How 555 IC works (Astable Operation & Monostable Operation) - Black keyhole

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How 555 IC works (Astable Operation & Monostable Operation)

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The 555 timer IC is an incredibly useful precision timer that can be used as either a timer or an oscillator. In timer mode it works as monostable mode the 555 simply acts as a “one-shot” timer; when a trigger voltage is applied to its trigger lead, the chip’s output goes from low to high for a duration set by an external RC circuit. In oscillator mode it works as astable mode the 555 acts as a
rectangular-wave generator whose output waveform (low duration, high duration, frequency, etc.) can be adjusted by means of two external RC charge/discharge circuits.

It can be used in an stunning number of applications. for example with the aid of a 555, it is possible to create digital clock waveform generators, LEDand lamp flasher circuits, tone-generator circuits (sirens, metronomes, etc.), one-shot timer circuits, bounce-free switches, triangular-waveform generators, frequency dividers, etc.

Astable Operation

   Below block diagram showing what is inside a typical 555 timer IC. that represents the astable 555 configuration.
The 555 gets its name from the three 5-kW resistors shown in the block diagram. These resistors act as a three-step voltage divider between the supply voltage (VCC) and ground.
The top of the lower 5-kW resistor (+ input to comparator 2) is set to 1⁄3VCC, while the top of the
middle 5-kW resistor (- input to comparator 1) is set to 2⁄3VCC. The two comparators output either
a high or low voltage based on the analog voltages being compared at their inputs. If one
of the comparator’s positive inputs is more positive than its negative input, its output logic
level goes high; if the positive input voltage is less than the negative input voltage, the output logic level goes low. The outputs of the comparators are sent to the inputs of an SR (set/reset) flip-flop. The flip-flop looks at the R and S inputs and produces either a high or a low based on the voltage states at the inputs.

Monostable Operation
     The monostable mode has only one stable state. This means that for the output to switch states, an externally applied signal is needed. 

    In the monostable configuration, initially (before a trigger pulse is applied) the 555’s output is low, while the discharge transistor is on, shorting pin 7 to ground and keeping C discharged. Also, pin 2 is normally held high by the 10-k pull-up resistor. Now, when a negative-going trigger pulse (less than 1⁄3VCC) is applied to pin 2, comparator 2 is forced high, which sets the flip-flop’s
Q‾ to low, making the output high (due to the inverting buffer), while turning off the discharge transistor. This allows C to charge up via R1 from 0 V toward VCC. However, when the voltage across
the capacitor reaches 2⁄3VCC, comparator 1’s output goes high, resetting the flip-flop and making the output low, while turning on the discharge transistor, allowing the capacitor to quickly discharge toward 0 V. The output will be held in this stable state (low) until another trigger is applied.

 555 ICs are available in both bipolar and CMOS types. Bipolar 555s, like the ones you used in the preceding examples, use bipolar transistors inside, while CMOS 555s use MOSFET transistors instead. These two types of 555s also differ in terms of maximum output current, minimum supply voltage/current, minimum triggering current, and maximum switching speed. With the exception of maximum output current, the CMOS 555 surpasses the bipolar 555 in all regards. A CMOS 555 IC can be distinguished from a bipolar 555 by noting whether the part number contains a C somewhere within it (e.g., ICL7555, TLC555, LMC555, etc.). (Note that there are hybrid versions of the 555 that incorporate the best features of both the bipolar and CMOS technologies.)

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