NE555 timer

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The integrated circuit NE 555 enables the astable and monostable functions of long duration which allow the realization of time delays ranging from a few microseconds to a few hours.

 Its performance and ease of use have opened up areas long reserved for electromechanics. The NE 555, which is in the form of an integrated circuit with 8 terminals, represents today one of the most used solutions for the generation of rectangular signals.


ic 555 pin diagram

As shown in the pinout in Fig1, the 8 terminals of circuit NE 555 are:

1 ground (0 V supply)

2 trigger input

3 the main outlet

4 Reset input (Reset to Zero)

5 control output

6 threshold input

7 unloading exit

8 power supply (voltage VCC)



the internal principle of 555:

the Circuit NE 555 therefore has 3 inputs, 3 outputs, and 2 power supply terminals.

We will call V2 the voltage present on terminal 2 (trigger), V6 the voltage on terminal 6 (threshold), V5 the voltage on terminal 5 (control), and Vo the voltage on terminal 3 (at the circuit output). Inside the NE 555 are several components. The internal electronic circuit of the NE 555 is as follows:

In Fig2, the internal electronic circuit shows us that the NE 555 is made up of:

3 resistors of the same value R

2 voltage comparators C1 and C2

an RS flip-flop, with 2 setting inputs (S and Set)

an inverter whose output is connected to terminal n ° 3 of the NE 555

a transistor whose collector is connected to terminal n ° 7 of the NE 555


In monostable mode


In monostable configuration, the 555 can generate a pulse of a defined duration only (the signal will not be symmetrical) using a single resistor R and a capacitor. The term monostable means that the timer has only one state which is stable.

A pulse is generated following the application of a falling edge at the input of the circuit (TRIG), Immediately after the application of the falling edge, the internal latch as well as the output are activated. At the same time, the discharge transistor is deactivated allowing the capacitor C to charge through the resistor R. 

The waveform at the terminals of the capacitor is that of a first-order circuit RC facing a voltage step, c is to say an increasing exponential. When this exponential reaches a value equal to 2/3 of the supply voltage Vcc, the internal flip-flop is deactivated, bringing the output and the capacitor to zero

In Astable multivibrator mode



This configuration allows you to use the 555 as an oscillator that will generate a square wave on its output. The word astable means that the timer has no stable state.

The resistors R1 and R2 as well as the capacitor C allow to modify the oscillation frequency and the  duty cycle .

A full oscillation is performed when the capacitor charges to 2/3 Vcc and discharges to 1/3 Vcc (voltage applied to the input of the 555). When charging, resistors R1 and R2 are in series with the capacitor, but discharge occurs through Rb only. This is how the duty cycle can be changed.



sources:

https://lecafedugeek.fr/le-ne555-ce-petit-circuit-integre-indispensable/

http://www.gecif.net/articles/genie_electrique/cours/terminale/exercices/le_circuit_integre_ne555.pdf

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