IC timer 555

The 555 timer has been one of the popular microcircuits in the electronics world for more than 50 years. The microcircuit allows you to form a single pulse or a series of repetitive pulses with stable time characteristics.

Based on the 555 timer, you can create the most unusual devices with your own hands, ranging from the simplest one that controls the blinking of a light bulb to a multi-stage security alarm. And all this without a microcontroller and programming.

Where is the 555 timer chip used?

The microcircuit allows you to assemble light and sound alarms, an illumination meter, a voltage converter, a combination lock and other useful devices with your own hands. To get started, we recommend using our lessons with the 555 timer .

Want to try building 555 devices, but don't know where to start? Pay attention to our electronic designer Omka , in which the 555 microcircuit is one of the key components.

History of creation

The 555 timer begins its history in 1971, when the Signetics Corporation released the NE555 chip called the Integral Timer.

Initially, the microcircuit was produced in two types of cases: plastic and round metal. Now on sale microcircuits in a metal case no longer exist - there are only plastic ones.

The 555 timer became a unique "timer" microcircuit available to ordinary consumers, which made it possible to assemble miniature and inexpensive devices due to the dense arrangement of elements in the microcircuit crystal. Subsequently, other manufacturers from around the world began to copy it: Texas Instruments , STMicroelectronics , Diodes Inc and others. In the USSR, they released their own version of the timer - KR1006VI1 .

What does a microcircuit consist of?

Inside the crystal, the microcircuit contains about 20 transistors, 15 resistors, 2 diodes. The composition and number of elements may vary slightly depending on the manufacturer.

The principle of operation of the timer 555 chip

Conventionally, the NE555 microcircuit consists of six functional blocks:

voltage divider (1);
two comparators (2 and 3);
RS flip-flop (4);
inverting power amplifier (5);
open collector transistor at the output (6).

A resistive voltage divider (1) is located at the input, which forms two reference voltages for the comparators (2 and 3). The output contacts of the comparators are fed to an RS flip-flop (4) with an external output for reset, and then to a power amplifier (5). The last node is an open collector transistor (6), which can perform several functions, depending on the task at hand.

Chip marking and designation

The NE555 microcircuit and its analogs are available in an eight-pin DIP-8, TSSOP or SOIC package. The pin layout is standard regardless of the case.

Pin number	Designation	Appointment	Description
1	GND	Common Ground	The pin is connected to the common wire of the circuit or the minus of the power supply.
2	TRIG	Launch	Comparator input "B". When an arbitrary waveform with voltage less than ⅓ from "VCC" arrives at the "TRIG" input, the integral timer is started, as a result of which a logical unit appears at the "OUT" output.
3	OUT	Output	Timer output signal. The OUT pin generates one of two voltages, approximately corresponding to the GND or VCC levels, depending on the state of the timer.
4	RESET	Reset	When the supply voltage is less than 0.7 V at the “RESET” pin, the timer is reset and, accordingly, a logical zero will appear on the “OUT” pin. If a reset mode is not needed in the projected circuit, then it is desirable to connect the "RESET" pin to the plus of the power supply.
5	CTRL	Control	Additional timer control. When applying to the input "CTRL" a signal from 45 to 90% of "VCC", you can control the duration of the pulses at the output. This eliminates the need for an external RC circuit. If you are adjusting timing with an RC circuit, connect the CTRL pin to the negative of the circuit through a 10nF capacitor.
6	THR	Stop	Comparator input "A". When an arbitrary waveform with a voltage of more than ⅔ from "VCC" arrives at the "THR" input, the integral timer is stopped, as a result of which a logical zero appears at the "OUT" output.
7	DIS	Discharge	Output "DIS" to the collector of the output transistor. Used to discharge the timing of the driving capacitor between intervals. The states of this output repeat the states of the main output OUT, therefore it is used to increase the load capacity of the timer.
8	VCC	supply	Power supply of the microcircuit. The pin connects to the power wire of the circuit with a voltage of 4.5 to 16 volts.

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Connection diagrams Timer 555

The 555 timer supports three main modes of operation:

monostable mode (one-shot);
self-oscillating mode (multivibrator);
bistable mode (Schmitt trigger).

To calculate the timing of the timer, it is necessary to perform the mathematical calculations of the RC-chain. We advise you to use the 555 program (in the Documents folder), which will calculate the denomination of all components for you.

Monostable mode (one shot)

The mode is suitable for turning on the timer for a fixed time interval. For example, turning on the light for a specified time interval when a motion sensor is triggered.

The stable state of the timer is off, i.e. there is a logical zero on the "OUT" leg. By an arbitrary signal at the TRIG input, the microcircuit generates a single rectangular pulse (logical unit) at the OUT output. The pulse duration is determined by an external RC-chain of one capacitor and a resistor according to the formula:

T = 1.1 * R * C, where:

Letter	Description	Measurement scale
T	Pulse time	Second
R	Resistance rating	Ohm
WITH	Capacity rating	Farad

T is the pulse time in seconds;
R - resistance in Ohms;
C is the capacity in Farads.

Example of work

Let's consider a monostable mode using a simple example. Connect LED1 to the OUT output through the current-limiting resistor R2. And to start the timer, connect the SW1 clock button to the "TRIG" pin

Given:
R1 = 10 kΩ = 10000 Ω.
C1 = 220 μF = 220 * 0.000001 F = 0.00022 F.

Solution:
T = 1.1 * 10000 * 0.00022 = 2.2 seconds.

Algorithm

In normal mode, the LED1 LED is off.
When you click on the SW1 button, the signal on the "TRIG" pin starts the timer, a logical unit appears at the "OUT" output and, accordingly, the LED1 LED lights up.
After 2.2 seconds, the timer turns off and the LED1 LED goes out, waiting for the next click on the SW1 button.

Self-oscillating mode (multivibrator)

The multivibrator mode allows you to blink LEDs, control the speed of motors and generate sounds. The microcircuit produces a sequence of rectangular pulses at the "OUT" output, the parameters of which are determined by the RC chain of capacitor C1 and two resistors R1 and R2.

f = 1 / 0.693 * C * (R1 + 2 * R2)
t = 1 / f
t = t1 + t2
t1 = 0.693 * C * (R1 + R2)
t2 = 0.693 * C * R2, where:

Letter	Description	Measurement scale
f	Pulse frequency	Hertz
t	Pulse period	second
t1	Pulse length (logical unit)	second
t2	Pause length (logical zero)	second
R1, R2	Resistance rating	Ohm
WITH	Capacity rating	Farad

Example of work

Let us consider the self-oscillating mode using an example. Connect LED1 to the OUT output through the current-limiting resistor R3.

Given:
R1 = 10 kΩ = 10,000 Ω
R2 = 100 kΩ = 100,000 Ω
C1 = 22 μF = 0.000022 F

Solution:

Pulse frequency: f = 1 / 0.693 * C1 * (R1 + 2 * R2) = 0.693 * 0.000022 * (10000 + 2 * 100000) = 1 / 3.2 = 0.31 Hertz.
Pulse period: t = 1 / f = 1 / 0.31 = 3.2 seconds.
Pulse duration (logical unit): t1 = 0.693 * C1 * (R1 + R2) = 0.693 * 0.000022 * (10000 + 100000) = 1.67 seconds.
Pause length (logical zero): t2 = 0.693 * C1 * R2 = 0.693 * 0.000022 * 100000 = 1.52 seconds.

Algorithm

The LED with the current values of the RC-chain will light up for 1.67 seconds, go out for 1.52 seconds, and so on in a circle. The LED blinking frequency is 0.31 Hz and the period is 3.2 seconds.

Bistable mode (Schmitt trigger)

The latter is the bistable mode or Schmitt trigger. The mode is suitable for simply turning on and off devices by clicking on the buttons. There are no time slots in it, i.e. No RC chain needed.

Example of work

Let us consider the bistable mode using an example. Connect LED1 to the OUT output through the current-limiting resistor R3. And to control the light bulb, connect two buttons SW1 and SW2 to the "TRIG" and "RESET" pins.

Algorithm

In normal mode, the LED1 LED is off.
When you click on the SW1 button, the signal on the "TRIG" pin starts the timer, a logical unit appears at the "OUT" output and the LED1 LED lights up.
When you click on the SW2 button, the "RESET" signal resets the timer, a logical zero appears at the "OUT" output and, accordingly, the LED1 LED goes out.