555 Timer - eclubiitk/EClub-Handbook GitHub Wiki

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications. Its one of the most used ICs in the industry. It has 8 pins. The pin diagram is shown below

A 555 timer is operated in 3 modes - Astable, Monostable, Bistable.

Since this is a pretty long tutorial, the contents are as follow

Inside the IC

The IC consists of 2 comparators, a discharge transistor, potential dividers (resistor in this case) and a RS flip flop. The block diagram is given below

To understand how a 555 timer works, its necessary to understand how flip flop and Comparators work

Comparator

If the voltage at Vin > Vref, the output voltage Vout is HIGH, in this case, Vcc, otherwise the output is 0V. Here the resistors act as potential dividers. They split Vcc into Vcc/2 at the junction between the two resistors. Therefore Vref becomes Vcc/2.

Summary:

  • When Vin > Vref, Vout = Vcc.
  • When Vin < Vref, Vout = 0V.

Flip Flops

A flip flop is basically a SR Latch.

In simple terms, when S (called SET) is HIGH, The ouput Q is HIGH. The important thing about flip flop is that it remembers that configuration. Now it doesn't matter whether S is HIGH or LOW. Q will remain as it is, i.e, HIGH.

Now, in order to turn Q LOW, we have to turn R high. Q remains LOW until again we turn S high.

Both R and S cannot be HIGH at the same time. This is considered an invalid configuration.

How does a 555 Timer work in Astable Mode of Operation?

  • At the beginning, after we make all the connections, the potential dividers do their work and split the voltages into VCC/3 and 2/3VCC.
  • Supposing initially the Trigger Pin and Threshold Pin is set low. The lower comparator gives a HIGH output and the upper comparator gives LOW output. This configuration drives the flip flop to turn Q high because R is LOW and S is HIGH.
  • As the capacitor charges, the Trigger pin and Threshold Pin give higher voltages as input. As soon as trigger input exceeds 1/3VCC, Lower comparator gives LOW output. This doesn't change the value of Q because R has not yet been turned on.
  • After a while, when charge on capacitor increases further, the upper comparator flips too. Earlier, it gave a LOW output. Now it gives HIGH output. This feeds into the R pin of flip flop and Q turns 0.
  • As Q turns 0, Q' turns HIGH. This Q' is connected to the discharge transistor. Q' being HIGH, turns on the discharge transistor. This forces the capacitor - resistor circuit to now discharge through the transistor.
  • As it does so, the charge on the capacitor decreases. The voltages on Threshold and Trigger Pins decrease. They eventually turn so low that Trigger pin becomes less than 1/3VCC due to which the output of the lower comparator goes HIGH which turns S HIGH. This in turn, sets Q HIGH.
  • As Q is set high, the discharge transistor turns off. The capacitor starts charging once again and the cycle repeats on and on.

This is how an Astable 555 Timer works. The cycle is periodically repeated after a set interval. Through output PIN 3 (which is basically Q) we recieve pulses of HIGH voltage periodically. Now how fast the capacitor charges depends solely on the resistors and capacitors. Adjusting their values adjusts the frequency of pulses.

The waveform of pulses is given below

Following are the standard formula:

How does a 555 Timer work in Monostable Mode of Operation?

A monostable 555 timer gives HIGH output only for a specific period after which it goes to LOW output and stays there. This time interval where it stays HIGH is dependant on value of resistor and capacitor. A switch is used to turn it ON.

We shall assume out Flip flop is in initial latch state of Q = 0. This means that when S and R are both LOW, Q is LOW.

  • Initially when the switch is open, comparators give outputs 0. Therefore R and S are LOW. Since Q' feeds into the discharge transistor, its HIGH state turns the transistor on and Vcc along with any charge on capacitor discharges.
  • After the switch is pressed, Lower comparators gives HIGH output. Q goes HIGH. Q' goes LOW which turns off the discharge transistor. This allows the capacitor to get charged.
  • As soon as the charge on capacitor is high enough, the upper comparator turns ON, and sets R high. This turns Q low. Q' goes HIGH which turns on the discharge transistor. This drains the capacitor. Eventually the upper comparator goes to 0.
  • Now both comparators are at 0 and Q is LOW. This is the initial state of the timer. This is repeated after we again press the switch

Therefore its up to the user to decide when to turn on the timer. The duration for which it stays on is dependant on the resistor and capacitor.

Formula for duration of HIGH state: Time = 1.1 x R x C

How does a 555 Timer work in Bistable Mode of Operation?

This is the simplest state of operation. It is called bistable because it remains stable in both ON and OFF state. The flip flop has a RESET pin which is Pin 4 of the IC. This brings the flip flop back to initial state ( when both R and S are LOW).

  • As we press S1 switch, Trigger pin goes LOW which results in S being HIGH. This, in turn, results in Q being HIGH which is our output. The flip flop remains in this state indefinitely. Therefore the ON state is stable.
  • To turn it off, press S2 which turns the RESET pin high. The RESET pin is in an active LOW state which means it takes LOW input to get active. The RESET button brings the flip flop back to initial state. Therefore Q becomes 0. The output turns LOW.

Applications

  1. Monostable multivibrator
  2. Voltage controlled oscillator
  3. Ramp generator

The datasheet is available here