Introduction to an IC 555 TIMER!!!!…

We have seen that Multivibrators and CMOS Oscillators can be easily constructed from discrete components to produce relaxation oscillators for generating basic square wave output waveforms. But there are also dedicated IC’s especially designed to accurately produce the required output waveform with the addition of just a few extra timing components. One such device that has been around since the early days of IC’s and has itself become something of an industry “standard” is the 555 Timer Oscillator which is more commonly called the “555 Timer”.

Now let’s see what a 555 Timer is?

A 555 timer is a type of integrated circuit, also known as a chip, which is extremely popular for a variety of purposes. Its main use is as a timer, but it can also be used as a multivibrator.

The 555 timer takes its name from the fact that the original model had three resistors, each rated at five thousand ohms. The first model was formally known as the SE555/NE555 and marketed as the IC Time Machine. At the time of its 1971 release, the 555 timer was the only commercially available timer circuit. Today there are many different models from different companies, though nearly all manufacturers include 555 as part of the model number because it’s such a recognizable name.

There are two different types of 555 timer, both being eight-pin chips. The most common one is the rectangular ‘V’ package or an 8-pin mini-DIP package, which has four pins down each side. The other version, which used to be the most common but has now fallen from favor somewhat, is the circular ‘T’ package or an 8-pin round TO3-style. The figure for an 8-pin mini-DIP package is shown above.

The 555 timer chip is extremely robust and stable 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of such as turning a light on for a certain length of time, or it can be used for creating a warning light that flashes on and off. We can use it to produce musical notes of a particular frequency, or we can also use it to control positioning of a servo device etc., in fact any circuit that requires some form of time control as the list is endless.

Types of 555 Timers

As we had explained that there are three different types of 555 Timers. We will give a brief introduction to each one of them

Monostable operation is when the output signal simply switches between the default off position and a temporary on position at regular intervals, which is most commonly used for timers.
Astable operation is when the output voltage rises and falls in a set pattern, making it an oscillator. As the pattern can be varied, it can be used for any purpose which requires a particular tone pattern.
Bistable operation is when the signal can be held in one of two positions, meaning the 555 timer can act as the smallest possible unit of computer memory.

Explanation of Terminals for 555 Timer

Pin 1: Grounded Terminal: All the voltages are measured with respect to this terminal.

Pin 2: Trigger Terminal: This pin is an inverting input to a comparator that is responsible for transition of flip-flop from set to reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.

Pin 3: Output Terminal: Output of the timer is available at this pin. There are two ways in which a load can be connected to the output terminal either between pin 3 and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between pin 3 and the supply pin is called the Normally on Load and the load connected between pin 3 and ground pin is called the Normally off Load.

Pin 4: Reset Terminal: To disable or reset the timer a negative pulse is applied to this pin due to the fact it is referred to as reset terminal. When this pin is not to be used for reset purpose, it should be connected to + VCC to avoid any possibility of false triggering.

Pin 5: Control Voltage Terminal: The function of this terminal is to control the threshold and trigger levels. Thus either the external voltage or a pot connected to this pin determines the pulse width of the output waveform. The external voltage applied to this pin can also be used to modulate the output waveform. When this pin is not used, it should be connected to ground through a 0.01 micro Farad to avoid any noise problem.

Pin 6: Threshold Terminal: This is the non-inverting input terminal of comparator 1, which compares the voltage applied to the terminal with a reference voltage of 2/3 VCC. The amplitude of voltage applied to this terminal is responsible for the set state of flip-flop.

Pin 7: Discharge Terminal: This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called Discharge terminal because when transistor saturates, capacitor discharges through the transistor. The capacitor charges at a rate determined by the external resistor and capacitor, when the transistor is cut-off.

Pin 8: Supply Terminal: A Supply voltage of + 5 V to + 18 V is applied to this terminal with respect to ground (pin 1).

Internal Circuitry

A simplified “block diagram” representing the internal circuitry of the 555 timer is given below

The single 555 Timer chip consists of some 25 transistors, 2 diodes and about 16 resistors arranged to form two comparators, a flip-flop and a high current output stage.

The 555 Timer consists of three 5kΩ resistors connected together internally producing a voltage divider network between the supply voltage at pin 8 and ground at pin 1. The voltage across this series resistive network holds the negative inverting input of comparator two at 2/3Vcc and the positive non-inverting input to comparator one at 1/3Vcc.

The two comparators produce an output voltage dependent upon the voltage difference at their inputs which is determined by the charging and discharging action of the externally connected RC network. The outputs from both comparators are connected to the two inputs of the flip-flop which in turn produces either a “HIGH” or “LOW” level output at Q based on the states of its inputs. The output from the flip-flop is used to control a high current output switching stage to drive the connected load producing either a “HIGH” or “LOW” voltage level at the output pin.

How to Use the 555 Timer Output

The output pin (pin 3) of an electronic 555 timer circuit can be in one of two states: high and low. In the high state, the voltage at the pin is close to the supply voltage. The low state is 0 V.

There are two ways you can connect output components to the output pin: Source and Sink. To illustrate these two configurations, consider a configuration using an LED as the output device with a resistor included in the circuit to limit the current flow. Without the resistor, current will flow through the circuit unimpeded, which will quickly burn out the LED and probably ruin the 555 as well.

In the circuit on the left, current flows through the LED circuit when the output is high. The current flows from the output pin through the LED and resistor to ground. This output configuration is called Sourcing because the 555 is the source of the current that drives the output.

In the circuit on the right, current flows through the LED circuit when the output is low. The current flows from the Vcc supply, through the LED and resistor, and into the 555 where it’s internally routed to ground through pin 1. This output configuration is called Sinking because the current is sent into the 555.

Whether we source or sink our output circuit depends on whether we want our output circuit to turn on when the output is high or low.

We can also combine both Sourcing and Sinking in a single circuit. Here, two LEDs are connected to the output pin. One is sourced; the other is sunk. In this circuit, the LEDs alternately flash as the output switches from high to low. LED1 lights when the output is low, LED 2 when the output is high.

The output circuit of a 555 timer can handle as much as 200 mA of current, which is actually much more current than most integrated circuits can source or sink. If we need to drive a device that requires more than 200 mA, we can isolate the output device from the 555 by using a transistor.

Simple example of a 555 Timer

When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of the 555 Timer oscillator, the internal comparator, (comparator No1) detects this input and “sets” the state of the flip-flop, changing the output from a “LOW” state to a “HIGH” state. This action in turn turns “OFF” the discharge transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1.

This action allows the timing capacitor to start to charge up through resistor, R1 until the voltage across the capacitor reaches the threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage divider network. At this point the comparators output goes “HIGH” and “resets” the flip-flop back to its original state which in turn turns “ON” the transistor and discharges the capacitor to ground through pin 7. This causes the output to change its state back to the original stable “LOW” value awaiting another trigger pulse to start the timing process over again. Then as before, the 555 Timer has only “ONE” stable state.

The 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully. Once triggered, the 555 Timer will remain in this “HIGH” unstable output state until the time period set up by the R₁ x C₁ network has elapsed. The amount of time that the output voltage remains “HIGH” or at a logic “1” level is given by the following time constant equation.