555 Timer

This is very popular and most commonly using ic used for various purposes in the electronics area. 555 timer ic is widely used in many electronic 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 15v with 200ma power.

The 555 timer is a chip and most versatile ic that can be used to create pulses of various durations and the most accurate 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 the 555 timer gets from the three 5kΩ resistors used in its circuit to generate the two comparators reference voltage, which 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 use 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

Pin diagram of 555 timer

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 suppress 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 used. Vth = Voltage given to Control Voltage when it is used.

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 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 generating 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

fosc= 1.4/(R1+2R2)C1

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, switch 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.
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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 is 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, the 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 C1 network 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 a time delay duration from a few microseconds to hours depending on the capacitor value of C and Resistor value of R.

555 timer monostable multivibrator operation

It is a 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 the particular time.

T = 1.1 * R * C

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.

Bistable mode

A Bistable Mode is 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 to its original state. Bistable multivibrators are also called as flip-flops or latches.

(FAQ)Frequently asked questions of 555 timer

Q1: What is a 555 timer multivibrator? – A 555 timer multivibrator is an integrated circuit (IC) that is commonly used to generate various types of waveforms, such as square waves, astable (free-running) multivibrator, monostable (one-shot) multivibrator, and bistable (flip-flop) multivibrator. It is a versatile and widely used IC in electronic circuits.

Q2: How does a 555 timer multivibrator work? – The 555 timer multivibrator consists of three main functional blocks: a comparator, a voltage divider network, and a flip-flop. It operates based on the charging and discharging of an external capacitor connected to the IC. The timing of the charging and discharging is controlled by the voltage levels at specific pins and the resistors connected to them.

Q3: What are the applications of a 555 timer multivibrator? – The 555 timer multivibrator has numerous applications in electronics, including:

Oscillators: Generating square, triangular, and sawtooth waveforms.
Timers: Producing precise time delays in monostable mode.
Frequency dividers: Dividing input frequency by factors of 2.
PWM (Pulse Width Modulation): Controlling the duty cycle of a square wave for applications like motor speed control, LED dimming, etc.
Tone generation: Creating various audio tones and signals.
Flip-flops: Constructing sequential logic circuits.

Q4: How can I calculate the frequency and duty cycle of an astable multivibrator? – The frequency (f) of an astable multivibrator can be calculated using the formula: f = 1.44 / ((R1 + 2 * R2) * C). The duty cycle (D) can be approximated as (R1 + R2) / (R1 + 2 * R2).

Q5: What is the difference between an astable and a monostable multivibrator? – An astable multivibrator is a free-running oscillator that continuously switches between two states, generating a square wave output with no stable state. A monostable multivibrator, on the other hand, generates a single pulse of a fixed duration when triggered, returning to its stable state afterward.

Q6: What is an astable multivibrator? – An astable multivibrator is a type of multivibrator circuit that generates a continuous square wave output without any stable state. It oscillates between two unstable states, continuously switching back and forth.

Q7: How do I calculate the frequency and duty cycle of an astable multivibrator using a 555 timer? – The frequency (f) can be calculated using the formula: f = 1.44 / ((R1 + 2 * R2) * C). The duty cycle (D) can be approximated as (R1 + R2) / (R1 + 2 * R2).

Q8: Can I adjust the frequency and duty cycle of the astable multivibrator? – Yes, the frequency and duty cycle can be adjusted by changing the values of the timing components (resistors and capacitor). Increasing the values of R1, R2, or C will decrease the frequency and affect the duty cycle.

Q9: Can I use different types of capacitors in the astable multivibrator circuit? – Yes, you can use different types of capacitors (such as ceramic, electrolytic, or film capacitors) as long as their voltage ratings are suitable for the circuit’s requirements. The choice of capacitor type may also affect factors such as stability and temperature characteristics.

Q10: What is a monostable multivibrator? – A monostable multivibrator is a circuit that generates a single pulse of a fixed duration when triggered, after which it returns to its stable state. It is also known as a one-shot multivibrator.

Q11: How does a monostable multivibrator using a 555 timer work? -In a monostable multivibrator using a 555 timer, a trigger input initiates the timing cycle. When triggered, the output transitions to a high state for a fixed duration determined by the timing components connected to the 555 timer. After this duration, the output automatically returns to its stable low state.

Q12: What are the essential components required for a monostable multivibrator using a 555 timer? – The essential components required are:

555 timer IC
Resistor (R)
Capacitor (C)
Trigger input
Power supply (Vcc)
Ground connection (GND)
Output load or circuit

Q13: How do I calculate the pulse duration in a monostable multivibrator using a 555 timer? – The pulse duration (t) can be calculated using the formula: t = 1.1 * R * C, where R is the resistance and C is the capacitance connected to the timing components of the 555 timer.

Q14: Can I adjust the pulse duration of the monostable multivibrator? – Yes, the pulse duration can be adjusted by changing the values of the timing components (resistor and capacitor). Increasing the values of R or C will result in longer pulse durations, while decreasing them will yield shorter durations.

Q15: Can I trigger the monostable multivibrator using an external signal? – Yes, the monostable multivibrator can be triggered using an external signal. The trigger input of the 555 timer can be connected to an external source such as a switch, sensor, or another circuit.

Q16: What is the maximum pulse duration I can achieve with a 555 timer monostable multivibrator? – The maximum pulse duration of a 555 timer monostable multivibrator is determined by the values of the timing components (R and C) used. It can range from microseconds to seconds, depending on the chosen values.

Q17: Can I use different types of capacitors in the monostable multivibrator circuit? – Yes, different types of capacitors (such as ceramic, electrolytic, or film capacitors) can be used in the monostable multivibrator circuit, as long as their voltage ratings and capacitance values meet the circuit’s requirements.

Q18: Can I use a variable resistor (potentiometer) for the timing resistor (R) in the monostable multivibrator? -: Yes, you can use a variable resistor (potentiometer) for the timing resistor (R) to make the pulse duration adjustable. This allows you to vary the duration of the output pulse by adjusting the resistance using the potentiometer.

Q19: Are there any precautions I should take while using a 555 timer monostable multivibrator? – Yes, a few precautions include:

Ensure that the power supply voltage (Vcc) does not exceed the maximum rating of the 555 timer IC.
Use appropriate decoupling capacitors to stabilize the power supply.
Consider the current limitations of the 555 timer when connecting external loads.
Provide proper triggering conditions to avoid unexpected triggering or false triggering of the monostable multivibrator.

Q20: Can a 555 timer multivibrator be used in low-power applications? – Yes, the 555 timer can be used in low-power applications by reducing the supply voltage and using appropriate resistor and capacitor values. Additionally, the 555 timer IC has low power consumption when in standby mode.

Q21: What are the limitations of a 555 timer multivibrator? – Some limitations of the 555 timer multivibrator include limited output current, restricted

Q22: What is a bistable multivibrator? – A bistable multivibrator, also known as a flip-flop or a latch, is a circuit that has two stable states and can remain in either state until it is triggered to transition to the other state.

Q23: How does a bistable multivibrator using a 555 timer work? – In a bistable multivibrator using a 555 timer, two external resistors (R1 and R2) form a voltage divider network that sets the trigger levels. The 555 timer acts as a flip-flop, changing its output state when a trigger input is applied. The output remains in the changed state until a reset input is provided.

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