555 timer ic

How 555 Timer IC Testing Circuit Works?


  • Introduction
  • A Brief Note on 555 Timer IC
  • Principle
  • Simple 555 Timer IC Testing Circuit Diagram
  • Components Required
  • Circuit Design
  • How to Check the 555 Timer IC?
  • Working of 555 Timer IC Tester Circuit


The 555 Timer IC Testing Circuit is undeniably one of the most widely employed integrated circuits, renowned for its versatility in handling various timing functions within electronic circuits. It offers a plethora of experimentation opportunities, making it a favorite among electronics enthusiasts.

However, before integrating the 555 Timer IC into your project, it’s essential to ensure its proper functionality. To address this requirement, I’ve developed a straightforward circuit for evaluating the operational status of the 555 Timer IC. This circuit serves as a 555 Timer IC Testing Circuit, allowing you to confirm whether the 555 IC is operating correctly or not.

555 Timer IC Testing Circuit

If you are starting with 555 Timer IC, then read this beginner’s tutorial on 555 TIMER IC.

A Brief Note on 555 Timer IC 

I won’t delve too deeply into the specifics of the 555 Timer IC, but there are a couple of key points to grasp before delving into the functionality of the 555 Timer IC Testing Circuit. Firstly, the 555 IC is available in an 8-pin Dual-in-line Package (DIP), and this packaging is the primary and most essential characteristic, at least for the purposes of this project.

Secondly, the 555 Timer IC has the capability to operate in three distinct modes: astable, monostable, and bistable. In this particular project, the circuit primarily employs the 555 Timer IC in its Astable Mode of Operation.


This simple 555 IC testing circuit can be employed to assess your entire collection of 555 timer ICs. It’s advisable to verify the functionality of your IC before incorporating it into any project, ensuring its reliability. This is achieved by configuring the IC to function as an oscillator, specifically setting the 555 to operate in its Astable mode.

The 555 tester circuit offers a rapid means of determining the operational status of the timer. A notable feature of this circuit is its capability to identify whether a 555 timer is either malfunctioning or not oscillating.

Simple 555 Timer IC Testing Circuit Diagram

Components Required

  • 555 IC (IC under test)
  • 8 Pin IC Holder
  • 2 X 10KΩ Resistors
  • 2 X 1KΩ Resistors
  • 47μF Capacitor (Electrolytic)
  • 0.01μF Capacitor (Ceramic Disc)
  • 2 X LEDs
  • 12V Power Supply 
  • Mini Breadboard 
  • Connecting Wires

Circuit Design

As previously mentioned, I will be employing the 555 IC in its Astable Mode of operation. If you are well-versed with this mode, you can readily design the circuit on your own.

To begin, establish connections by linking Pins 4 (Reset) and 8 (VCC) to a +12V power supply, while connecting Pin 1 (GND) to the ground (GND). Short Pins 2 (TRIG) and 6 (THRESHOLD). Next, attach a 10KΩ resistor between VCC and Pin 7 (DISCHARGE), designating this resistor as R1.

Additionally, establish a connection with a second 10KΩ resistor between Pin 7 and Pin 6, denoting this resistor as R2. Introduce a 47μF capacitor, referred to as C1 hereafter, connecting it between Pin 2 and the ground (GND).

You have the option to include an additional connection, which entails linking a 0.01μF capacitor between Pin 5 (CONTROL) and GND. To complete the circuit, connect two LEDs as illustrated in the circuit diagram to Pin 3 (OUT) of the 555 Timer IC.

How to Check the 555 Timer IC?

To initiate the process, handle the IC with care and ensure it is properly seated in its socket (if one is utilized) to avoid any damage to the pins of the 555 timer. Then, power on the supply to observe the outcome. If your 555 timer functions correctly, both LEDs (in my instance, red LEDs) will exhibit alternate illumination. However, if either of the LEDs fails to light up or if both LEDs remain unlit, it indicates a malfunction in your 555 timer IC.

Working of 555 Timer IC Tester Circuit

The 555 IC is utilised as an astable multivibrator in this circuit, and when power is applied to the circuit, the LEDs begin to blink, indicating that the IC is operational. Increase or decrease the values of resistor R1 and R2 as well as capacitor C1 to adjust the blinking rate of LEDs.

You can calculate the time duration with the help of formulae given below.

ON Time (HIGH) in Seconds = 0.693 * (R1 + R2) * C1
OFF Time (LOW) in Seconds = 0.693 * R2 * C1
Total Time Period in Seconds = 0.693 * (R1 +2R2)*C1
Frequency = 1.44 / ((R1 + 2R2) * C1)

As per our circuit, R1=10KΩ, R2=10KΩ and C1=47μF. If you substitute these values in the above equations, you will get the following results.

Frequency = 1.023 Hertz
ON Time = 0.651 Seconds
OFF Time = 0.326 Seconds
Time Period = 0.977 Seconds

When the power is turned on, C1 will begin charging through R1 and R2 as soon as the power is turned on. The internal Flip Flop toggles when the voltage across C1 exceeds 2/ 3 of the supply voltage. Pin 7 becomes low as a result, and C1 begins to discharge.

The internal Flip Flop resets and pin 7 turns high when the voltage across C1 falls below 1/ 3 of the supply voltage. The C1 begins to charge once more. All of this will only happen if your IC is in good working order. The output will stay HIGH or LOW depending on the charging and discharging durations of the Capacitor (as determined by R1, R2, and C1), and the LEDs will blink accordingly. We can deduce if the 555 Timer IC is defective or not based on these findings.


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