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555 Timer

The 555 timer is one of the most versatile IC’s ever produced.  It is a mixture of analogue and circuitry and its applications are virtually limitless in the world of digital pulse generation.  It can be used for producing accurate time delays or oscillation.  It can be used for applications such as precision timing, pulse generation, sequential timing, time delay generation and pulse width modulation (PWM)

The eight pins carry out the following functions:

  1. Ground, which acts as a safety measure as with electrical plugs
  2. Trigger, which passes on voltage to start the timing operations
  3. Output, which carries voltage to the device using the timer
  4. Reset, which is used to end the timing operation
  5. Control voltage, an optional pin used for controlling the timer from outside the main circuit set-up
  6. Threshold, which determines how long the timer should output voltage in each on/off cycle – in other words, how long the timing interval should be
  7. Discharge, connected to a capacitor which also influences the timing interval
  8. V+, which is the voltage input

There are three different types of use for a 555 timer.  The monostable operation as discussed in the ‘What is a digital circuit category’  is when the output signal simply switches between the default off position and a temporary on position at regular intervals, this is most commonly used for timers.

Astable operation is when the output voltage rises and falls in a set pattern, making it an oscillator. As the pattern can be varied, it can be used for any purpose which requires a particular tone pattern.

Bistable operation is when the signal can be held in one of two positions, meaning the 555 timer can act as the smallest possible unit of computer memory.

All IC timers require an outside capacitor to determine the on off times of the output pulses, it takes a period of time for a capacitor to charge or discharge through a resistor and theses times can be alternated by calculating the values of resistance and capacitance the time is easily worked out through this equation


So if I was to assume that a resistor had a value of 1 mega ohm and the capacitor a value of 1uF (microfarad) then the calculation would be

time =1,000,000 x 0.000001 and so for instance the time would be one second.

Categories: Integrated Circuits
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