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Ř IC 555 TIMER :











These specifications apply to the NE555. Other 555 timers can have different specifications depending on the grade (military, medical, etc.).

Supply voltage (VCC)

4.5 to 15 V

Supply current (VCC = +5 V)

3 to 6 mA

Supply current (VCC = +15 V)

10 to 15 mA

Output current (maximum)

200 mA

Maximum Power dissipation

600 mW

Power consumption (minimum operating)

30 mW@5V, 225 mW@15V

Operating temperature

0 to 70 °C






The 555 has three operating modes:


§  Monostable mode: in this mode, the 555 functions as a "one-shot" pulse generator. Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) and so on.


§  Astable: free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on. Selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor: the period of the output pulse is determined by the temperature. The use of a microprocessor based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.



§  Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.


Astable Mode :




In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:

f = \frac{1}{\ln(2) \cdot C \cdot (R_1 + 2R_2)}


The high time from each pulse is given by:

\mathrm{high} = \ln(2) \cdot (R_1 + R_2) \cdot C


and the low time from each pulse is given by:

\mathrm{low} = \ln(2) \cdot R_2 \cdot C

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.


The power capability of R1 must be greater than (VCC)2 / R1.

Particularly with bipolar 555s, low values of R1 must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above.

To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 towards the capacitor. This bypasses R2 during the high part of the cycle so that the high interval depends only on R1 and C.