555 Timer Projects

555 Timer IC – Features, Pinout, Circuit, Operation

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The 555 timer is an integrated circuit that has many uses in electronics. It is used for timing, oscillation, pulse generation, and other functions. This article discusses how to use the 555 timer IC.

The IC was designed in 1971 at Signetic Corporation by “Hans Camenzind” and was initially known as the SE or NE 555 timer. Compared to other ICs in the same regions, such as OP-Amp, these sorts of ICs are inexpensive and dependable in terms of cost and size.

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These integrated circuits are utilized in digital logic probes, DC-DC converters, tachometers, analogue frequency metres, voltage regulators, and temperature-controlled & measuring devices like astable and mono-stable multi-vibrators.

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The SE 555 Timer IC operates in the temperature range of -55°C to 125°C, whereas the NE 555 Timer IC works in the temperature range of 0° to 70°C. This IC is widely used in electronic circuits as a timer, pulse generator, and oscillator. To understand how the NE555 IC works, you may look at its datasheet.

555 Timer IC Internal Circuit Diagram & Working Principles

The 555 timer IC is a chip found in various fields and applications, such as a pulse generator, oscillator, and timer. The standard 555 Timer integrated circuit contains 25 transistors, two diodes, and fifteen resistors.

555 Timer IC Internal Circuit Diagram
555 Timer IC Internal Circuit Diagram

The 555 timer IC comprises a flip-flop, voltage divider, and comparator. This IC’s primary job is to create an accurate timing pulse for different devices.

Inside the IC is a voltage divider consisting of three 5k resistors. These resistors create two reference voltages. The reference voltages are 1/3 and 2/3 of the supplied voltage range from 5 to 15V.

Also, two comparators compare two analogue voltages. If the input voltage at the positive terminal is higher than the input voltage at the negative terminal, the comparator will output high.

Similarly, if the voltage at the negative input terminal is higher than that at the positive terminal, the comparator will output low.

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Note that high represents logic 1, and low represents logic 0.

As seen above, the first comparator’s negative (-ve) input terminal is connected to the 2/3 reference voltage at the voltage divider and the external control pin.

In contrast, the positive (+ve) input terminal is connected to the external Threshold pin.

The second comparator’s inverting (-ve) input terminal is connected to the Trigger pin, while the non-inverting (+ve) input terminal is connected to the 1/3 reference voltage at the voltage divider.

So we can control the output of the two comparators using the three pins Trigger, Threshold, and Control. The output is subsequently sent into the flip-flop’s R and S inputs. When R is 0 and S is 1, the flip-flop will output 1, and vice versa.

The flip-flop may be reset using the external Reset pin, which can override the two inputs and reset the entire timer at any moment.

The flip-flop’s Q-bar output is routed to the output stage or output drivers, which can either source or sink a current of 200mA to the load. The flip-flop output is also coupled to a transistor, which connects the Discharge pin to the ground.

555 Timer IC Pinout

Even though the 555 Timer IC comes in various packages and labels, it has eight (8) distinct pins. We have attached a diagram with all the pins and their functions below.

Pinout of the 555 Timer IC
555 Timer IC Pinout

Pin 1: Ground (GND)

The ground pin is directly linked to the power source’s negative terminal. It would be best if you did not connect resistors between the ground and the voltage source to avoid stray voltage.

Pin 2: Trigger Pin

The trigger pin is used to initiate the IC’s timing cycle so that it may operate. It is a low signal pin, and the timer is activated when the voltage falls below one-third of the supply voltage (1/3 V). It is connected to the comparator’s inverting input within the IC and accepts negative signals for operation.

Pin 3: Output Pin

Pin 3 is the output pin of the 555 Timer IC. When the IC is activated, the output pin goes high for the duration of the timing cycle. It is a sinking current with a voltage larger than zero in a situation where the output is zero (0 V). In the case of logic high output, however, it is sourcing current with an output voltage less than Vcc.

Pin 4: Reset Pin

To reset the IC, we use the reset pin. It must be connected to the positive terminal for the IC to function correctly. If this pin is grounded, the IC will not function at all. For the timer circuit to work, a reset voltage of 0.7 volts and a current rating of 0.1mA are required.

Pin 5: Voltage Control

It is employed for reliable timing. When the control pin is not in use, connect it to the ground via a capacitor; otherwise, the IC will exhibit inconsistent reactions (deviating from the desired value).

Pin 6: is the Threshold Pin

When the voltage on the timing capacitor exceeds 0.66Vcc, it detects it. The timing cycle is only completed when the voltage on this specific pin equals or exceeds 2/3 of Vcc.

Pin 7: is the discharge pin

When the output is low, it provides a discharge channel from the timing capacitor to the ground. To avoid damage to the 555 timer IC, ensure the discharging current should be less than 50 mA.

Pin 8: VCC (Supply Terminal)

To power up the 555 timer IC, the positive (VCC) terminal is connected to the positive terminal of the power source. It receives power supply voltage from the power source and uses it to operate. The operating voltage is between 5 and 15V.

Operating Modes of 555 Timer IC

The 555 integrated circuits can operate in various modes. But the most common modes are the astable mode, the bistable mode, and the mono-stable mode.

Astable Mode

Astable multivibrator
Astable multivibrator

In this mode, the circuit of the IC 555 timer produces continuous pulses with exact frequencies based on the value of the resistors and capacitors. Here, the charging and discharging of the capacitors depend on voltage.

The output pin generates a square wave to turn on and off a load at precise intervals. An example is the blinking of an LED. In the astable mode, two external resistors and one capacitor control the duty cycle and frequency to achieve the required operation.

Mono-stable Mode

Mono-Stable Multivibrator circuit
Mono-Stable Multivibrator

When the timer receives an indication from the trigger button’s input, the circuit generates only a single pulse in mono-stable mode. The resistor and capacitor values generally determine the duration of a pulse.

Suppose it is pushed to the circuit’s input via a push-button. In that case, the capacitor charges, and the timer circuit extends a high pulse, which remains high until the capacitor is completely discharged. If it is essential to increase the time delay, a greater rate of capacitor and resistor is required.

Bi-stable Mode (Multivibrator)

555 timer Bistable mode
555 timer Bistable mode

The Bi-stable mode is utilized when a load is repeatedly switched on and off with a push button. It is sometimes called the Schmitt Trigger.

The circuit here generates two steady-state output signals; low and high states. The output is controlled by applying an appropriate input signal to the set and reset pins.

If a low signal is applied to the set pin, the 555 timer IC output rises to a high level. On the other hand, the circuit’s output will be low if a low signal is applied to the RST pin.

Features of the 555 Timer IC

• It can operate at a voltage between +5V and +18V.
• The source and sink current of the output pin is about 200mA.
• Trigger Voltage is 1.6V when operating at +5V.
• It can withstand operating temperatures up to 70 degrees Celsius.
• It is available in 8-pin PDIP, SOIC, and VSSOP packages.
• The 555 timer IC has a variable duty cycle that can be variable as per user need.


The 555 timer is one of the digital electronics’ most widely used integrated chips. Some common uses and applications of 555 timers IC are:

  • It is used for Pulse, Waveform, and square wave generation.
  • It is also used in time delay generation, precision, and sequential timing.
  • It can be used as a mono-stable multi-vibrator and an astable multi-vibrator.
  • It is mostly used in PWM (Pulse Width Modulation) & PPM (Pulse Position Modulation).
  • Used in Tachometers & temperature measurement.
  • Commonly used in DC Voltage Regulators.
  • Used for conversion like Voltage to Frequency Converter.
  • It is also used in Frequency Divider.
  • It is used in the Pulse detector.
  • The 555 timer IC is also used to design a Timer Switch.

We’ve used the 555 Timer IC in several applications. To learn more about the 555 Timer IC, see the 555 Timer Based Projects area. All of the projects offered will assist you in expanding your expertise.

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Boateng Oduro is passionate about learning new technologies and working on them. He is a teacher and an engineer who loves to read, write, and teach. He's always curious about things and very determined to track the latest technologies and the trends for the future.

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