Engr.

Che Peter Suh Series Lab-SUHTRONICS

555 TIMER

GENERAL INTRODUCTION

The 555 Timer, designed by Hans Camenzind

in 1971, can be found in many electronic
devices starting from toys and kitchen
appliances to even a spacecraft. It is a highly
stable integrated circuit that can produce
accurate time delays and oscillations. The 555
Timer has three operating modes, bistable,
monostable and astable mode. Other used
include LED and lamp flashers, pulse
generation, logic clocks, tone generation,
security alarms, pulse-position modulation

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Engr. Che Peter Suh Series Lab-SUHTRONICS

BLOCK DIAGRAM OF 555 TIMER

The voltage divider consists of three identical 5k resistors which create two reference voltages at 1/3 and 2/3 of the supplied voltage, which can
range from 5 to 15V.

Next are the two comparators. A comparator is a circuit element that compares two analogue input voltages at its positive (non-inverting) and
negative (inverting) input terminal. If the input voltage at the positive terminal is higher than the input voltage at the negative terminal the

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Engr. Che Peter Suh Series Lab-SUHTRONICS

comparator will output 1. Vice versa, if the voltage at the negative input terminal is higher than the voltage at the positive terminal, the
comparator will output 0.

The first comparator negative input terminal is connected to the 2/3 reference voltage at the voltage divider and the external “control” pin, while
the positive input terminal to the external “Threshold” pin.

On the other hand, the second comparator negative input terminal is connected to the “Trigger” pin, while the positive input terminal to the 1/3
reference voltage at the voltage divider.

So using the three pins, Trigger, Threshold and Control, we can control the output of the two comparators which are then fed to the R and S
inputs of the flip-flop. The flip-flop will output 1 when R is 0 and S is 1, and vice versa, it will output 0 when R is 1 and S is 0. Additionally the
flip-flop can be reset via the external pin called “Reset” which can override the two inputs, thus reset the entire timer at any time.

The Q-bar output of the flip-flip goes to the output stage or the output drivers which can either source or sink a current of 200mA to the load.
The output of the flip-flip is also connected to a transistor that connects the “Discharge” pin to ground.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

BISTABLE MODE

The Trigger and the Reset pins of the IC are connected to VCC through the two resistors, and it that way they are always high. The two
pushbuttons are connected between these pins and the ground, so if we hold them pushed the input state will be low.

Initially, the two comparators outputs are 0, thus the flip-flop output as well as the output of the 555 Timer are 0.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

If we press the Trigger pushbutton, the state at the Trigger input will become Low, so the comparator will output High and that will make flip-
flip Q-bar output go Low. The output stage will invert this and the final output of the 555 Timer will be High.

The output will remain high even when the trigger pushbutton is not pushed because in that case the flip-flop inputs R and S will be 0 which
means that the flip-flop won’t change the previous state. For making the output Low we need press the Reset pushbutton, which resets the flip-
flop and the entire IC.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

MONOSTABLE MODE

The trigger input is held High by connecting it to VCC through a resistor. That means that the trigger comparator will output 0 to the S input of
the flip-flop. On the other hand, the Threshold pin is Low and that makes the Threshold comparator out 0 as well. The Threshold pin is actually
Low because the Q-bar output of the flip-flop is High, which keeps the discharge transistor active, so the voltage coming from the source is
going to ground through that transistor.

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In order to change the 555 Timer output state to High we need to press the pushbutton on trigger pin. That will ground the trigger pin, or the
input state will be 0, thus the comparator will output 1 to the S input of the flip-flip. This will cause the Q-bar output to go Low and the 555
Timer output High. At the same time, we can notice that the discharge transistor is turned off, so now the capacitor C1 will start charging
through the resistor R1.

The 555 Timer will remain in this state until the voltage across the capacitor reaches 2/3 of the supplied voltage. In that case, the Threshold input
voltage will be higher and the comparator will output 1 to the R input of the flip-flip. This will bring the circuit into the initial state. The Q-bar
output will become High, which will activate the discharge transistor as well as make the IC output Low again.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

So we can notice that the amount of time the output of the 555 Timer is High, depends on how much time the capacitor needs to charge to 2/3 of
the supplied voltage, and that depends on the values of both the capacitor C1 and the resistor R1. We can actually calculate this time with the
following formula, T=1.1*C1*R1.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

ASTABLE MODE

Next, let’s see how the 555 Timer works in an astable mode. In this mode the IC becomes an oscillator or also called Free Running
Multivibrator. It doesn’t have a stable state and continuously switches between High and Low without application of any external trigger. Here’s
an example circuit of the 555 Timer operating in astable mode.

We only need two resistors and a capacitor. The Trigger and Threshold pins are connected to each other s o there is no need of external trigger
pulse. Initially, the voltage source will start charging the capacitor through the Resistors R1 and R2. While charging the Trigger comparator will
output 1 because the input voltage at the Trigger pin is still lower than 1/3 of the supplied voltage. That means that the Q-bar output is 0 and the
discharge transistor is closed. At this time the output of the 555 Timer is High.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

Once the voltage across the capacitor reaches 1/3 of the supplied voltage, the Trigger comparator will output 0 but at this point that won’t do any
change as both R and S inputs of the flip-flop are 0. So the voltage across the capacitor will keep rising, and once it reaches 2/3 of the supplied
voltage, the Threshold comparator will output 1 to the R input of the flip-flop. This will active the discharging transistor and now the capacitor
will start discharging through the resistor R2 and the discharging transistor. At this moment the output of the 555 Timer is Low.

While discharging, the voltage across the capacitor starts to decline, and the Threshold comparator right away starts to output 0, which actually
doesn’t do any change as now both R and S inputs of the flip-flop are 0. But once the voltage across the capacitor drops to 1/3 of the supplied
voltage, the Trigger comparator will output 1. This will turn off the discharge transistor and the capacitor will start to charge again. So this
processes of charging and discharging between 2/3 and 1/3 of the supplied voltage will keep running on its own, thus producing a square wave
on the 555 Timer output.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

We can calculate the time the output is High and Low using the shown formulas. The High time depends the on the resistance of both R1 and
R2, as well as the capacitance of the capacitor. On the other hand, the Low time depends only on the resistance of R2 and the capacitance of the
capacitor. If we sum the High and Low times we will get the Period of one cycle. On the other hand, the frequency is how many times this
happens in one second, so one over the Period will give use the frequency of the square wave output.

If we make some modifications to this circuit, for example, change the R2 resistor with a variable resistor or a potentiometer, we can instantly
control the frequency and the duty cycles of the square wave. However, more on this in my next video where we will make a PWM speed
controller using 555 timer.

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Engr. Che Peter Suh Series Lab-SUHTRONICS

Parameter specification

556 dual 558 quad timer

Partial list of differences between 558 and 555 chips:

One VCC and one GND, similar to 556 chip.
Four "Reset" are tied together internally to one external pin (558).
Four "Control Voltage" are tied together internally to one external pin
(558).
Four "Triggers" are falling-edge sensitive (558), instead of level
sensitive (555).
The dual version is called 556. It The quad version is called 558 Two resistors in the voltage divider (558), instead of three resistors
features two complete 555 timers and has four reduced-functionality (555).
in a 14-pin package; only the two timers in a 16-pin package
power-supply pins are shared designed primarily for monostable
between the two timers. multivibrator applications

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