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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 user-friendly integrated circuit for both monostable and astable applications. Since this device was firstmade 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 left off.
This primer is about this fantastic timerwhich 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 see this 555 timer used in advanced and high-tech electroniccircuits, 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 will use a more simplified (but accurate) block diagram toexplain the internal organizations of the 555. So, lets start slowly and build it up from there.






The first type-number, in Table 1 on the left, represents the type which was/is preferred for military applications which have somewhat improved electrical and thermal characteristics over their commercial counterparts, but also a bit more expensive, and usually metal-can or ceramiccasing. 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 a14-pin DIP package, the 558 is a quad version with four 555's also in a 14 pin DIP case.




I nside the 555 timer, at fig. 3, are the equivalent of over 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 moreattractive 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 also withstand 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 1milli-amp (mA) or less. The initial monostable timing accuracy is typically within 1% of its calculated value, and exhibits negligible (0.1%/V) drift with supply 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. Asyou 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 are clearly defined and can be calculated given the values of resistance and capacitance.
The basic RC charging circuit is shown in fig. 4. Assume that the capacitor is initially discharged. When the switch is closed, the capacitor...