The 555 is also a common CMOS multivibrator used in timing applications. This IC is manufactured as L555, XR-L555, ICM7555, NE555 etc, (both linear and CMOS versions are available). It is also packaged in dual (556) and quad versions (558). This timer can be used as an astable multivibrator or as a monostable multivibrator

The 555 timer functions as an astable multivibrator, square wave generator, or signal source.

The standard 555, 556 and 558 timers are compatible with their CMOS versions, and can be used directly to clock CMOS circuits without an interface. However, the CMOS version consumes les than 5% of the power required by the standard 555.
The 555 Timer integrated circuit is quite possibly one of the most revolutionary pieces of technology ever seen. Introduced in 1971 by Signetics, it is still in common usage today in an incredible variety of equipment, and just about all major semiconductor manufacturing companies produce a version of the chip.

The popularity and longevity of the 555 is due to the fact that it is simple to use (only 8 pins), cheap to produce and buy and incredibly versatile. It can be used as a "one-shot" monostable timing device, where it will act as a switch causing some other item to be switched on or off for a programmable length of time. It can also be used as an astable multivibrator, whereby it sends out a regular sequence of on-off pulses at a specified frequency. With the use of these two options a great variety of timing and counting functions can be produced.

Pin Specification

               +-------+
       Ground -+ 1   8 +- Vcc
               |       |
      Trigger -+ 2   7 +- Discharge
               |       |
       Output -+ 3   6 +- Threshold
               |       |
        Reset -+ 4   5 +- Control Voltage
               +-------+

In monostable mode, the IC should be connected with a resistor R between pins 6 and 8, a capacitor C between 1 and 7, and pins 6 and 7 should be directly connected. Pins 4 and 5 should be left unconnected. When a trigger pulse is applied to pin 2 the 555 will generate an output pulse on pin 3 of duration approximately equal to:

T = 1.1 x R x C (in seconds)

The lower limit is around 10 microseconds, the upper limit can be considered infinity (in practice bounded only by limits of C and R).

In astable operation the pins should be connected as follows: A resistor R1 between pins 7 and 8, a second resistor R2 between 6 and 7, a capacitor C between 6 and 1, and pin 2 should be directly connected to pin 6. This is probably far easier to understand if you draw the relevant connections on a diagram!

No trigger pulse is required in this operating mode as the IC wlil begin generating pulses as soon as power is applied. The pulse frequency in Hertz is calculated with:

f = 1 / (0.693 x C x (R1 + 2 x R2))

A 555 IC Timing Chip is a widespread electronic component, used mainly in timing circuits and to debounce an input signal. There are two main configurations.

Astable

An astable setup has the output changing constantly between on and off. Pin configuration:

  1. Linked to 0V.
  2. 'Trigger'. Linked to pin 6.
  3. Output. Commonly linked to a BC108 Transistor, the activation of which turns on a relay.
  4. Linked to the positive rail.
  5. Unused.
  6. Linked to 2, R2, and the positive leg of C1.
  7. Linked to the other side of R2, and to R1.
  8. Linked to positive rail.

Additional components in Astable Configuration:

  • R1 - connected to positive rail, to R2 and to pin 7.
  • R2 - connected to R1, C1, pin 6 and 7.
  • C1 - connected to R2, pin 6 and 0V.

The frequency of the output changing can be worked out with the following formula: f (Hz) = 1.44/(((2*R2)+R1)*C1). Please note that the 'on' and 'off' times will be microscopically different.

Monostable

This configuration, when triggered, goes 'on' for a certain length of time, before going 'off' again. Once 'off', it will remain off until triggered again. This can be used to replace a Schmitt Trigger in debouncing a signal. Pin configuration:

  1. Linked to 0V.
  2. 'Trigger'. Linked to SW1.
  3. Output. Commonly linked to a BC108 Transistor, the activation of which turns on a relay.
  4. Linked to the positive rail.
  5. Unused.
  6. Linked to 7.
  7. Linked to R1, pin 6 and C1.
  8. Linked to positive rail.

Additional components in Monostable Configuration:

  • R1 - connected to positive rail, pin 7 and C1.
  • C1 - connected to R1, pin 7 and 0V.
  • SW1 - connected to pin 2.

The time in seconds of the output remaining on can be worked out with the following formula: t (s) = 1.1*R1*C1.

Here's the circuit for an astable multivibrator using the ubiquitous IC 555

    ____________________________  +Vcc
   |            |     |
   |            |     |
   \        ____|_____|____
R1 /       |    4     8    |
   \       |               |
   /       |               |
   |       |               |
   |-------|7              |
   |       |               | Output
   \       |    IC 555     |       __   __ 
R2 /       |              3|---- _|  |_|  |_
   \       |               |
   /       |               |
   |_______|6              |
   |   |___|2   5     1    |
   |       |_______________|
 ----- C1       |     |
 -----      C2 ---    |
   |           ---    |
   |____________|_____|__________  Gnd

The pin names are as follows :
The circuit operates thus:
  • The threshold pin is initially at ground because C1 is charging through R1 + R2. The output pin is at +Vcc.
  • As soon as the potential of the threshold pin reaches 2/3 of Vcc , the output pin switches to the off state and the discharge pin goes to ground potential.
  • This causes the capacitor C1 to discharge through R2.
  • When the trigger pin goes below 1/3 of Vcc , the output pin goes back to the on state and the discharge pin goes to a floating (disconnected) state.
  • The entire cycle repeats indefinitely.

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