sexta-feira, 24 de julho de 2009

O Eterno 555

O CI timer 555 existe à mais de 30 anos.  180px-Signetics_NE555N

O chip foi originalmente fabricado pela Signetics.  Nos primeiros três meses após a sua introdução (1972), foram vendidos mais de meio milhão deles.   Além disso, tem sido um sucesso: desde aquela época que o 555 tem sido CI mais popular vendido a cada ano!

Actualmente, faz sentido utilizar a versão CMOS deste CI, uma vez que consome muito menos energia.

Praticamente tudo o que está relacionado com o 555 pode ser encontrado em Pode ser descarregado deste site um programa, que calcula facilmente os valores para os componentes RC. O programa é adequado para ambos os modos ‘astable’ e ‘bistable’. Os botões de "ajuste" são usados para alternar entre a versão única 555 e a versão dupla (o 556). Quando um valor diferente é escolhido para C1, as resistências mudam automaticamente.555ast


Source:  [Link]






 555 timer IC 

NE555 Astable Multivibrator Frequency and Duty Cycle Calculator

555 and 556 Timer Circuits:


Duty cycle

The duty cycle of an astable circuit is the proportion of the complete cycle for which the output is high (the mark time). It is usually given as a percentage.

For a standard 555/556 astable circuit the mark time (Tm) must be greater than the space time (Ts), so the duty cycle must be at least 50%:

Duty cycle  =      Tm       =  R1 + R2

                       Tm + Ts       R1 + 2R2



To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 as shown in the diagram. This bypasses R2 during the charging (mark) part of the cycle so that Tm depends only on R1 and C1:

Tm = 0.7 × R1 × C1 (ignoring 0.7V across diode)
Ts  = 0.7 × R2 × C1 (unchanged)


Duty cycle with diode  =      Tm        R1 

                                         Tm + Ts     R1 + R2

Use a signal diode such as 1N4148.

PWM Speed Motor Controller:


This 555 timer based PWM controller features almost 0..100% pulse width regulation using R1, while keeping the oscillator frequency relatively stable. The frequency is dependent on values of R1 and C1, values shown will give a frequency range from about 170 to 200 Hz. Any 555 chip will do, CMOS is fine as well. Diodes are not critical, I used 1N4148. Total cost of parts is about $2. As the whole thing is quite trivial, it's very easy to build on prototyping board like I did (as you can see, one of capacitors - C1 is replaced with different value, 0.047 uF to be precise).

A Simple PWM Circuit Based on the 555 Timer:


One of the most fundamental problems in robotics is DC motor speed control. The most common method of speed control is PWM or pulse width modulation. Pulse width modulation is the process of switching the power to a device on and off at a given frequency, with varying on and off times. These on and off times are referred to as "duty cycle". The diagram below shows the waveforms of 10%, 50%, and 90% duty cycle signals.

When the circuit powers up, the trigger pin is LOW as capacitor C1 is discharged. This begins the oscillator cycle, causing the output to go HIGH.

When the output goes HIGH, capacitor C1 begins to charge through the right side of R1 and diode D2. When the voltage on C1 reaches 2/3 of +V, the threshold (pin 6) is activated, which in turn causes the output (pin 3), and discharge (pin 7) to go LOW.

When the output (pin 3) goes LOW, capacitor C1 starts to discharge through the left side of R1 and D1. When the voltage on C1 falls below 1/3 of +V, the output (pin 3) and discharge (pin 7) pins go HIGH, and the cycle repeats.

Pin 5 is not used for an external voltage input, so it is bypassed to ground with an 0.01uF capacitor.
Note the configuration of R1, D1, and D2. Capacitor C1 charges through one side of R1 and discharges through the other side. The sum of the charge and discharge resistance is always the same, therefore the wavelength of the output signal is constant. Only the duty cycle varies with R1.

The overall frequency of the PWM signal in this circuit is determined by the values of R1 and C1. In the schematic above, this has been set to 144 Hz.

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