One of the most versatile chips ever made is 555 IC timer.  It has two modes of operation:

	1. Monostable Mode
	2. Astable Mode

In monostable mode the 555 IC will act as a one shot timer. It will send out " one " pulse of a variable length. In the astable mode the 555 IC will send a stream of pulses. Take a look at the basic timer information below.

Monostable Mode:

     When the timer is set to basic monostable mode it will be triggered when a negative pulse is applied to pin 2.  When the timer is triggered it will output a pulse on pin 3.  The length of the pulse is determined by the C1 and R1 in the diagram bellow. Like the Ohm's law there is a simple formula to help you understand the length of the pulse 555 timer produces. "T = R1 * C1" . If R1 = 100K and C1 = 10uF then T = 1 second. Now lets say you wanted to make the length of the pulse 10 seconds and you had 100K resistor.  Here you would use the following formula C1 = T / R1. Now you know that you need a 100uF capacitor. To calculate the value of R1 you would use R1 = T / C1 . Finally lets take a look at how we would wire up a timer in monostable mode to output a desired pulse.

Astable Mode:

    In the astable mode the pins 2 and 6 are connected together to cause the timer to retrigger itself. To calculate T1 ( output high ) we would use the following formula: T1 = .693( R1 + R2 )C1. To calculate the T2 ( output low ) we would use the following formula: T2 = .693( R1*C1 ). Now let's take a look at the astable timer.