555 Timer/Oscillator

Basics of 555 Timer/Oscillator

555 timer is 8 pin IC for accurate time delays and oscillations in circuit. By controlling the capacitor time constant, the time delay and square wave generation time period is controlled.

The time delay is controlled by single resistor and capacitor circuit and the square wave form is controlled by two resistors and a capacitor.

Pin configuration of 555 IC:

This device has two output and 6 input pins. The pin configuration along with a little working/description of this IC is shown table below:

Pin Configuration of 555 IC

Common Mode of Operation:

Following are some of common mode of operations of 555 timer IC which is used in most of circuit.

  1. Astable mode                   (for square wave production)
  2. Mono Stable Mode        (producing single pulse when triggered)
  3. Bistable Mode                (a simple memory which can be set and reset)
  4. Buffer

These are discussed in detail in below:

1. Astable Mode:

In this mode, the capacitor is charged through two resistors from Vcc to ground and discharged through a resistor from pin 7 (discharge pin) in order to create a continuous train of the square waveforms. A resistor (R1) is place between supply voltage and pin 7 (discharge pin). The pin 2 and pin 6 are nod together and a resistor (R2) is placed between pin 7 and nod point of these two pins. A Capacitor from the common node is place between it and ground. Now, the capacitor is charged from supply voltage through both of the resistors and discharged through R2 resistor and discharge pin. Since, discharge pin provide low impedance to ground so the capacitor is discharge through it.

Astable Mode Circuit of 555 timer
Figure 1: Astable Mode Circuit
output of Astable mode
Figure 2: Output of Astable Mode of 555 timer

When the capacitor is charging, the output is high because the voltage is between 1/3 and 2/3 of Vcc. When it reaches the 2/3 of Vcc, it started discharging through resistor R2 by mean of discharge pin 7. When it again fall to 1/3 of Vcc again it toggles the output of timer IC and capacitor is again charging. The ON time (charging of capacitor) and OFF time (discharging of capacitor) for the square wave can be adjusted by tuning the values of R1 and R2 or the value of capacitor. The formulae for them along with the frequency of square wave pulses are given below:

f=1.44 / {(R1+R2) x C}                            (Frequency of pulse stream)
high time= 0.69 (R1+R2) x C                (Charging time of capacitor)
low time = 0.69 (R2 x C)                        (Discharging time of capacitor)

duty cycle = high time / pulse period time

2. Mono-stable Mode:

In this mode the 555 timer IC gives a pulse of specific time period one time when it is triggered i.e. when the signal is given to the trigger pin (pin 2) it the output of the circuit goes high for the time of time period determined by the RC time constant. The pin 7 (discharge) and threshold pin (Pin 6) makes a common node and a resistor is placed between this node and the supply voltage and a capacitor is placed between this node and ground.

Mono-stable Mode Circuit of 555 timer IC
Figure 3: Mono-stable Mode Circuit

The capacitor remains discharged in normal conditions, when a pulse is provided to the trigger pin, the capacitor starts charging from zero and the output goes high and remain high until it is charged to 2/3 of Vcc.

As it is connected to pin 7 which toggles the output at this point by discharging the capacitor through it by providing the low impedance path. The width of the square wave can be controlled by controlling the time for charging the capacitor.

It must be keep in mind that the triggering time between two triggers must be greater than its discharge time. Once the capacitor started charging, it remains so until it goes to 2/3 of Vcc and back to zero volts (after discharge). The triggering signals between this intervals does not matters. This sometimes is considered as disadvantage of this mode. So a switch is placed with the reset pin (pin 4). When reset got the potential it reset the circuit and output remain in low state until next trigger is applied.

Output of Mono stable circuit
Figure 4: Mono-stable Mode Graph output
Switch on/off for output
Figure 5: Output for Monostable mode with switch

It can be seen that when a pulse is provided to pin 2 the output pin goes high for the time the capacitor charge to 2/3 of Vcc and goes to zero after that. And can be seen that the trigger pulses during the output is high does not affect the output or charging of the capacitor.

3. Bistable Mode:

In Bistable mode, the 555 timer acts as a basic flip-flop. Which means with a triggered pulse the output goes high and remain high. In this mode the trigger (pin 2) and reset (pins 4) are held high by Pull-up resistors while the threshold input (pin 6) is simply grounded. When the button is pressed connected to pin 2, it make contact between pin 2 and ground thus this pin got momentarily pulse which triggers the timer IC and output goes high. This condition can be said as ‘set’ condition.  The output of the IC at pin 3 remains at set state until reset button is pressed which acts in the same way as the set or trigger button.

Bi Stable Mode of 5551
Figure 6: Bi Stable Mode of 555 timer Circuit

The circuit is shown in figure. No capacitor or combination of capacitor-resistor is required in this mode. The pin 7 left floating in this mode and control pin is grounded through a small capacitor.

Bi Stable output
Figure 7: Output of Bi Stable Mode of 555 timer IC

Applications of 555 timer IC:

Some widely used circuits are:-

  • Precision timing
  • Pulse generation
  • Pulse Width Modulation (PWM)
  • Time Delay Generation

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