555 Timer Ic

Topics: 555 timer IC, Pulse-width modulation, Integrated circuit Pages: 5 (1492 words) Published: April 2, 2011
555 timer ic

The 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed by Hans R. Camenzind in 1970 and brought to market in 1971 by Signetics (later acquired by Philips). The original name was the SE555 (metal can)/NE555 (plastic DIP) and the part was described as "The IC Time Machine".[1] It has been claimed that the 555 gets its name from the three 5 eke resistors used in typical early implementations,[2] but Hans Camenzind has stated that the number was arbitrary.[3] The part is still in wide use, thanks to its ease of use, low price and good stability. As of 2003[update], it is estimated that 1 billion units are manufactured every year.[3] Depending on the manufacturer, the standard 555 package includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).[4] Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the 558 (a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR falling edge sensitive instead of level sensitive). Ultra-low power versions of the 555 are also available, such as the 7555 and TLC555.[5] The 7555 requires slightly different wiring using fewer external components and less power. The 555 has three operating modes:

* Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) etc * Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc. * Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc. * The connection of the pins is as follows:

Pin| Name| Purpose|
1| GND| Ground, low level (0 V)|
2| TRIG| OUT rises, and interval starts, when this input falls below 1/3 VCC.| 3| OUT| This output is driven to +VCC or GND.|
4| RESET| A timing interval may be interrupted by driving this input to GND.| 5| CTRL| "Control" access to the internal voltage divider (by default, 2/3 VCC).| 6| THR| The interval ends when the voltage at THR is greater than at CTRL.| 7| DIS| Open collector output; may discharge a capacitor between intervals.| 8| V+, VCC| Positive supply voltage is usually between 3 and 15 V.| * Monostable mode

* In the monostable mode, the 555 timer acts as a “one-shot” pulse generator. The pulse begins when the 555 timer receives a signal at the trigger input that falls below a third of the voltage supply. The width of the pulse is determined by the time constant of an RC network, which consists of a capacitor (C) and a resistor (R). The pulse ends when the charge on the C equals 2/3 of the supply voltage. The pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C.[6] * The pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by *

* where t is in seconds, R is in ohms and C is in farads. See RC circuit for an explanation of this effect. * ] Bistable Mode
* In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via pull-up resistors while the threshold input (pin 6) is simply grounded. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground (low state). No...
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