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 IC555 Timer IC Testing Circuit is one of the most popular and most frequently used integrated circuits. It performs an array of timing tasks in the electronic circuits and there is a huge list of experiments which can be performed with 555 IC.  That is why it is very popular among electronics hobbyists.

But before using the 555 Timer IC, you should check it i.e. whether it is working properly or not. So, in this project, I have designed a simple circuit that can be used as a 555 Timer IC Testing Circuit and determine whether the 555 IC is functioning or not.

555 Timer IC Testing Circuit

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

A Brief Note on 555 Timer IC 

I won’t go into too much depth regarding the 555 Timer IC, but there are a few things you should know before trying to figure out how the 555 Timer IC Testing Circuit works. The 555 IC is offered in an 8-pin Dual-in-line Package (DIP) which is the first and most crucial feature (or atleast it is the one I will be using in this project).

The second feature of the 555 Timer IC is that it operates in three modes: astable, monostable, and bistable. The circuit used in this project is essentially a 555 Timer IC in Astable Mode of Operation.


You can use this basic 555 IC testing circuit to test your whole 555 timer IC collection. So, test your IC before utilising it in any project to ensure that it is good or terrible. This is accomplished by setting the IC to act as an oscillator, i.e. the 555 is set to operate in the Astable mode.

The 555 tester circuit will quickly inform you whether or not the timer is working. This circuit’s main characteristic is that it can detect whether a 555 timer is shorted 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 mentioned earlier, I am going to use the 555 IC in its Astable Mode of operation. If you are familiar with this mode, then you can easily design the circuit yourself. 

First, connect Pins 4 (Reset) and 8 (VCC) to +12V Supply and Pin 1 (GND) to GND. Short Pins 2 (TRIG) and 6 (THRESHOLD). Now, connect a 10KΩ Resistor between VCC and Pin 7 (DISCHARGE). This resistor will be called R1. 


Also, connect another 10KΩ Resistor between Pin 7 and Pin 6. This resistor will be called R2. A 47μF Capacitor (here after called as C1) is connected between Pin 2 and GND. 

An optional connection is to connect a 0.01μF Capacitor between Pin 5 (CONTROL) and GND. Finally, connect two LEDs as shown in the circuit diagram to Pin 3 (OUT) of the 555 Timer IC.

How to Check the 555 Timer IC?

To begin, carefully place the IC in the socket (if one is used) so that no 555 timer pins are damaged. Switch on the power supply to see the result. If your 555 timer is working properly, both LEDs (in my case, red LEDs) will alternately glow. Your 555 timer IC is malfunctioning if either of the LEDs are turned off or if both LEDs are not shining.

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

You can see this in the following video.

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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