The 555 Timer IC is a highly popular and versatile integrated circuit used in timing, multivibrator, and tone generating applications.  Although the IC is already several decades old (having been invented in 1970), it remains to be widely used in circuits today because of its low cost, ease of use, and high functionality.

The 555 has 8 pins and is usually packaged in an 8-lead Plastic Dual-in-Line Package (PDIP) as shown in Figure 1. Table 1 shows the typical characteristics of a 555 timer IC. The 555 is also available in the SOIC, MSOP, and metal can packages.

Figure 1.  Photo of a 555 Timer IC

The pin assignments of the 555 timer IC are as follows: 

Pin 1 - Ground

Pin 2 - Trigger Input

Pin 3 - Output

Pin 4 - Reset Input

Pin 5 - Control Voltage

Pin 6 - Threshold Input

Pin 7 - Discharge Pin

Pin 8 - Vcc (+4.5V to +15 V)

The 555 timer may be operated in three primary modes: 1)  as a monostable multivibrator, wherein it outputs a single pulse of predetermined pulse width; 2) as an astable multivibrator, wherein it outputs a continuous square wave of defined frequency; and 3) as a bistable multivibrator, wherein it operates like a flip-flop.

Table 1.  Typical Characteristics of the 555 Timer IC

The dual version (two 555 timers in one IC package) of the 555 is called the 556.  The quad version (four 555 timers in one IC package) of the 555 is called the 558.

Learn more about the 555 Timer IC and its applications...

See also:  555's Block Diagram;  How a 555 IC Operates Internally;

555 Astable Multivibrator;  555 Monostable Multivibrator

HOME