This tiny tester uses two transistors which resembles tiny switches to check the wires quality.
You only need a battery and an LED light.
while touching the battery with one tester and a wire with the other.
If the wire is in good shape electrical current will flow and the light will illuminate.
This indicates that electricity is flowing continuously.
If the wire is broken no electricity will flow and the light will not turn on.
Designed from common parts this simple tester is a great tool for detecting wire defects
What is a Continuity Tester Circuit:
A continuity tester is an instrument for quickly testing if an electrical circuit is still working.
Checking for a break in the circuit or whether a specific electrical line is complete (i.e., a continuous connection) is helpful.
The integrity of wires, cables, switches, fuses and other electrical components can be checked and fixed via continuity testing.
Circuit Working:
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:
Using some general principles and calculations one can choose the right component values and ensure that the circuit operates as designed for the continuity tester circuit shown in the above picture.
The LEDs current limiting resistor (100Ω):
R = Vsource−VLED / ILED
where,
- The battery voltage 3V is Vsource.
- The forward voltage of an LED is represented by VLED which is usually around 2V for a red LED.
- The required current flowing through the LED is represents by ILED which is around 20mA for ordinary LEDs
Base Resistor(1M) for Transistor BC547 :
R = Vsource−VBE / IB
where,
- The voltage of the battery is 3V Vsource.
- The base emitter voltage VBE of the BC547 transistor is commonly measured at 0.7V.
- IB is the required base current.
One can 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 should have a IB of around 2mA.
Note:
How much the resistor values differ will depend on the particular features of the transistors and LED used.
Although accurate formulas help with value selection, component availability and actual testing often have an impact on the final choice.
How to Build:
To build the two transistor continuity tester circuit one 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:
- Make sure your connections are correct.
- After putting the battery in, tap the particular wires with the continuity testing probes.
- If the cables are running continuously the LED will light up.
- Keep in mind that the power supply voltage and the properties of the components may require adjusting the exact resistor values.
- Always check the transistor and LED datasheets for exact needs.
Conclusion
In the end the circuit detects continuity in the tested wire using the NPN transistor BC547.
If continuity is found current is allowed to pass via the PNP transistor BC557 which activates the LED to show continuity.
The LED stays off if there is a break.
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