555 IC


  • Developed in the 1970s.
  • Very inexpensive.
  • Can be wired in three forms: astable, bistable and monostable.
  • Remember that when working with equations the values must always be in the SI unit:
    • For instance, capacitance must always be in farads (F) and resistance in ohms (Ω), etc.
  • Can be used to:
    • Switch something ON or OFF at a precise time.
    • Switch something ON or OFF after a certain time delay.
    • Generate a constant pulse (as a pulse generator) i.e. used to provide a series of clock pulses for a counter.
  • All ICs are made of silicon or another such semi-conductor and can be encased within plastic packages.
    • The 555 IC contains:
      • 2 comparators.
      • A bistable.
      • Resistors.
      • Transistors.
  • It is available in a dual package which has two 555s on one chip called the 556 IC.
  • It is possible to ‘tune’ astable and monostable circuits by changing the value of the resistors that matter (preferably using variable resistors first and then replacing them with accurately selected fixed-value resistors once the tuning is complete) and capacitors whilst looking at the output on an oscilloscope which will reveal Time High’s and Time Low’s.
  • If the 555’s output is not high enough it can be used to switch ON a relay to power a higher power circuit.

Pins 2 and 3:

  • Pin 2:
    • This is where the current and voltage enter into the chip to start the counting sequence.
  • Pin 3:
    • This is the output pin where the current comes out after the chip finishes counting.

Typical Characteristics for the 555 IC:

  • Different 555s may have different characteristics due to the way in which they were manufactured and the precision of the techniques used to manufacture them.
  • Max. supply voltage – 16V.
  • Max. output current – 200mA.
  • Max power – 600mW.
  • Reset Current – 0.1mA.
  • Trigger voltage – 1.6-5v.
  • If the voltage on the trigger pin (2) is less than 1/3 of the supply voltage, the output pin (3) goes high and pin 7 is disconnected from the ground.
  • When the voltage on the threshold pin (6) is greater than 2/3 of the voltage input the output goes low and pin 7 is grounded.


  • In astable format the leg 6 of the chip is joined to its leg 2 thus allowing the output to discharge a cycle of pulses with a time constant determined by the resistors R1, R2 and the capacitor C.


  • Astable sequence of operation (speculation):
    • The current is switched on.
    • If the voltage on the trigger pin (2) is low the output (3) begins sourcing current and pin 7 is disconnected from the ground.
    • This results in the capacitor charging up through the two resistors in series to it.
    • When it is charged, pin 6 goes high thus pin 3 begins sinking current and pin 7 is grounded.
    • This allows the capacitor to discharge through the 555 IC.
    • When it is fully discharged pin 2 goes low again and the cycle repeats itself.
  • The equation for the frequency is:
    • Frequency (EQ)
  • Period is 1/F:
    • It is the time taken for each complete cycle.
    • It should therefore also equal to TH + TL.
  • The equation to work out the time high (time ON) for such a circuit is:
    • TH
  • The equation to work out the time low (time OFF) for such a circuit is:
    • TL
    • It is because of these formulae that TH and TL can never be equal.
      • Even though the formulas allow for R1 to be equal to 0Ω, the actual circuit does not allow for this.
      • Because of this MSR can never equal to 1 in reality but can in theory.
  • The formula for the Mark Space Ratio is:
    • MSR



  • In monostable format the leg 6 of the chip is connected with its leg 7 thus allowing the output to discharge a single pulse of electrical power when switched on with a time period determined by the resistor R and the capacitor C.
    • The timing mechanisms on this chip include the 10kΩ Resistor, R, and the capacitor, C.
    • The equation for the time period of this circuit is:
      • Monostable (EQ)


  • The remaining resistor connected in series to the push-to-make switch is to keep the current going through that branch of the circuit under control when the switch is pressed.
    • This is to ensure that the wire in that branch does not heat up to dangerous temperatures.
    • And it is also for the protection of the 555 when the switch is not pressed.
  • Monostable sequence of operation (must be remembered):
    • When the circuit is turned on the voltage at the trigger pin (2) is high. As a result the output will be low and sinking current:
      • This means that conventional current will be drawn into the IC.
    • When the push-to-make switch is pressed, the voltage at the trigger pin goes low:
      • This causes the output pin to begin sourcing current:
        • This means that a conventional current starts to flow out of the IC:
          • This current should not flow towards the positive pole of the battery as there is no potential difference between those two places.
          • But, just in case, there should be a diode or an equivalent component (i.e. maybe an LED) connected that leads from the positive pole as in the diagram below:
            • This prevents the buzzer in the diagram below from sounding.
    • After this, the capacitor charges through the resistor R1 for the time constant because when pin 2 is high pin 7 is disconnected from the ground.
    • After it finishes charging pin 6 goes high and the capacitor discharges through pin 7.
    • This then causes pin 3 to go low again.
  • It is possible to tune a monostable circuit or an astable circuit by looking at their output on an oscilloscope and by substituting the fixed resistors within with variable resistors.


  • Please note that using a 555 chip to make a bistable is a waste of components and is rarely done.
  • In bistable format, the sixth and seventh legs of the chip are not connected thus allowing two triggers to control the chip in such a way that it provides a bistable output.
    • Since this circuit is bistable, there are no equations or timing mechanisms associated with it.

Bistable 555

 Bistable Transistors


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