A mini continuity tester

Introduction

Many a time it may become absolutely necessary to know the continuity of the circuit elements or any other electrical components in practical electronics. To check the continuity of two points it is necessary to use a reliable and at the same time simple continuity tester. It is always handy to have it ready to conduct the check of circuit elements to ascertain the worthiness of it before putting it into use.

I have experience in adding a component especially resistors with open-circuit and resulting malfunction of the circuit after assembling. This has caused time, energy and money in reality. As such it is a wise idea to check the components before assembling, that is putting them into the circuit. Transformer winding, blown fuses, and sensitive components like transistors and other semiconductors are worthy examples for the check. 

(Components under test

A simple electrical fuse, electrical bulb or any other illuminating object, transformer windings, coils, relay coils, low resistance resistors, etc are some of the examples to be required to be tested for continuity before putting into the circuit assembly.   

With ingenuity even you can test diodes, LEDs and transistors for their working by making use of this circuit.)

The three golden steps such as knowing your circuit, knowing your design and knowing your assembling are to be followed in constructing an electronic project for its optimum performance and utility and satisfaction. 

You can understand the need for this three steps approach is discussed here.

Know your circuit

As such, the first step is to know your circuit: 

How about this: "Check out this cool Mini Continuity Checker!"

I describe here a versatile, simple and at the same time low-cost mini continuity checker to assemble and use. As I said earlier a good continuity checker is a must for one who aspires to assemble their own circuit schematics for satisfactory and successful project building. The circuit schematic is given below. 

Circuit  schematic

Parts list

You may require very few components to assemble this mini continuity checker.

You will need the following electronic components 

BC147	NPN Transistors	2 nos.
22K ohm	¼ W Resistor	1no
15K ohm	¼ W Resistor	1 no
680 ohm	¼ W Resistor	1 no
LED	Red LED	1 no
10K ohm	Potentiometer	1 no
3V	1.5 V battery	2 nos
SW1	On-off switch	1 no.

 

Kindly go through the circuit schematic of the mini continuity checker and you can use it as a handy tool in your suit case. Two NPN transistors are used along with LED for simplicity and low cost.

Know your design

How to use it

Just switch on the gadget, connect terminal A and terminal B to two points under test, whose continuity has to be checked, slightly adjust the potentiometer P1, the red LED lights up indicating that continuity has been established. If LED does not lit, it shows no connectivity. That’s all, very simple.

As an illustration, if you want to test the continuity of an electrical fuse, then connect the two test probes A and B of the circuit to the two points P and Q of the fuse. Adjust P1, LED lit, continuity is OK, fuse is good. Otherwise, fuse is blown out.       

 Working of the circuit

The working principle of the circuit is very simple. Very low resistance of the circuit element connected between the terminals A and B helps to bias Q1 which in turn activates Q2 and amplifies the signal and helps to conduct   current that passes through R3 and LED. Thus LED is lit in short condition. No continuity, no bias and absence of current flow through LED resulting no lit up of it.

Know your assembling

Assembly hints

It is very easy and simple to assemble. Any novice can do it with ease. The circuit can be easily assembled on vero-board or any general purpose PCB. It is possible to assemble it even on the back side of the 2 pen torch battery holder. Some ingenuity idea can make the job simpler. 

I actually built this circuit on a very small vero board and placed over the 2 pen torch cell holder box. 

If you want to build Light operated toy organ click and know more.

Concluding Remarks 

High resistance components may not show continuity and this is the only limitation of the circuit. Otherwise commonly used electronics components as described above will give satisfactory results. 

Please record your comments so that the idea of presentation may be improved.