555 Timer

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555 timer
  An Overview of the 555 Timer The 555 Integrated Circuit (IC) is an easy to use timer that has many applications. It is widely used in electronic circuits and this popularity means it is also very cheap to purchase. A 'dual' version called the 556 is also available which includes two independent 555 ICs in one package. The following illustration shows both the 555 (8-pin) and the 556 (14-pin). In a circuit diagram the 555 timer chip is often drawn like the illustration below. Notice how the  pins are not in the same order as the actual chip, this is because it is much easier to recognize the function of each pin, and makes drawing circuit diagrams much easier. For the 555 to function it relies on both analogue and digital electronic techniques, but if we consider its output only, it can be thought of as a digital device. The output can be in one of two states at any time, the first state is the 'low' state, which is 0v. The second state is the 'high' state, which is the voltage Vs (The voltage of your power supply which can be anything from 4.5 to 15v. 18v absolute maximum). The most common types of outputs can be categorized by the following (their names give you a clue as to their functions):    Monostable mode: in this mode, the 555 functions as a one-shot . Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) etc     Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse  position modulation, etc.    Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.    Pin Configuration of the 555 Timer    Here is the identification for each pin:       When drawing a circuit diagram, always draw the 555 as a building block, as shown  below with the pins in the following locations. This will help you instantly recognise the function of each pin:       Pin 1 (Ground):  Connects to the 0v power supply. Pin 2 (Trigger):  Detects 1/3 of rail voltage to make output HIGH. Pin 2 has control over pin 6. If pin 2 is LOW, and pin 6 LOW, output goes and stays HIGH. If pin 6 HIGH, and pin 2 goes LOW, output goes LOW while pin 2 LOW. This pin has a very high impedance (about 10M) and will trigger with about 1uA. Pin 3 (Output):  (Pins 3 and 7 are in phase. ) Goes HIGH (about 2v less than rail) and LOW (about 0.5v less than 0v) and will deliver up to 200mA.    Pin 4 (Reset):  Internally connected HIGH via 100k. Must be taken below 0.8v to reset the chip. Pin 5 (Control):  A voltage applied to this pin will vary the timing of the RC network (quite considerably). Pin 6 (Threshold):  Detects 2/3 of rail voltage to make output LOW only if pin 2 is HIGH. This pin has a very high impedance (about 10M) and will trigger with about 0.2uA. Pin 7 (Discharge):  Goes LOW when pin 6 detects 2/3 rail voltage but pin 2 must be HIGH. If pin 2 is HIGH,  pin 6 can be HIGH or LOW and pin 7 remains LOW. Goes OPEN (HIGH) and stays HIGH when pin 2 detects 1/3 rail voltage (even as a LOW pulse) when pin 6 is LOW. (Pins 7 and 3 are in phase. ) Pin 7 is equal to pin 3 but pin 7 does not go high - it goes OPEN. But it goes LOW and will sink about 200mA. Pin 8 (Supply):  Connects to the positive power supply (Vs). This can be any voltage between 4.5V and 15V DC, but is commonly 5V DC when working with digital ICs.   You may wonder what is inside the 555 timer chip or what makes it work. Well, the 555 timer chip an Integrated Circuit (IC) and therefore it contains a miniaturized circuit surrounded by silicon. Each of the pins is connected to the circuit which consists of over 20 transistors, 2 diodes and 15 resistors. The illustration below shows the functional block diagram of the 555 timer IC. Do you notice the three 5k resistors? This is how the chip got it's name.  555 Timer Operating Modes The 555 has three main operating modes, Monostable, Astable, and Bistable. Each mode represents a different type of circuit that has a particular output. Astable mode  An astable circuit has no stable state - hence the name astable . The output continually switches state between high and low 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 mode  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 . This 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 (or Schmitt Trigger)  A bistable mode or what is sometimes called a Schmitt Trigger, has two stable states, high and
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