Battery tester

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 see if a wire is connected from one end to the other, we can use the continuity tester’s probes and place them at the ends of the wires to be examined. If the wires are connected, the circuit will create a sound, showing that the wires are connected in the middle without any breaks.

We can also use this tool to construct a tester to check for wire discontinuity. When connecting components on a printed circuit board or a bread board, there is always the potential that the components will become linked due to faults in the printed circuit board or bread board, or due to mistakes made during the circuit assembly. Whatever the case may be,


  • Circuit Diagram of Continuity Tester
    • Components
    • Circuit Expalnation
  • Polarity cum Continuity Tester Circuit
    • Polarity Cum Continuity Tester Circuit Diagram
    • Components Used in this Circuit
    • Polarity Cum Continuity Tester Circuit Description

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

Many circuit testers have already been devised, but this one has the advantage of making a pleasant sound instead of the irritating buzzer sound that is intolerable to hear when the circuit detects the connection. This circuit also performs it in an efficient manner, employing integrated circuits whenever possible to keep the hardware to a minimum and the circuit size to a minimum.

  • 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

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

To begin, connect the component you want to test with the test probe in any way possible. Any LED in this circuit that glows indicates that the cathode terminal is active.

When you turn on the circuit for the first time, both LEDs will begin to shine. Now connect the component across the probe for testing.

If LED1 starts to glow, it means the component’s cathode side is connected to probe 1, and if LED2 starts to glow, it means the component’s cathode side is connected to probe 2.
The circuit is completed with the help of two transistors, which are connected in an astable multivibrator configuration.

The circuit outputs from both transistors are not in phase with one another, which means that if the first transistor output is high, the other transistor output will be low, or if the output of the second transistor is higher, the first transistor output would be low.

When T1 transistor is turned on and T2 transistor is turned off, LED 1 will begin to glow. Similarly, when the T1 transistor is in the “off” state and the T2 transistor is in the “on” state, LED 2 will shine.

When a component, such as a diode or LED, is placed between the probes provided for testing, it bypasses to one of the LEDs dependent on polarity. The test current will begin flowing through a component in the circuit in place of the bypass LED.

To increase the forward voltage drop, a series grouping of LEDs and diodes is connected in the circuit with the T1 and T2 transistors at the collector terminal.

This ensures that the voltage drop across any of the LEDs is greater than the forward drop of the component under investigation.

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.

When both probes are connected to each other, a buzzer will sound, indicating that your circuit is in good working order.

When probes C1 and C2 are not connected to each other, transistor T3 is closed and no base current is present. The voltage levels at the emitter and base of transistor T3 are currently equal.

The transistor became open when you started the continuity test, and because the emitter base of the transistor was at a higher level than the emitter base of the transistor, the buzzer connected to it began to ring. This circuit can be used to inspect various types of cables and PCBs.


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