555 Timer Applications

IC 555 is named after the three 5 kilo-ohm resistances that act as the voltage divider inside the IC circuitry. The IC was first invented in 1970 and since then, the 555 timer applications have been found to be endless. However, with the advent of other ICs like the MC 1455 manufactured by Motorola, the use of this IC has become less profound.

It is a timer IC and can be used for triggering applications. It can also be used as a square wave generator. However, the 555 timer application can be broadly classified into two types; astable multivibrator and monostable multivibrator. Before we start off with the applications of this IC, let us try to understand its pin configuration.

The IC 555 is available in the form of an 8 pins DIP and can be listed as follows: 

Pin 1: Ground
Pin 2: Trigger
Pin 3: Output
Pin 4: Reset
Pin 5: Controlled Voltage
Pin 6: Threshold
Pin 7: Discharge
Pin 8: V_cc

A monostable multivibrator produces a stable, logic low level output as long as no trigger is applied to the multivibrator. The IC 555 can ideally work in such a manner.

Circuit Design

• A 10 kilohms resistance is connected between Pin 8 and Pin 7.
• Two capacitors of 0.01 microfarad are connected to Pin 2 and Pin 5 respectively, and the other terminals of the capacitors are grounded.
• Pin 1 is grounded.
• Pin 4 and Pin 8 are connected to the 5 Volt power supply.

Now, if a trigger voltage is applied to Pin 2 (Trigger), then the output voltage from Pin 3 rises for a certain duration of time, which depends on the external RC circuit. IC 555 when operating in monostable mode, can be used for the purpose of pulse width modulation.

An astable multivibrator is generally used for the purpose of generating pulses. In such a circuit, both the high and low levels of output produced by the multivibrator are unstable. The output thus keeps vibrating between both the levels and hence a pulse wave is generated. The IC 555 can be configured very easily to work as an astable multivibrator.

Circuit Design

• Two 10 kilohm resistances are connected between Pin 8 and Pin 7, and Pin 7 and Pin 2 respectively.
• 100 nanofarad capacitor is connected to Pin 6 and the other terminal of the capacitor is grounded.
• Pin 6 is shorted with Pin 2 and Pin 1 is grounded.
• Pin 4 and Pin 8 are connected to the 5 Volt power supply.

The output obtained from Pin 3 (output pin) will keep changing between the logic 0 and logic 1 level. The duration for which the output voltage remains at a particular voltage, depends on the RC circuit that has been connected.

The IC 555 is used widely in different types of alarm circuits. Let us try to understand one of these circuits, which can be used for light detection.

Circuit Design

• A 50 kilohm resistance is connected between Pin 8 and Pin 7 and a 1 kilohm resistance is connected between Pin 7 and Pin 6.
• Pin 2 is shorted to Pin 6.
• The positive terminal of a 0.047 microfarad capacitor is connected to Pin 6 and the other end of the capacitor is grounded.

• The speaker is connected to the output Pin 3 through a 4.7 mircofarad capacitor. The other terminal of the speaker is grounded.
• For detecting light, an LDR (Light Dependent Resistor) is used. The LDR is connected between Pin 5 and Pin 8 using DPDT (Double Pole, Double Throw) switch.

• Another DPDT switch is connected in between Pin 4 and Pin 8 with a 10 kilohm resistance in between.
• Finally we connect Pin 8 to the 5 Volt power supply and Pin 1 is grounded.

Now, whenever the LDR is exposed to direct light or when the LDR is place in complete darkness, the speaker will produce an alarm. This is just a simple alarm circuit using the 555 timer. IC 555 can be used similarly, in major alarm systems. 

The 555 timer applications that have been explained, are the miniature versions of their actual applications. The circuits are used in large numbers by major consumer electronics goods manufacturers. Using this IC simply eliminates the need of an external timer circuit.