Today, there’s a party at your home. And you are busy detecting faults in the decorative lights hanging over the boundaries of your house. You want to finish the job before evening. But daylight adds to your frustration by making it difficult to observe whether the neon bulb inside the tester is glowing or not.
Conventional continuity testers for live-voltage testing use a neon bulb to indicate the presence of live voltage. But their brightness is significant only indoors. If it were an ultra-bright LED or buzzer, it could be observed in daylight outside the home too.
The live-wire scanner presented here not only uses an LED but also a piezobuzzer that produces a chirping sound like a bird on detecting the point of break in a wire inside the sheath.
Continuity tester circuit
The circuit comprises a few discrete components and a piezobuzzer wired around CMOS Johnson decade counter CD4017 (IC1). This decade counter/divider has ten decoded outputs (Q0 through Q9) with a divided (divideby-10) output available at its pins 3, 2, 4, 7, 10, 1, 5, 6, 9 and 11, respectively, and a carryout bit at pin 12. The count of CD4017 advances by one at the positive edge of each input pulse at its pin 14. This decade sequence repeats after Q9 at pin 11 goes high.
Since the input impedance of a CMOS IC is extremely large and it is a voltage-controlled device, it can be triggered easily with weak stray signals, such as the electric field of a 220V live-wire. This circuit works on the same principle. The potential across clock input pin 14 of IC1, when connected to a small metallic strip, can swing to high and low logic levels even if live wire is 20 cm away from the metallic strip. This triggers the counter and a divide-by-10 (50/10=5Hz) squarewave output is obtained at pin 12 of IC1.
Now, this high sesitivity of the clock input can be reduced to the desired extent by connecting a voltage divider made of passive components of proper value, which, in turn, decreases the input impedance and hence the sensitivity. The device can respond from a distance of up to 10 cm away from the live-wire. This configuration can be used to scan the wire for any breakage (discontinuity) inside the sheath.
Whenever output pin 12 of IC1 goes high, it sends a pulse via capacitor C3 to make transistor T1 conduct and also charge capacitor C4. When pin 12 goes low, transistor T1 remains conducting for a short duration because of charged C4, which discharges via resistor R3. During discharging of C4, transistor T1 exponentially shifts from saturation to active region and then to cut-off region. The voltage across the buzzer and hence the pitch of the buzzer vary accordingly. The buzzer finally stops sounding. This process repeats at a frequency of 5 Hz and a chirping sound is produced. LED1 also blinks in the process.
This continuity tester circuit does not need any earth terminal (as in conventional testers) to touch with finger. So it is quite safe in case of a faulty contact inside the circuit. Assemble all the components, along with a lithium battery, on a general purpose PCB and house in a plastic cabinet.
The article was first published in December 2008 and has recently been updated.