555 timer

555 Timer

     The 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and multivibratorapplications. The IC was designed by Hans R. Camenzind in 1970 and brought to market in 1971 bySignetics (later acquired by Philips). The original name was the SE555 (metal can)/NE555 (plastic DIP) and the part was described as "The IC Time Machine. It has been claimed that the 555 gets its name from the three 5 kΩ resistors used in typical early implementations, but Hanz Camenzind has stated that the number was arbitrary. The part is still in wide use, thanks to its ease of use, low price and good stability. As of 2003, it is estimated that 1 billion units are manufactured every year.

     Depending on the manufacturer, the standard 555 package includes over 20 transistors, 2 diodes and 15resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the 558 (a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR falling edge sensitive instead of level sensitive).

DOCTRONICS Safety Lights construction kit

Here is the circuit on prototype board:

Use the design formula, or component selector program to calculate the frequency of pulses you would expect to obtain with this circuit. Monitor the output pulses with an oscilloscope to check that your calculation is correct.

In an electronic die, provided the output pulses are fast enough, it is impossible to 'cheat' by holding down the button for a definite length of time. This is the circuit used in the DOCTRONICS electronic die construction kit:

Think about how you could use this circuit together with a bistable as part of a burglar alarm. Under normal conditions, the output of the bistable is LOW and the astable is stopped. If the alarm is triggered, the output of the bistable goes HIGH and the pulses start, sounding the alarm.

Pin connections

555 timer

     is an extremely versatile integrated circuit which can be   used to build lots of different circuits. You can use the 555 effectively without understanding the function of each pin in detail.

   Frequently, the 555 is used in astable mode to generate a continuous series of pulses, but you can also use the 555 to make a one-shot or monostable circuit. The 555 can source or sink 200 mA of output current, and is capable of driving wide range of output devices.

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                  555 Timing:  R_a R_b

Duty Cycle >50%
Normally the 555 timer is unable to produce a duty cycle of 50% or less. This is due to the fact that the first half of the cycle both Ra and Rb determine the charging interval (T1); where Rb alone determines the discharge interval (T2).

Duty Cycle <50%

  To allow a Duty Cycle of 50% or less, a Diode D1 is placed in parallel with Rb such that during the charging cycle (T1) Rb is bypassed. This allows Ra and Rb to act independently, allowing a duty cycle of nearly 0% to nearly 100%.

                   Triggering of a 555

    The Triggering process starts when the negative differentiated pulse edge "dips" below 1/3 Vcc, the capacitor starts charging.If the trigger is held below 1/3 Vcc longer than the charge time, the output will remain high even though the capacitor charging cycle is complete; and then only goes low when the trigger rises above 1/3 Vcc. It can be seen, therefore, that it is desirable to have the negative going trigger pulse to be shorter than the charge time. 

This site/ link below will help you to construct a circuit consist of 555 timer.

http://www.doctronics.co.uk/555.htm