Circuitos Timer Itm

555 Timer/Oscillator Tutorial

© by Tony van Roon

The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/
NE555 and was called "The IC Time Machine" and was also the very first and only commercial timer ic available. It provided circuit designers and hobby tinkerers with a relatively cheap, stable, and userfriendly integrated circuit for both monostableand astable applications. Since this device was first made commercially available, a myrad of novel and unique circuits have been developed and presented in several trade, professional, and hobby publications. The past ten years some manufacturers stopped making these timers because of competition or other reasons. Yet other companies, like NTE (a subdivision of Philips) picked up where some leftoff.

This primer is about this fantastic timer which is after 30 years still very popular and used in many
schematics. Although these days the CMOS version of this IC, like the Motorola MC1455, is mostly used, the regular type is still available, however there have been many improvements and variations in the circuitry. But all types are pin-for-pin plug compatible. Myself, every time I seethis 555 timer used in advanced and high-tech electronic circuits, I'm amazed. It is just incredible.

In this tutorial I will show you what exactly the 555 timer is and how to properly use it by itself or in
combination with other solid state devices without the requirement of an engineering degree. This timer uses a maze of transistors, diodes and resistors and for this complex reason I willuse a more simplified (but accurate) block diagram to explain the internal organizations of the 555. So, lets start slowly and build it up from there.

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555 Timer/Oscillator Tutorial

The first type-number, in Table 1 on the left, represents the type which was/is preferred for military applications which havesomewhat improved electrical and thermal characteristics over their commercial counterparts, but also a bit more expensive, and usually metal-can or ceramic casing. This is analogous to the 5400/7400 series convention for TTL integrated circuits.

The 555, in fig. 1 and fig. 2 above, come in two packages, either the round metal-can called the 'T'
package or the more familiar 8-pin DIP 'V' package.About 20-years ago the metal-can type was pretty much the standard (SE/NE types). The 556 timer is a dual 555 version and comes in a 14-pin DIP package, the 558 is a quad version with four 555's also in a 14 pin DIP case.

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555 Timer/Oscillator Tutorial

I nside the 555 timer, at fig. 3, are the equivalent ofover 20 transistors, 15 resistors, and 2 diodes, depending of the manufacturer. The equivalent circuit, in block diagram, providing the functions of control, triggering, level sensing or comparison, discharge, and power output. Some of the more attractive features of the 555 timer are: Supply voltage between 4.5 and 18 volt, supply current 3 to 6 mA, and a Rise/Fall time of 100 nSec. It can alsowithstand quite a bit of abuse. The Threshold current determine the maximum value of Ra + Rb. For 15 volt operation the maximum total resistance for R (Ra +Rb) is 20 Mega-ohm.

The supply current, when the output is 'high', is typically 1 milli-amp (mA) or less. The initial
monostable timing accuracy is typically within 1% of its calculated value, and exhibits negligible (0.1%/ V) drift withsupply voltage. Thus long-term supply variations can be ignored, and the temperature variation is only 50ppm/°C (0.005%/°C).

All IC timers rely upon an external capacitor to determine the off-on time intervals of the output
pulses. As you recall from your study of basic electronics, it takes a finite period of time for a capacitor (C) to charge or discharge through a resistor (R). Those times...