From the past until now, the working principle component of this type does not change but each manufacturer makes the IC design and different technology. Almost all manufacturers make this type of component, although with different names. For example, National Semiconductor LM555 call it, Philips and Texas Instruments named SE/NE555. Motorola / ON-Semi designed with CMOS transistors are small enough so that komsusi powernya and named MC1455. Philips and Maxim make its CMOS version with the name of the ICM7555. Although the name is different, but the function and pin compatible with each other diagram with one another (functional and pin-to-pin compatible). It's just that there are several different specific characteristics such as power consumption, maximum frequency and so forth. More detailed specifications are usually listed on each manufacturer's datasheet. Number first, the casing is made with 8-pin T package (tabular from tin-like transistors), but is now more common with 8 pin DIP IC packaging. IC monostable circuit is designed in such a way that only requires a few external components to work. Among the main external resistors and capacitors (external). This IC is working with utilizing the principle of charging (charging) and discharging (discharging) of the capacitor through the external resistor. To explain the principle works, consider the diagram of IC 555 image with external resistors and capacitors below. This circuit is nothing but a timer circuit (timer) monostable. The principle of this circuit will produce a single pulse with a certain time on output pin 3, where pin 2 of this component is triggered. Notice in the IC, there are two comparators that Comp A and Comp B. Note also in the IC have 3 internal resistor R which are the same size. With such a composition series of VCC and GND, an internal resistor circuit is a voltage divider. This arrangement provides a reference voltage, each of magnitude 2 / 3 VCC at the comparator negative input of A and 1 / 3 VCC at the comparator positive input B.
in the absence of input, output pin 3 is 0 (ground or normally low). The transistor Q1 is in the IC is always ON and prevent external capacitor C of the process pengisisian (charging). When there is a trigger signal from 1 to 0 (VCC to GND) which is fed to pins 2 and less than 1 / 3 VCC, then necessarily the comparator B sets the output flip-flop. This in turn triggers the transistor Q1 becomes OFF. If the transistor Q1 OFF will pave the way for an external resistor R to start charging a capacitor C (charging). At the same time the output from pin 3 to high (VCC), and kept high until a certain time as desired. Call it the length is t seconds, the time needed to charge the capacitor C voltage reaches 2 / 3 VCC. C voltage is connected to pin 6 which is the positive input A. comp So if the voltage is 2 / 3 VCC is reached, the comparator A will to reset flip-flop and not necessarily the internal transistor Q1 becomes ON again. At the same time the output pin 3 of IC 555 is returned to 0 (GND). How long pulses generated very depending on the value resitor and attach an external capacitor. From the formula exponentially charging capacitor is known that: Vt = VCC (1 - et / RC) ... .. (1) Vt is the voltage at time t. If t is the exponential time required to fill the capacitor until Vt = 2 / 3 VCC, then the formula (1) can be substituted with this value: 2 / 3 = 1-et/RC 1 / 3 = et / RC ln (1 / 3) = -t/RC and so on can be obtained t = (1.0986123) RC are rounded to t = 1.1 RC This is the formula for counting the length of a single pulse output can be generated by monostable circuit of the IC 555. Few Astable circuit berdeda with monostable circuit, astable circuit is made by changing the composition resitor and capacitors outside the IC 555 as shown below. There are two resistors Ra and Rb as well as an external capacitor C is required. The principle is made to trigger astable circuit itself over and over again so that this circuit can produce oscillations in the output signal. When the power supply circuit in turn, began to fill the capacitor C through resistor Ra and Rb until the voltage reaches 2 / 3 VCC. At this voltage is reached, it is understandable comparator A of the IC 555 began working to reset flip-flop and beyond to make the transistor Q1 ON. When transisor ON, resitor Rb as a short-circuited to ground thus removing the load capacitor C (discharging) through the resistor Rb. At this time the output pin 3 to 0 (GND). When discharging, the voltage on pin 2 continue to fall until it reaches 1 / 3 VCC. When this voltage is reached, can be understood comparator B's turn to work and re-trigger the transistor Q1 becomes OFF. This causes the output pin 3 back into high (VCC). So forth repeatedly so that the output oscillation signal formed pin3. Seen here a trigger signal (trigger) the two comparators are worked alternately on the voltage between 1 / 3 VCC and 2 / 3 VCC. This is the limit to find the pulse width and period of oscillation is generated. For example it is assumed t1 is the time that the capacitor charging process in the content via resistors Ra and Rb than 1 / 3 VCC to 2 / 3 VCC. Assumed also is the time t2 the capacitor discharging through a resistor Rb from voltage 2 / 3 VCC to be 1 / 3 VCC. With the exponential calculation with the restriction 1 / 3 VCC and 2 / 3 VCC, it can be derived: t1 = ln (2) (Ra + Rb) C = 0693 (Ra + Rb) C and t2 = ln (2) RBC = 0693
Oscillator period is to be known by calculating T = t1 + t2. Percentage duty cycle of the oscillation signal generated was calculated from the formula t1 / T. So if the desired duty cycle oscillator for (near) 50%, it can be used resistor Ra is relatively much smaller than the resistor Rb. Closing One thing that is interesting from the components of IC 555, both timer monostable circuit applications as well as the frequency of oscillation of the astable circuit is independent of how the value of working voltage VCC is given. Working voltage IC 555 can vary between 5 to 15 VDC. The accuracy of the time (timing) generated depends on the value and tolerance of the external resistors and capacitors are used. For time series that are categorized as critical, typically used precision capacitors and resistors with a small tolerance. In many application notes, is usually also added a 10 nF capacitor on pin 5 to ground to ensure stability of the voltage reference 2 / 3 VCC. Many other applications can be made * with IC 555, one of the other popular applications is a series of PWM (Pulse Width Modulation). PWM circuit easily realized by slightly altering thefunction the monostable timer circuit . Namely by triggering the trigger pin (pin 2) is continuous, in accordance with the desired clock period, while the pulse width can be adjusted to provide variable voltage to control voltage pin (pin5). In the market many are also encountered two timers 555 are packed within a single IC such as IC LM556 or MC1456.
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