This is very popular and most common using ic used for various purposes in the electronics area. 555 timer ic is widely used in many electronics circuits for generating pulses, timer/ delay, oscillation, tone generator, led light flasher, multivibrator, etc. This ic especially designed to accurately produce the required output waveform with the requirement of just a few extra components. This ic is easily operated with a voltage range from 5v to 15 v with 200ma power.
The 555 timer is a chip and most versatile ic that can be used to create pulses of various durations and most accurately and highly stable timing cycle, it can produce output at continuous waveforms of adjustable pulse width and frequency, and to toggle between high and low states in response to inputs.
The name of 555 timer gets from the three 5kΩ resistors used in its circuit to generate the two comparators reference voltage, is a very cheap and accurate timing ic. It can be used for various timing applications from microseconds to hours range. By connecting it with resistors and capacitors in various ways, we can get three different modes.
A dual version of 555 is 556 is also available which has two independent 555 in one IC and 558 which has 4 555 timers. The single 555 Timer chip in its basic form is a Bipolar 8-pin mini Dual-in-line Package (DIP) device consisting of some 25 transistors, 2 diodes, and about 16 resistors arranged to form two comparators, a flip-flop and a high current output.
Block diagram of 555 Timer
A comparator is a circuit used to compare two voltages to which is high. Compares input with a required reference voltage and outputs of the circuit is a LOW or HIGH signal based on whether the input is a higher or lower voltage than the reference. Comparators constructed in 555 by uses of some transistors, so in The comparator connected to pin 2 compares the “trigger” input to a reference voltage of 1/3Vcc and the comparator connected in pin 6 compares the “threshold” input to a reference voltage of 2/3Vcc from the voltage divider.
The output of comparators are directly given to the inputs of SR Flip Flop. Thus the output of the SR Flip Flop will be set as per the outputs signals of the comparator which depends on the Threshold and Trigger inputs.
Pin diagram of ic 555
Internal Circuit of 555 Timer
The timed interval is controlled by a single external resistor and capacitor network. the frequency and duty cycle can be controlled independently with two external resistors in a-stable mode operation.
Pin 1. –Ground: The ground pin connects the 555 timers to the negative (0v) supply voltage.
Pin 2 -Trigger:when the input at pin 2 falls below 1⁄2 of CTRL voltage (typically 1⁄3VCC, CTRL being 2⁄3VCC by default if CTRL pin is left open), the output goes high and output pin 3 goes high then the timing interval starts. More simply, as long as the trigger is kept at low voltage, OUT will be high. The output of the timer totally depends upon the amplitude of the external trigger voltage applied to this pin.
Pin 3. –Output, This is the output pin of the timer. It can source supply up to 200 mA of current and also sink of 200ma current. The output pin can drive any TTL circuit and is capable of up to 200mA of current for sourcing and sinking. The output voltage is equal to approximately Vcc – 1.5V, so a small speaker, LEDs, or very small motors can be connected directly to the output.
Pin 4 – Reset: a timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG, which overrides THR.
Pin 5 – Control Voltage, This pin is used to control the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. When a voltage is applied to this pin the output signal width can be changed independently of the RC delay or timing network. When the control voltage pin is not used it is connected to the ground signal or with a negative voltage of the battery through a 10nF capacitor to prevent and suppression any noise.
Pin 6 –Threshold -This pin resets the timer (makes output low) when the voltage given to it becomes greater than Vth provided the Trigger input remains HIGH. Vth = 2/3 Vcc when the Control Voltage at pin 5 is not using. Vth = Voltage given to Control Voltage when it is using.
Pin 7 –Discharge, The discharge pin is internally connected directly to the Collector of an NPN transistor which is used for“discharging”. This is used to discharge by providing a path from the timing capacitor to the ground when the output is low.
Pin-8 – This is the positive supply terminal of the IC. It can be connected to any voltage between 4.5 V through 15 V .
Astable Multivibrator mode
This is also called Free running or self-triggering mode. It has no any stable state, it has two quasi-stable states (HIGH and LOW). No external triggering is required and it automatically interchanges its two states continuously on a particular interval, hence generates a rectangular waveform. This time duration of HIGH and LOW output has been determined by the external resistors and a capacitor. The astable mode works as an oscillator circuit. In astable multivibrator mode, the output is High and Low Continuously.
Astable multivibrator using 555
In the beginning, the voltage is low at C1, the trigger pin, and the threshold pin. Whenever the trigger pin voltage is low, the output is on, and the discharge pin is off. Since the discharge pin is off, current flows through resistors R1 and R2, and then capacitor C1 charging.
When capacitor C1 charges to 2/3 Vcc, the output is switched off by the threshold pin. When the output becomes off, switches on the discharge pin. This allows the charge stored on capacitor C1 to drain to the ground.
once the voltage across C1 drops to 1/3 Vcc, the trigger pin turns off the discharge pin, so C1 can start charging again.
An astable mode is used in led and lamp flashers, pulse generation, logic clocks, tone generation, security alarms etc
This model also used to give the clock signal to 4017 counter for led sequencer etc. .
Monostable multivibrator mode
The circuit of this mode is called a one-shot circuit. In the monostable mode of the 555 timer circuit. In this mode, The output produced a duration of a fixed set time (T). The output voltage becomes high for a set duration (T) when the pulse detected at pin 2 of ic. The pulse detected at pin 2 is called a triggered pulse. The triggered pulse as a falling edge voltage.
When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer oscillator, the internal comparator 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.
When a negative trigger is applied on the Trigger input pin 2 of 555, output goes high and the capacitor starts charging through the resistor. When the capacitor voltage becomes greater than 2/3 Vcc, output goes low and the capacitor starts discharging through the Discharge pin of 555 Timer. A time period of the unstable state is given the expression, T = 1.1 x RC. And awaiting another trigger pulse to start the timing process over again. Then as before, the Monostable Multivibrator has only “ONE” stable state.
Once triggered, the 555 Monostable will remain in this “HIGH” unstable output state until the time period set up by the R1 x C1network 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.
T =1.1R1C1 The monostable mode circuit can generate pulses to the time delay duration from a few microseconds to hours depending on the capacitor value of C and Resistor value of R.
It is the multivibrator in which one state is stable, but the other state is unstable. When a trigger occurs then the multivibrator goes into an unstable state and it again returns into a stable state after a duration of particular time.
The monostable circuit can be used as a simple touch switch. A touch plate can be connected to the trigger pin, when touching the plate which will ground. This will produce a pulse at the output whose width is determined by the R and C combination.
A Bistable Mode sometimes called a Schmitt Trigger. In this mode has two stable states, one state is high and the other is low. Taking the Trigger input at Pin 2 low makes the output of the circuit go into the high state. It remains in the same state until and unless an external trigger input is applied. Generally, a bistable multivibrator stays low until a trigger signal is applied and it stays high until a reset signal is applied. After it returns in the original state. Bistable multivibrators are also called as flip-flops or latches.
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