The 555 timer IC was invented by Hans camenzid in 1971. It is one of the most popular IC in the electronics industry. Over 300 billion of 555 timer ICs were manufactured annually. 555 timers are use to generate high frequency clock signals that can be use to turn a servo, flash a bulb based on time or flash an LED repeatedly under a fixed time interval or use it to play a musical note of a particular frequency. The 555 timer is the most popular integrated circuit ever developed.
It finds wide applications in electronics such as controlling motor speed, controlling traffic lights, inverters, creating pulse width modulation (PWM) and generating high frequency alternating current. A 555 timer IC is made of two diodes, fifteen resistors and twenty five transistors packaged in 8 pin dual in-line form. This IC got it name from three 5 kilo ohm resistors in series connected to form a voltage divider. Manufacturers producing 555 timer ICs often incorporate their model number examples are NE555, CA555, MC14 555, SE 555 etc. This are ICs aren't different from standard 555 timer IC, they only differ by numbers. Two 555 timers sandwiched or embedded into a single block of package is called a 556 timer. Modern chips can contain more than four 555 timers embedded in them.
Pinout diagram of a 555 timer.
Above is a labelled diagram of a 555 timer. It has eight pins for connecting to external circuits. In circuits diagram you will come across 555 timer connected in circuits with it pins. It's very important for any diving in the world of electronics to understand the name of the pins and their functions for you to design your own circuits with it or understand how to connect them in circuits. The pins are labelled 1,2,3,4,5,6,7 and 8. Shown below is a typical 555 timer. The pin nearest to the notch is the pin1 followed by pin2, pin3, pin4, pin5, pin6, pin7 and pin8.
For us to label the pins we have to start counting from the notch (an hole made at the edge of the IC). The notch is near the left hand side of the IC. so when identifying the pin numbers the IC should be positioned in an upright position as shown above.
555 timers and pins.
1. Ground: This is the first pin (pin 1) and it's usually connected to the ground or negative(-ve) voltage.
2.Vcc: This is pin8 of The IC. It must connected to the positive supply voltage. The input voltage must be at least 4.5v and should not be greater than 15v. Therefore a 555 timer can't be run on voltage above 15v or else you will get the IC destroyed. It's common to power 555 timer with four AAA or AA batteries in series producing 6v or by using a single 9v battery.
3. Output: Pin3 of the IC is the output pin i.e the pin where current or voltage flows out of the IC. The voltage output can either be low or very close to 0v or high which is close to the voltage supplied to the pin 8. The shape of the output current i.e how long it produce voltage equals to that applied to the pin 8 or how long it's high depends on how the remaining five pins are connected.
4. Trigger: The pin2 is the trigger pin. It triggers the 555 timer into work. The timer starts when the voltage on pin2 drops to below one-third of the power supply voltage. When the IC is triggered through pin2, the output on the pin3 becomes high.
5. Discharge: The pin7 is called the discharge pin. It's use to discharge an external capacitor that works in conjunction with a resistor to control the timing interval. In most circuits, pin7 is connected to the power supply by a resistor to the ground through a capacitor.
6.Threshold: Pin6 is called the threshold. The function of this pin is to monitor the voltage across the capacitor that is discharged by pin 7. When this voltage reaches two third of the power supply voltage (Vcc), the timing cycle ends and the output on pin3 goes low.
7.Control: The 5th pin "pin5" is the control pin. In most 555 timer circuits, the control pin is usually connected to the ground (-ve voltage) commonly through a capacitor of little capacitance of about 0.01uF. The capacitor level out or reduce any flunctuation in the power supply voltage capable of affecting the operation of the IC.
8.Reset: This 4th pin of the IC otherwise called pin4. It is use to restart the 555 timer's operation. Just like the trigger input, reset is an active low input. Thus, pin4 must be connected to the supply voltage for the IC to operate. If pin4 is grounded, the 555 timer's operation is interrupted and won't start again until it's triggered again with pin2.
Operating modes of 555 timer.
Astable vibrator.
In this mode, the output voltage isn't stable. The voltage swings between high and low without the application of an external trigger. In this mode, the trigger and threshold pins are connected together and it doesn't require an external trigger pulse.
An Astable Circuit has no stable state hence the name "astable". The output continually switches state between high and low without without any intervention from the user, called a 'square' wave. This type of circuit could be used to give a mechanism intermittent motion by switching a motor on and off at regular intervals. It can also be used to flash lamps and LEDs, and is useful as a 'clock' pulse for other digital ICs and circuits.
•Monostable vibrator.
A Monostable Circuit produces one pulse of a set length in response to a trigger input such as a push button. The output of the circuit stays in the low state until there is a trigger input, hence the name "monostable" meaning "one stable state". his type of circuit is ideal for use in a "push to operate" system for a model displayed at exhibitions. A visitor can push a button to start a model's mechanism moving, and the mechanism will automatically switch off after a set time.
•Bistable mode.
A Bistable Mode or what is sometimes called a Schmitt Trigger, has two stable states, high and low. Taking the Trigger input low makes the output of the circuit go into the high state. Taking the Reset input low makes the output of the circuit go into the low state. This type of circuit is ideal for use in an automated model railway system where the train is required to run back and forth over the same piece of track. A push button (or reed switch with a magnet on the underside of the train) would be placed at each end of the track so that when one is hit by the train, it will either trigger or reset the bistable. The output of the 555 would control a DPDT relay which would be wired as a reversing switch to reverse the direction of current to the track, thereby reversing the direction of the train.
Circuit applications of 555 timer.
Timer 555 timer is most important integrated circuit (chip) used widely in digital electronics. Some common uses and application of 555 timer IC are as follow:
- PWM (Pulse Width Modulation) & PPM (Pulse Position Modulation)
- Duty Cycle Oscillator Lamp Dimmer To provide Accurate time delays
- As a flip-flop element Digital logic probes Analog frequency meters Quad Timer applications Pulse,
- Waveform, and square wave generation Stepped tone & tone burst generator & linear ramp generation
- Tachometers & temperature measurement
- It can be used as monostable multivibrator and astable multivibrator
- DC to DC Converters
- DC Voltage Regulators
- Voltage to Frequency Converter
- Frequency Divider Schmitt
- trigger Cable tester Pulse
- detector Wiper speed control
- Timer Switch
- Time delay generation,
- precision timing and sequential timing.
The 555 Timer IC are widely used in most of interesting electronic circuits and project like Traffic Light Circuit using 555 Timer, LED Flashing circuits, police siren, LED dice, Music Box, Metal detector, Joystick and game paddles, & low cost line receiver, Clap switch activated circuit and lots of other projects and circuits designs.
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Power Electronics