Know All about 555 Timer Circuits with Applications

A 555 timer IC is developed in the year 1971 by American corporation Signetics. But, now it is designed by many organizations in the original bipolar and also in low power CMOS types. In the year 2003, they expected that one billion ICs are manufactured every year. A 555 timer is a well known integrated circuit in many electronic circuits. These circuits are designed with a basic electrical and electronic components. But most of the people do not know the internal structure and function of different pins present on the IC. The 555 timer have three main operating modes, namely astable, monostable and bistable. Each mode denotes a different type of circuit that has a specific output. The 555 timer chip can be used as an oscillator, the flip-flop element and it provides time delays.

555 Timer Pin Configuration

A simplified “pin configuration” signifying the internal circuitry of the IC 555 timer is given below with a brief description of each of its connecting pins to give a clearer understanding of how it works.

Pin-1 is Ground Pin

This pin connects the 555 timer to the 0V supply

Pin-2 is Trigger Pin

A negative pulse on this trigger pin sets the inside of the Flip-Flop. When the voltage falls below 1/3 Vcc then it causes to switch from a low state to a high state.

Pin-3 is O/P Pin

The o/p pin can drive any transistor transistor logic circuit or sinking up to 200mA of current at an o/p voltage equal to about 1.5V Vcc. So directly EDs, small speakers or motors can be interfaced directly to the o/p.

Pin-4 is Reset Pin

The reset pin  is used to change the internal Flip-Flop to control the state of the o/p pin3. This is an active low i/p & is generally connected to a logic 1 level when not used to avoid any unwanted rearranging of the o/p.

Pin-5 is Control Voltage Pin

The control voltage pin is used to control the timing of the 555 IC by overriding the 2/3 Vcc level of the voltage divider n/w. When the voltage is applied to this pin, then the width of the o/p signal varies independently of the RC timing n/w. It is connected to the ground terminal through a 10nF capacitor to eliminate the noise.

Pin-6 is Threshold  Pin

The threshold pin is used to change the Flip-Flop. When the voltage is applied to this pin exceeds 2/3 Vcc and this pin connects directly to the RC timing circuit.

Pin-7 is Discharge Pin

This pin is directly connected to the collector of the NPN transistor. When o/p of the pin3 is switches low, then it discharges the timing capacitor to the ground terminal.

Pin-8 is Supply Vcc

This is a power supply pin

Operating Modes of 555 Timer

555 timer has three operating modes, namely astable mode, monostable mode & bistable mode.

Astable Mode

In this mode, 555 timer can work as an oscillator. Uses contain LED & lamp flashers, logic clocks, pulse generation, security alarms, tone generation and PPM and so on. The 555 timer IC can be used as a simple analog to digital converter. For example, selecting a thermistor as a timing resistor permits the use of the 555 timer IC in a temperature sensor, the o/p pulse period is determined by the temperature.

Monostable Mode

In this mode, 555 timer IC  works as a one shot pulse generator.  Applications mainly include bounce free switches, timers, touch switches, missing pulse detection, frequency divider, pulse-width modulation (PWM), capacitance measurement, and so on.

Bistable Mode

In this mode, the 555 timer IC can work as a flip-flop, if the DIS-pin is not connected & no capacitor is used.  It is used in bounce-free latched switches.

The Important Features of the 555 Timer

• 555 timer IC works from a wide range of power supplies ranging from +5V to +18V.
• Sourcing or sinking load current is 200mA.
• The external components must be selected correctly, so that the timing intervals can be completed in several minutes along with the frequencies beyond several hundred kHz.
• The o/p of a 555 timer IC can drive TTl due to its high current o/p.
• It takes a temperature stability of 50 ppm/oC change in temperature (ppm means parts per million)
• The timer’s duty cycle is adjustable.
• The max power dissipation per package is 600 milliwatts & its reset and trigger i/ps has logic compatibility.

555 Timer Circuits

The 555 timer ICs are used to generate a precise square wave form and that is used in many circuits. This circuit is designed with transistors, diodes, resistors and flip flops, and this circuit can be worked in the range of 4.5-15V DC supply. The 555 timer circuit consists of three functional parts, namely flip-flops, a comparator and a voltage divider.

The main function of the comparator is to compare 2-i/p voltage levels such as inverting (-) and non-inverting (+) terminals. If the ‘V’ is high at the non inverting terminal, then the o/p is high. The /p resistance of perfect comparator is infinite.

As the i/p resistance is endless in the comparator, the voltage between all the 3-resistors are similarly divided and the value across each resistor is Vin/3

Flip/flops are digital electronic devices and it has a memory. If the i/p is high while low at R, then the o/p at Q is high. When S is high, o/p of the Q is also high & if R is high, then the o/p of the Q is low.

555 Timer IC based Projects for Engineering Students

The 555 timer IC is used in many electronic engineering projects to generate a pulse signal. Here we have discussed some major 555 timer IC based projects and these are very useful for engineering students.

Low-High Voltage DC Converter using IC 555

This project is used to develop the voltage nearly double to the i/p voltage using the voltage multiplier principle. For example, If the input voltage is around  5V DC, then the o/p we can get is around 10V DC. This project is designed with 555 timer, it operates in astable mode. In this project, the capacitors are connected in series and to charge these capacitors IC 555 timer delivers the clock pulses. This charged capacitors change the voltage which is nearly equal to the double the i/p voltage. The o/p can be calculated by using the multimeter.

Wire Loop Breaking Alarm Signal for Burglars

This project is used to identify when the thief attacks to break the window glass act to give an alarm. This project is designed with a 555 timer IC and this project is used as a security system. When the wire loop breaks, then the IC activates the buzzer to give an alert for indication.

Hidden Active Cell Phone Detector

This project is designed to identify any activated mobile phone from one and half feet distance to avoid the use of unauthorized cell phone in prohibited areas or defense security. This project is designed with 555 timer IC which operates in monostable mode. When any unknown person tries to make a call then the buzzer will give a signal presence of an active cell phone.

Touch Controlled Load Switch

This project is designed to regulate a load for the short time duration by using a touch sensitive switch and a 555 timer IC. This IC operates in monostable mode and activates by a touch plate that is connected to its trigger pin. The o/p of the IC provides logic high for a fixed time interval which is decided by the RC time constant. The o/p makes a relay to switch ON the load for that duration after that it switches OFF automatically.

This is all about 555 timer IC circuits, pin configuration of a 555 timer IC, operating modes of 555 timer and a 555 timer IC based projects for engineering students. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this topic, please give your feedback by commenting in the comment section below.Here is a question for you, what is the purpose of 555 timer circuit?