by Marcial Wills
The 555 timer IC was introduced in 70's by the Signetics Corporation as the "SE555/NE555 IC Time Machine" and was also the first and only commercial timer integrated circuit 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 first made commercially available, a myrad of novel and unique circuits have been developed and presented in several trade, professional, and hobby publications.
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.
This document will show you what the 555 timer is and how to properly use it by itself or in combination with other solid state devices. This timer uses a maze of transistors, diodes and resistors and for this complex reason we are going to use a more simplified block diagram to explain the internal organizations of the 555.
Figures 1 and 2. 555 packages.
The 555, in figure 1, come in two packages, either the round metal-can or 'T' package, and the 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. |
Figure 3. 555's Block Diagram.
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Inside the 555 timer, at fig. 3, are the equivalent of a lot of transistors, resistors, and some diodes. The equivalent circuit, in block diagram, provides 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 15 volt, supply current 3 to 7 mA, and a Rise/Fall time of 100 ns.
The Threshold current determine the maximum value of Ra + Rb, and, for 15 volt operation the maximum total impedance for R (Ra +Rb) is about 20 Mega-ohm
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All IC timers rely upon an external capacitor to determine the off-on time intervals of the output pulses.
Remember that 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 figure 5.
Assume that the capacitor is initially discharged. When the switch is closed, the capacitor begins to charge through the resistor. The voltage across the capacitor rises from zero up to the value of the applied DC voltage. The charge and the discahrge curves for the circuit are shown in figures 6 and 7 respectively.
The time that it takes for the capacitor to charge to 63.33% of the applied voltage is known as the time constant (t). That time can be calculated with the simple expression:
t = R X C
Assume a resistor value of 1 MegaOhm and a capacitor value of 1uF (micro-Farad). The time constant in that case is:
t = 1,000,000 X 0.000001 = 1 second
Assume further that the applied voltage is 6 volts. That means that it will take one time constant for the voltage across the capacitor to reach 63.33% of the applied voltage, it means, that the capacitor charges to approximately 3.79 volts in one second.
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