 of VCC so

with nine volts VCC that makes the input

three volts the negative input goes to

pin 2 the trigger if the voltage at that

pin is below three volts the output goes

high

if pin two goes above three volts

becoming the larger input the output

would go low and again as soon as pin

two drops below three volts the output

will revert to high the outputs of the

comparators go to the two inputs of the

flip-flop in a previous episode I

covered how SR flip-flops work but I'll

recap that for you real quick the

flip-flop has two inputs set and reset

marked by s and R and two outputs Q and

not Q naught U is always the inverse of

Q this SR flip-flop is active hi the set

input sets the output high while the

reset input resets the output back to

low when set goes high output Q goes

high with not Q being the inverse if

both inputs are low the outputs don't

change when reset goes high output Q

goes low with not you being the inverse

while flip-flops typically have two

outputs in the 555 timer only output not

Q is used let's look at our comparators

in action and how they set and reset the

flip-flop VCC is still nine volts pin

six needs to be lower than six volts for

comparator one toutput low when pin two

is lower than three

volts comparator two outputs hi this

sets the flip-flop and not Q output slow

the trigger at pin two acts like a

starter pistol starting the timer

setting the flip-flop but in order to

get more than one tick of the timer the

flip-flop needs to first be reset first

the voltage at pin two must go higher

than three volts so that compared to two

outputs low then when pin six goes

higher than six volts comparator one

outputs high this resets the flip-flop

and not Q goes high we learn in the flip

flop episode that we never want both

inputs SNR to go high at the same time

when that happens the outputs begin to

behave unpredictably so we never want

pin six to go high and pin two to go low

causing both comparators to output high

at the same time this state is

considered invalid so now the flip-flop

is outputting a digital signal high or

low and this goes to the output pin of

the 5-5-5 timer

first the signal passes through an

inverter then out to pin three this

effectively outputs what would be the

signal from output Q of the flip-flop

that covers the path of the signal from

input to output but we still have three

pins to cover pin five is control

voltage they connect to the negative

input of comparator one typically this

input has a voltage that's two-thirds

VCC but pin five can be used to adjust

or control that voltage in some use

cases it's ideal to have an easy way to

reset the circuit so we have pin four

pin four is connected to the base of a

transistor this transistor must be on

for the five high five to function so

pin four is usually connected to VCC to

hold it high if pin four is ever

grounded going low the transistor is

turned off this triggers the reset and

the five by five output goes low last is

pin 7 discharge it's used with external

capacitors that are integral in studying

the duration of the timer in the five

five five pin seven is connected to

another transistor whose base is

controlled by the flip-flop output when

the five five five output is high not Q

is low so the transistor is off this

usually causes the capacitor to charge

up and eventually triggers the reset

when the circuit is reset the transistor

turns on which connects the capacitor to

ground allowing it to quickly discharge

I've covered how the 555 timer works

internally but to fully understand how

some of the pins function they need to

be seen working in a circuit so in a

future episode I'm going to talk about

how the 555 timer can be used in various

types of circuits and modes like a

stable monostable and by stable if you

have any questions about what we learned

today

you can get help by posting to the

element14 community on element14.com

forward slash the learning circuit happy

learning

[Music]

you