Operating Modes 
The 555 timer a very popular and versatile integrated circuit that includes 23 transistors, 2 diodes and 16 resistors on in an 8-pin DIP (Dual In-line Package).  It has two main operating modes:
Monostable Mode - the 555 functions as a 'one-shot'.  Applications include timers, missing pulse detectors, bouncefree switches and touch switches.

Astable Mode - the 555 functions as an oscillator.  This mode is used for circuits such as LED and lamp flashers, pulse generators, logic clocks, tone generators and security alarms.

Theory of Operation
The 555 timer is a hybrid circuit containing both analog and digital components.  Two comparators with one input of each connected to an internal resistor ladder are included in the chip.  The first comparator causes a flip flop to reset when the voltage on its positive terminal reaches 1/3 Vref, while the second comparator resets the flip flop at 2/3 Vref. 

A block diagram of the 555 is shown in Figure 2.1 and a schematic of the timer used as an astable multivibrator is shown in Figure 2.2.

Figure 2.1  Block Diagram of the 555 timer1


Assume that the output is high ('1'), the charge on the capacitor low, and the 'discharge' transistor is not conducting.  The capacitor now begins charging through R1 and R2 in series toward the +5-volt supply.  When the voltage across the capacitor gets to 2/3 of the supply, the 'threshold' comparator senses this and flips the internal circuitry to the other state.  The output now goes low ('0'), and the discharge transistor turns on.  The capacitor now discharges through resistor R2.  Discharge continues until the capaciotor voltage drops to 1/3 of the supply voltage.  At this instant, the 'trigger' comparator senses the capacitor voltage and flips the circuit back to its initial state.  The cycle continuously repeats, and the output is a rectangular waveform.  The output is high while the capacitor is charging and low while the capacitor is discharging.   

Click on the diagram below to see an animation of how this works.


Circuit Design Tips
Astable Multivibrator (Signal Source) Mode:

Figure 2.2  The 555 connected as an astable 
multivibrator or signal source
The 555 timer can generate a very wide frequency range, depending on the values of R1, R2 and C.  Figure 2.3 shows how to choose the timing resistors. 

Figure 2.3  The 555 Astable Free Running Frequency
Design equations: 
Charge time  (output high):      0.693 (R1 + R2) C

Discharge time  (output low):        0.693 (R2) C

Period:                      0.693 (R1 + 2R2) C

Frequency:               1.44 / (R1 + 2R2) C

Duty cycle:       Time High / Time Low:  (R1 + R2) / R2

With a 5-volt supply, the resistors can range from 1K (minimum value - R1 or R2) through 3.3 megohms (maximum value - R1 and R2 in series).  This gives a potential adjustment range of 3300:1.  Best results are obtained with capacitors of 1000 pF or larger, but smaller values can be used with lower values of R1 and R2. 

In 555 circuits, the timing capacitor always has one end connected to ground, and a positive-only current is applied.  This lets you use large electrolytics for long timing periods.  The input is also a very high impedance, which lets you use high-value resistors and small capacitors for a given time constant.  You can easily get a 1000:1 frequency range out of a single capacitor simply by changing the series resistor. 

The maximum operating frequency is around 1 MHz, but best operation is obtained below 300 kHz.  The minimum operating frequency is limited only by the size and leakage of the capacitor you use.  For instance, a 10 µF capacitor and a 3.3 megohm resistor will give a time interval of 23.1 seconds if the leakage of the capacitor is low enough. 

By making R2 large with respect to R1, we can get an essentially symmetrical square-wave output.  For instance, if R1 is 1K and R2 is 1 megohm, the difference in charging and discharging resistance is only 0.1%, and good symmetry results.  Any symmetry you want from 50 through 99.9% can be obtained by a selection of the ratio of R1 and R2.  Since the '1' time always charges through two series resistors, it is always longer than the '0' time.1

Only a small frequency variation occurs due to power supply variation (0.1% per volt of supply voltage change).  Unfortunately, variation due to temperature changes is large, so any precise instrumentation projects require more stable crystal clocks. 

An internal buffer enables the 555 to provide a large output current which is able drive many TTL loads, and can power a small speaker directly. 

Since the frequency of this device can easily be adjusted, astables are often used as clocks in digital instrumentation and data communications. 

Monostable Multivibrator (Pulse Generator) Mode:

Monostable multivibrators have only one stable state which remains until an input pulse occurs.  A circuit diagram for a monostable multivibrator is shown in Figure 2.5 and a plot of the pulse widths as a function of R and C is shown in Figure 2.6.. 

Figure 2.5  The 555 as a Monostable 
A 555 timer can be used to build a monostable if long time intervals are required.  For pulse widths below one microsecond, the 555 will be inadequate.  The monostable maintains this unstable state for a time period given by 
T = 1.1 RC

where T is the period of the unstable output, R is the resistance, and C is the capacitance. 

Figure 2.6  Monostable Multivibrator
Timing Diagram

When using a 555 timer as a monostable, large resistance values can be used (up to 10 megohm).  This results in a timing range of over 7 decades.
1Lancaster, Don.  TTL Cookbook
Howard W. Sams & Co., Indianapolis, Ind,, 1989