Introduction 555 Timer

The 555 timer was introduced more than 40 years ago. Thanks to its relative simplicity, its ease of use and its low costs, it has been used in thousands of applications and is still widely used.

Working of 555 Timer IC

To understand the working of 555 timer IC we have to recall the comparator, S R flip flop, NPN transistor working. 555 Timer IC consist two comparator, one S R flip flop, voltage divider, loose transistor and output phase.

The voltage divider has three identical 5k resistors that make up two reference voltages at 1/3 and 2/3 of operating voltage at range 5 to 15V. Voltage divider connected to the two comparator as shown in the animated figure. Comparator is a circuit that compare the input voltage from the voltage divider circuit and then send to the flip flop circuit. In 555 timer two comparator consist comparator 1 and comparator 2. A negative input terminal of the first comparator is connected to the voltage divider and “control” at the external terminal with a reference voltage of 2/3, and the positive input terminal connected to the external terminal “Threshold”. Comparator above side which which connected tot the pin number 5 and 6 of 555 time IC and lower side comparator 2 connected to the pin number 2.  On the other hand, the second terminal is a negative input of the comparator connected to the “Trigger” pin, and the positive input terminal of the voltage divider connected to the reference voltage 1/3. First 555 timer IC start charging from the input supply voltage via resistance Ra, Rb and capacitor. When capacitor charge upto 2/3 of Vcc (Supply Voltage) then it trigger the positive voltage at comparator and reset the circuit so the output becomes low and above side LED get glows that connected to the supply voltage and pin number 3. Now at the same time capacitor start discharging via Rb resistance and reach upto 1/2 of Vcc. When input voltage reach 1/3 of Vcc then it trigger the lower comparator and it set the S R flip flop. Now the output Q becomes high and lower LED get glow at the output side that connected between pin number 3 and ground.

555 Timer Characteristics and Pin assignments (DIP 8 PIN)

Below Figure shows the pin configuration of 555 timer IC standard dual-line (DIP)  package.

Pin 1 – Ground (GND) this pin is connected to the ground circuit.

Pin 2 – Trigger (TRI) low voltage (less than 1/3 of the supply voltage) applied directly to the input triggers the output of the output (pin 3). The output remains high until the high voltage is applied to the threshold input (pin 6).

Pin 3 – Output (OUT) In a low output state, the voltage is close to 0 V. In a high-state output, the voltage at the supply voltage is below 1.7 V. For example, if the output voltage is 5 V, the output voltage is 3.3 volts. The output can reach 200 mA or plug in (the maximum depends on the supply voltage).

Pin 4 – Reset (RES)

A low voltage (less than 0.7 V) applied to reset the pin has the output (pin 3). This input must be linked to Vcc when it is not in use.

Pin 5 – Pin control (CON)

The threshold voltage (pin 6) can be controlled by a controlled input (set internally at 2/3 of the supply voltage). It can vary from 45% to 90% of the supply voltage. In this way, the length of the output pulse can be changed in the simulation mode or the output frequency in an unstable mode. When it is not in use, it is recommended that this input is connected to the capacitor by a 0.01 uF capacitor.

Pin 6 – Threshold (Th) With unstable and injectable methods, the capacitor voltage is synchronized with the portal input. When the voltage in this input exceeds the threshold, the output is up and down.

Pin 7 – Discharge (DIS) when the voltage across the capacitor exceeds the threshold. The simultaneous capacitor is released from this input

Pin 8 – Supply Voltage (Vcc) The positive supply voltage applied between this terminal and the ground terminal.

Some important features of the 555 timer

The 555 timer is now used in almost all electronic circuits. It has several feature in various application but here we cover some  most important functions of the timer are 555:

  • Works with a wide power range from 5 volts to 18 volts.
  • Reduction or delivery of a current load of 200 mA.
  • External components must be selected correctly so that time intervals can be set in minutes with hundreds of kilometers of frequencies.
  • The transistor output of the 555 transistor transistor (TTL) can be controlled due to the high power.
  • Temperature stability of 50 parts per million (ppm) per Celsius phase, equal to 0.005% / ° C.
  • The operating cycle of the timer is adjustable.
  • In addition, the maximum discharge capacity in each container is 600 mW and the release and reset of the inputs are logically exchanged.

In the 555 Timer Package:

Qty. Description Manufacturer P/N
1 Standard Timer Single 8-Pin Plastic Dip Tube NE555P
1 400-Point Solderless Breadboard 3.3″L x 2.1″W WBU-301-R
10 Resistor Carbon Film 10kΩ CF1/4W103JRC
1 9V Alkaline Battery ALK 9V 522
1 9V Battery Snap with 6″ 26AWG Leads BC6-R
2 3-Pin SPDT Slide Switch SS-12E17
2 Radial Capacitor 0.01µF 2.54mm Bulk SS-12E17
1 Radial Capacitor 4.7µF 2.5mm Bulk TAP475K025SCS-VP
10 Resistor Carbon Film 1.0MΩ 1/4 Watt 5% CF1/4W105JRC
10 Resistor Carbon Film 220Ω 1/4 Watt 5% CF1/4W221JRC
10 LED Uni-Color Red 660nm 2-Pin T-1¾ Box UT1871-81-M1-R
100 Resistor Carbon Film 3kΩ 1/4 Watt 5% CF1/4W302JRC
10 Resistor Carbon Film 330kΩ 1/4 Watt 5% CF1/4W334JRC
1 Radial Capacitor 1µF 25 Volt 2.5mm Bulk TAP105K025SCS-VP