555 timer ic operation

Posted on Feb 5, 2014

The 555 Timer is an integrated circuit used as a timer, a pulse generator, tone generator and many other circuit functions, all from one universal IC function. Each pin function is described and design recommendation are given when required. A Monostable Multivibrator setup is used as an example to describe the operation of the 555. The basic bloc

555 timer ic operation
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k diagram for the 555 timer is shown in the graphic below and referenced by the text. Note the term Vcc is used because the 555 will operate with any Vcc [pin 8] between 4. 5 volts and 16 volts. The actual voltage used by the 555 does not effect the timing circuit because both the charging rate of the capacitor and the threshold of the capacitor use the same voltage. As with any integrated circuit always Bypass the Vcc pin to ground with a 0. 1uF ceramic capacitor [not shown in the circuit below]. Pin 8 is Vcc and Pin 1 is ground; however the capacitor should be connector to Vcc and taken to the closest possible ground, which may not necessarily be pin 1 of the IC. A by-pass capacitor is also called a Decoupling Capacitor. The 555 IC may be operated with any voltage between 4. 5 volts and 16 volts [see data sheet info below]. Select the Vcc voltage based on the circuit being driven by the 555 timer. The [trigger] input to the 555 is only being used as a reference switch so the actual voltage used is not important as long as the input transitions past the reference point [more below]. The 555 uses two comparators to control the operation of the timer. The lower comparator is set to trigger at 1/3 Vcc while the upper comparator is set to trigger at 2/3 Vcc. Three internal 5k resistors placed in series between Vcc and ground are used to provide voltage references. The 1/3 Vcc voltage is generated between the bottom and middle...

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