Battery tester

Continuity Tester With Melody

A continuity tester is a vital device used for assessing the integrity of a given wire. It proves invaluable for identifying issues like severed wires or unintended wire connections.

To determine whether a wire maintains a continuous connection from one end to the other, one can employ the probes of a continuity tester by placing them at the wire’s endpoints. If the wire is indeed connected, the circuit will emit an audible signal. Confirming that there are no interruptions in the wire’s continuity.

Furthermore, this tool can be repurposed to create a tester capable of detecting wire discontinuities. When assembling components on a printed circuit board or breadboard. There’s always the possibility of unintended component connections due to defects in the board or errors in the circuit assembly process. Regardless of the specific circumstances,


  • 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

Numerous circuit testers have been developed, but this one offers the advantage of producing a pleasant sound instead of the annoying and intolerable buzzer sound commonly associated with circuit continuity detection. Furthermore, this circuit is designed for efficiency, employing integrated circuits wherever possible to minimize hardware and circuit size.

  • This circuit utilizes the 555 IC timer in buffer mode. When the circuit probes detect a shorted connection, the 555 IC outputs a DC voltage. This output is then directed to a UM66 music-generating IC, which takes its input from the 555 IC’s output. The output remains high only when the circuit detects that the probes are shorted; otherwise, it stays low.
  • The music generated by the UM66 integrated circuit is played through an 8-ohm mini loudspeaker. The circuit can be conveniently assembled onto a printed circuit board for easy access whenever circuit continuity or short circuits need to be tested.
  • For power, a battery power supply is recommended, as the circuit is designed to consume minimal power. This choice ensures longer battery life and enhances the circuit’s portability.

Polarity cum Continuity Tester Circuit

Here is another circuit designed to assess both circuit continuity and the polarity of components.

This straightforward circuit is capable of testing the polarity of various components, including diodes, LEDs (including Zener diodes), and infrared LEDs. It serves as a valuable tool because there are instances when individuals struggle to determine the correct polarity of a component, and as a result, they may incorrectly install it in the circuit, leading to potential damage to the component or even the entire circuit.

Furthermore, this circuit can be employed as a Continuity Tester, allowing for the verification of circuit integrity.

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 start, attach the component you wish to test to the test probe in any orientation. In this circuit, the illumination of any LED indicates that the cathode terminal is active.

Upon initially activating the circuit, both LEDs will illuminate. Subsequently, connect the component across the testing probe.

If LED1 begins to light up, it signifies that the component’s cathode side is connected to probe 1. Conversely, if LED2 starts to shine, it indicates that the component’s cathode side is connected to probe 2.

This circuit operates with the assistance of two transistors configured in an astable multivibrator setup.

The outputs from both transistors are out of phase with each other, meaning that when the first transistor’s output is high, the other transistor’s output will be low, and vice versa.

When T1 transistor is activated and T2 transistor is deactivated, LED 1 will illuminate. Similarly, when the T1 transistor is deactivated and the T2 transistor is activated, LED 2 will light up.

When a component, such as a diode or LED, is placed between the provided testing probes, it connects to one of the LEDs depending on its polarity. The test current is then routed through the component within the circuit instead of bypassing the LED.

To ensure that the forward voltage drop is sufficient, a series arrangement of LEDs and diodes is incorporated into the circuit at the collector terminal of T1 and T2 transistors.

This setup guarantees that the voltage drop across any of the LEDs exceeds 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.

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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