555 Timer

The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many projects. With just a few external components it can be used to build many circuits, not all of them involve timing!
A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package
Low power versions of the555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with a 9V supply) is too low for many standard 555 circuits.
The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).

The input and output pin functions are described briefly below and thereare fuller explanations covering the various circuits:
Astable - producing a square wave
Monostable - producing a single pulse when triggered
Bistable - a simple memory which can be set and reset
Buffer - an inverting buffer (Schmitt trigger)
The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducersdirectly, including LEDs (with a resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series), relay coils (with diode protection) and some motors (with diode protection).
A loudspeaker (minimum resistance 64) may be connected to the output of a 555 or 556 astable circuit but a capacitor (about 100µF) must be connected in series.

Relay coils and otherinductive loads
Like all ICs, the 555 and 556 must be protected from the brief high voltage 'spike' produced when an inductive load such as a relay coil is switched off. The standard protection diode must be connected 'backwards' across the the relay coil

An astable circuit produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that thedurations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'.
The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second. 
T = 0.7 × (R1 + 2R2) × C1   and  f =  |1.4           |
| (R1 + 2R2) × C1 |

T   = time period in seconds (s) 
f    = frequency in hertz (Hz) 
R1 = resistance in ohms () 
R2 = resistance in ohms () 
C1 = capacitance in farads (F)
The time period can be split into two parts: T = Tm + Ts 
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1 
Space time (output low): Ts  = 0.7 × R2 × C1
Choosing R1, R2 and C1
555 astablefrequencies |
C1 | R2 = 10k
R1 = 1k | R2 = 100k
R1 = 10k | R2 = 1M
R1 = 100k |
0.001µF | 68kHz | 6.8kHz | 680Hz |
0.01µF | 6.8kHz | 680Hz | 68Hz |
0.1µF | 680Hz | 68Hz | 6.8Hz |
1µF | 68Hz | 6.8Hz | 0.68Hz |
10µF | 6.8Hz | 0.68Hz
(41 per min.) | 0.068Hz
(4 per min.) |
R1 and R2 should be in the range 1k to 1M. It is best to choose C1 first because capacitors are available in just a fewvalues.
Choose C1 to suit the frequency range you require (use the table as a guide).
Choose R2 to give the frequency (f) you require. Assume that R1 is much smaller than R2 (so that Tm and Ts are almost equal), then you can use: 
R2 =  | 0.7   |
| f × C1 |
Choose R1 to be about a tenth of R2 (1k min.) unless you want the mark time Tm to be significantly longer than the space time Ts.
Ifyou wish to use a variable resistor it is best to make it R2.
If R1 is variable it must have a fixed resistor of at least 1k in series 
(this is not required for R2 if it is variable).

555/556 Monostable

A monostable circuit produces a single output pulse when triggered. It is called a monostable because it is stable in just one state: 'output low'. The 'output high' state is temporary....