The 555 Timer

The NE555 Timer



NE555 Timer


The 555 Timer IC(Integrated Circuit) is generally a versatile IC for timer operations. Designing a simple oscillator, timer, pulse generator, flip-flop can be done using this IC, also a well available cheap IC in the original bipolar & also in low power CMOS types. This revolutionary chip was introduced in 1971 by Hans Camenzind under contract to Signetics & became world-wide as electronics hobbyists & engineers found it’s vast area of applications. The IC reduces the space of 25 transistors or 2 comparetors, 2 diodes, 15 resistors. From the internal block diagram, there is three ‘R’ resistances present having value 5 Kilo-Ohm that’s why it is known as the 555 timer.

Internal Block Diagram

Pin Functions :

  1. PIN 1 : Pin 1 or the Ground pin is for ground reference voltage (0 V).
  2. PIN 2 : Pin 2 or the Trigger pin goes high and a timing interval starts when this input falls below 1/2 of Control voltage (hence TRIG is typically 1/3 VCC, CTRL being 2/3 VCC by default, if control is left open).
  3. PIN 3 : Pin 3 or the output pin is driven to approximately 1.7 V below +Vcc or GND.
  4. PIN 4 : Pin 4 or the Reset pin, 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 Trigger which overrides Threshold.
  5. PIN 5 : Provides “control” access to the internal voltage divider (by default, 2/3 VCC).
  6. PIN 6 : The timing (OUT high) interval ends when the voltage at Threshold is greater than that at CTRL (2/3 VCC if Control is open).
  7. PIN 7 : Output which may discharge a capacitor between intervals. In phase with output.
  8. PIN 8 : Positive supply voltage, which is usually between 3 and 15 V depending on the variation.

Modes Of Operation:

There are three modes of operations Monostable Mode, Astable(Free-Running) Mode & Bistable Mode or the Schmitt Trigger.

Monostable Mode :

Monostable Mode

When the circuit is turned on, the output is LOW and a brief negative pulse

on pin 2 will make the output go HIGH for a period of time determined by
the value of R and C. If pin 2 is low for longer than this period, the output
will remain HIGH while pin 2 is LOW and immediately go LOW when pin 2
goes HIGH.

Applications : Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, Pulse Width Modulation (PWM) etc.

Astable(Free-Running) Mode : 

Astable Mode

The capacitor C charges via R1 and

R2 and when the voltage on the
capacitor reaches 2/3 of the supply, pin
6 detects this and pin 7 connects to 0v.
The capacitor discharges through R2
until its voltage is 1/3 of the supply and
pin 2 detects this and turns off pin 7 to
repeat the cycle.
The top resistor is included to prevent
pin 7 being damaged as it shorts to 0v
when pin 6 detects 2/3 rail voltage.
Its resistance is small compared to R2
and does not come into the timing of
the oscillator.

Applications : The 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on. The 555 can be used as a simple ADC, converting an analog value to a pulse length. E.g. selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor: the period of the output pulse is determined by the temperature. The use of a microprocessor based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.

Bistable Mode :


The bi-stable 555 has two steady states. SET turns ON the LED and
RESET turns the LED off. The 555 comes on in reset mode as Pin2 does
not see a LOW to SET the 555.

Applications : The 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.

Example Project :

Here is a simple configuration of the 555 Timer used as a low frequency oscillator. Both values of resistances(R) is 10K & the capacitor value is 10uF,40 V. Or it can be used as a LED flasher circuit having delay of 2.2 seconds between each flash. If one 0f the resistor is replaced by a 10K pot then there will be also an option for reducing or increasing the time interval.

Low frequency oscillator or Timer Circuit (2.2s)
Breadboard Setup
Written by : Subhadeep Biswas