This little tester uses two transistors like tiny switches to see if your wires are good.
It just needs a battery and an LED light.
You touch one tester to a wire and the other to the battery.
If the wire is okay electricity can flow and the light will turn on.
This means there is a continuous path for electricity.
If the wire is broken the electricity can not flow and the light will not turn on.
This simple tester is easy to build because it uses common parts and is a great tool for finding problems in wires.
What is a Continuity Tester Circuit:
A continuity tester is a device used to quickly check the continuity of an electrical circuit.
It helps determine if a particular electrical path is complete (i.e. if there is a continuous connection) or if there is a break in the circuit.
Continuity testing is commonly used for troubleshooting and verifying the integrity of wires, cables, switches, fuses, and other electrical components.
Circuit Description:
Parts List:
| Category | Item | Quantity |
|---|---|---|
| Resistors | 1k 1/4 W CFR | 1 |
| 100Ω 1/4 W CFR | 1 | |
| 1M 1/4 W CFR | 1 | |
| Capacitor | Ceramic 0.1µF | 1 |
| Semiconductors | Transistor BC557 | 1 |
| Transistor BC547 | 1 | |
| Red LED 20mA 5mm | 1 | |
| Coin Cell 3V | 1 |
The circuit is powered by a 3V battery.
The emitter of the BC547 NPN transistor is connected to the ground line and the emitter of the BC557 PNP transistor is connected to the positive terminal of the battery.
The BC547 transistor is an NPN transistor and its base is connected to one of the continuity testing probes.
The collector of the BC547 is connected to the base of the BC557 through a resistor.
The base emitter junction of an NPN transistor like BC547 acts as a diode and in this case, it is forward biased by the voltage provided by the base probe.
To check continuity one probe is connected to the base of BC547 this is the probe that is touched to the wire being tested for continuity.
The other probe is connected to the positive terminal of the 3V battery.
When there is continuity in the wire i.e. the wire is not broken current can flow through the wire and the BC547 transistors base emitter junction.
This causes the BC547 to conduct.
The BC547 conducts allowing current to flow from its collector to emitter.
This current flows through the resistor and into the base of the BC557 PNP transistor.
The BC557 is a PNP transistor, and it is turned on when the base emitter junction is reverse biased.
When there is continuity the BC547 allows current to flow through the BC557 turning it on.
The collector of the BC557 is connected to the anode of the red LED and the cathode is connected to ground through a limiting resistor.
When the BC557 conducts current flows through the LED causing it to illuminate.
Formulas:
To choose the right component values and make sure the circuit functions as intended, you may apply certain general ideas and computations to the continuity tester circuit shown in the above figure.
The LEDs current limiting resistor (100Ω):
R = Vsource−VLED / ILED
where,
- The battery voltage 3V in our article, is Vsource.
- The forward voltage of an LED is represented by VLED, which is usually around 2V for a red LED.
- The intended current flowing through the LED is represents by ILED (often approximately 20mA for ordinary LEDs).
Base Resistor(1M) for Transistor BC547 :
R = Vsource−VBE / IB
where,
- The voltage of the battery is 3V V source.
- The base emitter voltage VBE of the BC547 transistor is commonly measured at 0.7V.
- IB is the intended base current.
You may estimate that the base current IB should be around one tenth of the LED current in order to operate the LED correctly.
A 20mA LED might have a IB of around 2mA.
Note:
The precise features of the transistors and LED utilized will determine how much of a variation there will be in the resistor values.
While precise formula aid in value selection, the ultimate decision is frequently influenced by component availability and real world testing.
How to Build:
To build the two transistor continuity tester circuit, you can follow these steps.
- Insert the BC547 and BC557 transistors into the PCB.
- Connect the emitter of BC547 to the ground.
- Connect the emitter of BC557 to the positive supply.
- Connect the base of BC547 to probe 1 (one of the continuity testing probes).
- Connect the collector of BC547 to the base of BC557 through a resistor R1.
- Connect the collector of BC557 to the anode of the LED.
- Connect the cathode of the LED to the ground through a limiting resistor R2.
- Connect probe 2 the other continuity testing probe to the positive supply.
Notes:
- Double check your connections insert the battery and touch the continuity testing probes to the wires you want to test.
- If the wires are continuous the LED should illuminate.
- Remember the specific resistor values may need adjustment based on the characteristics of your components and the power supply voltage.
- Always refer to the datasheets of the transistors and LED for accurate specifications.
Conclusion
To conclude, the circuit uses the NPN transistor BC547 to sense continuity in the tested wire and if continuity is detected, it allows current to flow through the PNP transistor BC557, turning on the LED to indicate continuity.
If there is a break the LED remains off.