This Continuity Tester Circuit uses two transistor like small switch for checking wire is good or bad and for that we just need a battery and one LED light.
Here, one side touches to battery other side touches the wire and if wire is okay, then current connects through and light will ON, that mean electricity flows without stopping.
Also, if the wire breaks, no current flows and the light stays OFF.
Moreover, this circuit uses normal parts, so we can easily build it and find wire problems.
What is a Continuity Tester Circuit:
Continuity tester circuit is a tool to check fast if circuit is still working or not, which helps to see if wire or line have break or full connection means electricity can flow.
Also, we can check wire, cable, switch, fuse and other electric parts with this and if problem found we can fix it.
Circuit Working:
Parts List:
| Components | Values | 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 |
To begin with, this circuit uses 3V battery for power.
First, BC547 transistor NPN emitter connects to ground and BC557 transistor PNP emitter connects to battery positive side.
Then BC547 base connect to one probe wire bundle for testing through resistor 1M and BC547 collector connect to BC557 base using 1k resistor.
NPN transistor like BC547 work like diode at base emitter and here it get power from probe side.
Furthermore, to test wire one probe touches wire and connect to BC547 base and other probe connects to positive of battery.
Also, if wire is good with no breaks, current connects through wire and BC547 base emitter, then BC547 start working.
Current flow from BC547 collector to emitter and this current connects to base of BC557 through resistor.
Moreover, the BC557 is a PNP transistor that turns ON when the base-emitter junction becomes forward-biased.
Therefore, when the wire is intact the BC547 conducts and allows current to flow, which in turn activates the BC557.
After that, BC557 collector connects to LED anode and cathode of LED connects to ground with 100Ω resistor.
So when BC557 works current flow through LED and LED lights ON.
Formulas:
Below we are using simple rule and small math to choose right parts so tester circuit work good.
LED resistor (100Ω):
Use this formula:
R = (Vsource − VLED) / ILED
where:
- Vsource is battery and in circuit it is 3V
- VLED is LED voltage and red LED usually need 2V
- ILED is LED current and normal LED need 20mA
So:
R = (3V − 2V) / 0.02A = 1V / 0.02A = 50Ω
But we used 100Ω for safety, LED still lights but with less brightness.
Base resistor (1M) for BC547:
Use this formula:
R = (Vsource − VBE) / IB
where:
- Vsource is 3V battery
- VBE is 0.7V for BC547
- IB is base current
We want IB around 1/10 of LED current and if LED takes 20mA base current IB = 2mA
So:
R = (3V − 0.7V) / 0.002A = 2.3V / 0.002A = 1150Ω
We use a 1 MΩ resistor in this circuit for low current testing and safety.
Note:
Value of resistors can change depending on parts used like LED or transistor and above formula help but actual testing and what parts we have also matter for final choice.
How to Build:
To build a Continuity Tester Circuit we can follow these steps.
- First, put BC547 and BC557 transistors on the PCB.
- After that connect BC547 emitter to ground line.
- Then connect BC557 emitter to battery positive.
- Next, take probe 1 and connect it to base of BC547.
- Also, connect BC547 collector to base of BC557 using resistor R1.
- Now connect BC557 collector to LED anode positive leg.
- LED cathode negative leg connects to ground using resistor R2.
- Finally, connect probe 2 to battery positive side.
Important Notes:
- Be sure all wires and parts connect in right place.
- After we add the battery, touch the wire with two probes and if the wire is good with no cut, the LED turns ON.
Remember:
- Resistor values may change if parts are different.
- Always check datasheet for LED and transistor to be sure it work well.
Conclusion:
To conclude, this Continuity Tester Circuit checks the wire using a BC547 NPN transistor, if the wire has good continuity, current flows through the BC547 and then through the BC557 PNP transistor.
This turns the LED ON to show the wire is good and if the wire breaks, current stops flowing and the LED stays OFF.