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Continuity Tester With Melody

A continuity tester is a device which tests the continuity of the wire at hand. It is an indispensable tool to check broken wires and undesired shorting of wires.

If we want to check if the wire is connected from one end to another, then use the probes of the continuity tester and put to the ends of the wires to be tested. If the wires are connected, then the circuit makes a sound indicating that the wires is continuous without any break in the middle.

We can also make use of this tool to make a tester to check discontinuity of wires. Many a times when we are connecting the components on the printed circuit board or the bread board, there is always a possibility of the components to get attached  due to defects in the printed circuit board or bread board or due to the mistakes which we may commit while assembling the circuit. What so ever the case is, the continuity tester helps us to debug our circuit with ease.

Circuit Diagram of Continuity Tester

continuity tester circuit


  • 555timer
  • 1N4728A diode
  • UM66 IC
  • R1-1k
  • R2-1k
  • Transistor-BC107
  • Speaker
  • Capacitor -0.01uf

Circuit Expalnation

There are many circuit testers already designed but this comes with an improvement that when the circuit detects the connection, it makes a melodious sound which is pleasing to hear instead of the irritating buzzer sound which is intolerable to hear. That too, this circuit does it using integrated circuits wherever possible in an efficient manner so that the hardware is kept to a minimum and the circuit size is also reduced.

  • The circuit uses the 555 IC timer in buffer mode. The output of the 555 IC is a DC voltage when the circuit probes detects the connection shorted.
  • This output is given to a music generating IC which is UM66. This music generating integrating circuit is then given the input from the output of the IC 555.
  • The output is high only when the circuit detects the probes are shorted. If not, the output is kept low.
  • The music which is generated by the integrated circuit um66 can he heard through the loudspeaker.
  • The loudspeaker used is a mini 8 ohms loudspeaker. The circuit can be conveniently assembled onto a printed circuit board so that it comes handy whenever we need to test a circuit for continuity or short circuits.
  • A battery power supply can be used for powering this circuit as the circuit is made to consume very little power due to which using a battery based power supply is ideal as the batteries will last longer maintaining the portability of the circuit.

Polarity cum Continuity Tester Circuit

Here is the another circuit which tests continuity of the circuit along with the polarity of the components.

The polarity of lots of components viz diodes, LED comprises Zener diode as well as infrared LED can be tested with the help of this very simple circuit

Although it happens many times that people are not  being able  to discovered the polarity of any component and mount them wrong in the circuit which leads to damage of the component or may damage  the  entire circuit.

The continuity of any circuit can also be tested with the help of this circuit, i.e. it also works as a Continuity Tester.

Polarity Cum Continuity Tester Circuit Diagram

Continuity Tester Circuit Diagram
Continuity Tester Circuit Diagram – ElectronicsHub.Org

Components Used in this Circuit

  • Resistor:It is a passive component and mainly employed in the circuit to direct the flow of current in the circuit. Resistors are broadly classified as fixed type and variable type.
  • R1, R4 (820E) – 2
  • R2, R3 (33K) – 2
  • R5 (270E) – 1
  • R6 (1M) – 1
  • D1, D2 (1N4148) – 2: It is a semiconductor device and it permit the current to flow merely in forward direction and block the current in backward direction.
  • LED1, LED2 – 2: LED is a semiconductor device and generally work as an indicator in any circuit and works on low voltage and current.
  • C1, C2 (. 1uF) – 2: Capacitors are mainly employed in the circuit to store the charges. A dielectric medium flows in it which is used to separate the two conducting plates inside it.
  • T1-T3 (BC548) – 3: Transistor is a semiconductor device mainly used to amplify the current.
  • Buzzer – 1: Also known as beeper and is an audio signal device

Polarity Cum Continuity Tester Circuit Description

Firstly in any manner connects the component which you want to test with test probe. Glowing of any LED in this circuit is the indicator of cathode terminal.

Both the LED in the circuit will start glowing when you first start the circuit. Now across the probe provide for testing connect the component.

If LED1 start glowing it means that the cathode side of the component is coupled to the probe 1 and if LED2 start glowing it implies that the cathode side of the component is coupled with probe 2.

The circuit is finished with the aid of two transistors and both the transistors in the circuit are connected as an astable multivibrator.

The circuit output which we get from the both transistors are not in phase with one another, it implies that if we get high output from the first  transistor then the other transistor output will be low or if the output of the second transistor is higher than the output from the first transistor will be low.

LED 1 will start glowing when T1 transistor is in “on” state and T2 transistor is in “off” state. In the same manner LED 2 will be glowing when the T1 transistor is in the “off” state while T2 transistor is in “on” state.

When the component like diode or LED which you want to test is put between the probes provided for testing it bypasses to any one of the LED based on the polarity.In place of the bypass LED the test current will start flowing via a component in the circuit.

Series grouping of LED as well as diodes are connected in the circuit with the T1 and T2 transistor at the collector terminal so that it can raise the forward voltage drop.This will make sure that the voltage drop across any one of the LED is bigger as compared with the  forward drop of the component being examined.

The result of the test when the component is connected-

  1. If LED 1 starts glowing this implies that at probe 1 of the tester cathode is coupled while at probe 2 of the tester anode is coupled.
  2. If LED2 starts glowing it implies that at probe 1 of the tester anode is coupled while cathode of the component is coupled with probe 2.
  3. If any of the LED will not glow it implies that the component coupled to the circuit is short circuit and due to that stable multivibrator of the circuit stop oscillating
  4. If  the components open  circuit  than both the LED in the circuit will start glowing.

If you want to check the continuity of the circuit coupled the circuit with the polarity cum continuity tester circuit with the help of probe C1 and C2 provided in the circuit.

A sound of the buzzer will start when both the probes are attached to each other it means that your circuit is in proper working condition.

When probe C1 and C2 are not connected with each other than at that time transistor T3 is closed and there is no base current. At this moment voltage level of transistor T3 at the emitter and base are of the same level.

Transistor gotten open at the time when you start the continuity test and as compared with the emitter base of the transistor is at a higher level so the sound started from the buzzer connected with it. Different types of cables and PCB can be checked with the help of this circuit.

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