Enhance This Inexpensive NCVT

Intro: Enhance This Inexpensive NCVT

An NCVT is a non-contact voltage tester. They can be very handy, although not a foolproof indicator no line voltage is present. Much depends on the sensitivity of the tester and the technique of the user. Recommended practice is to check the tester on a known good circuit before touching bare terminals with your fingers.

This NCVT was less than $7 from a mail order electronics parts company. A photo I saw suggests to me that Home Depot sells the same tester with a different color scheme on the case. This looks just like a FLUKE NCVT. The model number is even similar. But, it is not a FLUKE.

One of its problems is that it lacks the usual sensitivity expected from an NCVT based on those I have used. This Instructable will show how I increased the sensitivity so that it is a useful, dependable tool. I will also show how to circumvent another problem.

Materials

• 2,200 Ohm resistor
• Solder
• Paper

Tools

• Soldering iron
• Needle nose pliers
• Thin knife

STEP 1: Before You Power Up

A very nice feature of this inexpensive NCVT is that it is always ready to test for line voltage without activating a switch. Just bring it near to a wire carrying between 90 and 1,000 volts. It also shuts itself off so the battery is not depleted.

But, there is a problem that easily causes one battery to get very hot. See the text box in the first photo. I cut a strip of paper with a scissors and folded it over to make double thickness. Line the bottom of the battery compartment with the paper to keep the battery away from the two metal tabs. My batteries have a plastic coating the tabs pierced and the battery over them became very hot very fast. See the second photo.

STEP 2: Sensitivity Flaw

The photo shows a tool I made. It has a flat blade end and a wooden handle. In the middle is a steel shaft. I place it into an outlet and rest the tester on the taped shaft. The NCVT would alert to the live voltage. It would not indicate voltage is present if I merely put the end of the tester into a slot in the outlet. Two other NCVTs I have used indicate a voltage if the flat end of the tester is inserted into the live side of a 117 volt outlet of the style used in the USA. One of those NCVTs was also an inexpensive one.

STEP 3: Remove the Circuit Board From the Tester

See the text box in the photo. It shows where to place the point of a knife to lift the back end of the circuit board. Twist right and left to loosen the circuit board from the case of the NCVT. Pull the circuit board toward the back of the tester and out of its case.

STEP 4: The Antenna

The antenna on this NCVT is not a metal blade as I have seen on other NCVTs, but the wire on one side of a resistor. The resistor has a value of 1,000,000 Ohms, or 1 Megohm. It is soldered to the circuit board on one end.

STEP 5: Change the Resistor

I experimented and discovered a 2,200 Ohm resistor in place of the 1 Megohm resistor increases the sensitivity of the tester very nicely and brings it into a useful range I expect from an NCVT. I did not have a very small resistor and was able to use a standard size 1/4 Watt resistor. I had to do some trial and error to get the correct length on the leads. I bent the front lead into a "U" to conform to the place for it in the flat sensor portion of the translucent portion of the case. When inserting the circuit board into the tester’s case be careful that the bent lead of the resistor slides into the pocket for the antenna. It slides in more easily if you lightly sand the resistor lead on the left and right sides to make the lead thinner. Look at the end of the tester with a bright light in the background and you should be able to see the outline of the resistor lead in the pocket. If the resistor lead does not slide into its intended place, the soldered lead of the resistor can come into contact with other parts of the circuit to cause a short that keeps the tester from working.

STEP 6: Try It

In the photo I have pushed the flat sensor portion of the translucent NCVT case into the slot of an outlet that is the hot side. The red light indicates the tester has detected voltage. This tester is now much more useful to me.

See the second photo. I hoped this NCVT would alert on Christmas light strings. It does on this string, which means I can use this NCVT to troubleshoot problems by determining where the current flow ends. (See the red glow reflected off of the tree "needles.") Unfortunately, it does not alert on an older string also on this imitation tree.

STEP 7: A Precaution

I have decided I need to remove the AAA batteries from this NCVT when not in use. I had leakage from aging alkaline batteries. There was corrosion and the tester stopped working. The photo shows the IC and its legs on the bottom of the circuit board. A very small amount of corrosion was between two legs of the IC and I did not see it. I sprayed it with a Water Pik and plain water. Distilled water would have been preferred, but tap water worked. I let it dry completely, and my NCVT works again. I also cleaned some corrosion from other places. Some months after this happened, some corrosion continued at one of the contacts for the battery, and the tester did not work when I installed batteries that were still good. I cleaned and resoldered the battery contact to the circuit board.

STEP 8: Build Your Own?

You can build your own NCVT. Here is a link to an Instructable that offers three different circuits. One uses three common transistors. Another uses a 555 Timer chip. And a third uses a 4017 decade counter chip. In the photo is the NCVT I built using a 4017 decade timer. I built it as a curiosity. After experimenting with it, I decided it is a solid performer sensitive enough to troubleshoot strings of Christmas tree lights as well as line voltages. The circuits in the Instructable linked use 6 to 9 volts input power and have a buzzer as well as an LED for an indicator. The circuit I built called for 3 volts input power. The LED glowed very faintly. I placed a tube around it to concentrate the light emitted. I also eventually increased the voltage to 6 volts and it is much easier to see. An LED alone is fine for me. The antennae are often just some magnet wire in a small coil. I used a piece of #14 plastic covered household wire. It can take considerable abuse without bending or breaking. I put a ball of hot glue on ithe end of the wire so I do not accidentally send line voltage into my tester and perhaps my hand. I slit a short piece of PVC to act as a clamp for two 2032 wafer batteries. This tester now resides in my vehicle so I can test something on the spur of the moment when away from home. People sometimes ask me to look at something when I least expect it.

Comments

[АхмедА]

Thank you friend

[Phil B]

You are welcome. I really want to try this modified NCVT on AC mini-bulb lights for a Christmas tree. I have a Klein Tools NCVT I thought I had ruined when the batteries corroded, but I used a Water Pik water flosser for cleaning teeth to spray away the corrosion. I waited for it to dry thoroughly and it works again. The Klein has a lower voltage function. It would be a bonus if I can troubleshoot mini-bulb Christmas lights with it. As relates to safety concerns raised by some, I personally have never used this NCVT to test anything with more than 120 volts. You write your name with letters from the Cyrillic Alphabet, which means you may live where 220 volts is standard line voltage.

[cri8tor]

Mr. Phil B
Thank you so much for sharing your mods of this tool. Knowledge is power and it should never be censored because of the emotional outburst of the ignorant.
Cheers brother

[Phil B]

Thank you. Safety is important. It is also necessary that the user of a device employs safe practices in its use. The maker of a device cannot protect every user from every possible misuse of the device. Imagine no one has ever invented a knife until now. You get the idea for a knife, and you publish an Instructable on how to make and use a knife. You would probably have a squadron of commenters denouncing your idea of a knife because of the many ways people could injure themselves or be killed using your invention. Yet, we all use knives almost daily and accept the risk, but cover it in safe practices.

[raphan]

These kinds of tester only detect alternative voltages, like mains, they don't work for direct current which isn't their purpose. Of course, reducing the serie resistor's value will increase sensivity, but not that much as this decreases the input impedance, it's better to increase the amplifier gain, and safer as well.

[Phil B]

As I mentioned in response to another comment, there is a circuit disgram for an NCVT one can build at home that uses three cascading transistors. Each has an associated resistance. A note says lowering the value of any of the resistances increases the sensitivity of the NCVT. Some NCVTs have a sensitivity adjustment knob. Anything resembling an amplifier on this NCVT is probably encased inside an IC chip and not very accessible.

[emrude]

I have gone through a lot of these testers of different brands. I always do a pre-test to be sure meter is working. I do this for all meters. When the things don't pass the pretest they are pitched after checking for blown fuse, bad leads or faulty batteries. I don't think I hardly ever put a contact-less tester in an outlet. I use them to check for power in ceiling box, or along a wire run. Safe practices that may save a bad shock, or my life.
I carry one in my CERT bag.

[Phil B]

These non-contact testers are helpful, but not absolutely a reliable process. The one shown here pretty much is better now at indicating current is present when I bring the tip to touch the rubber covering on a cable connected to a wall outlet. It did not pick up the presence of current through the outer insulation of a cable before. But, the first time I used one of these I was replacing outlets in my daughter’s older house that she wanted to put on the msrket for sale. Most outlets in the house had nothing plugged into them. The little non-contact tester was not an expensive brand name, but helped a lot to know if an outlet was live. I think I did need to insert the tip a bit into the slot. Still, it is a good idea to double check results indicated by one of these testers with a voltmeter. What I really like about these, when they are sensitive enough, is the assist they give in fixing Christmas tree light string problems.

[ChrisS53]

You have just made what was maybe a safe tool into a lethal one. That 1 M ohm resistor is there for your safety. In the event that the plastic cover fails and the end of the resistor comes into contact with 120 vac, the 1 M ohm resistor will limit the potential current you could be exposed to to ~0.1 mA. The 2200 ohm resistor you have chosen would limit the current to ~ 55 mA - well inside the instant death zone by fibrillation - generally taken as 10 mA to 2 A. I paid $50 for the Fluke - I figure my life is worth that.
By the way, when I said "maybe a safe tool" I meant it - is it actually approved? My Fluke has full European, US and Canadian approvals and I can verify that on line anytime I want.

[Pyrojason]

The very design of these testers - Klein, Fluke, this one - all have the
"potential" to be dangerous if they were broken and completed a circuit
between you and.. what else are you touching? A lot could go wrong -
but there's no need to insult the author.
Can you use the other
resistor in front of the stock resistor to achieve the best of both
worlds? More "safety" and ability to sense current more easily?

[Phil B]

Thank you for your comment. Concerning the use of both resistors, there are two unbending rules. One is: resistors joined one after the other like rail cars (in series) adds their individual resistances for a new, larger cumulative resistance. Since the original design of my tester chose a 1 million Ohm resistor, likely to stop the slim possibility of line voltage flowing to the user as a safety feature, and that resistance also greatly reduced signal sensitivity, adding even more resistance would reduce signal sensitivity even more. Rule # 2 of resistors is: resistors parallel to one another like cars attempting to pass on a race track, reduce the total resistance. The formula is 1/R1 + 1/R2 + 1/R3 (et cetera) = 1/R total. (If you have two resistances of 100 Ohms each, joining them in parallel produces a new resistance of 50 Ohms. It gets more complicated when more than two resistances are involved and when they are not of equal value, but several different values. Now there are apps. and on-line calculators to make it very easy to calculate. But, sometimes you need a 50 Ohm resistance and you have only a handful of 100 Ohm resistors.)

[Phil B]

Thank you for your concern. If the tip broke off and left the antenna lead exposed, I would no longer use the tester unless I could properly insulate it. Even if the resistor lead did contact a live wire, there is no possibility of the user’s hand touching anything that would carry that live current to the user’s skin. I have not looked up its pedigrees, but I am sure KLEIN took care of that.

I do have a KLEIN NCVT-2 as well and I have peaked inside it. It does not have a resistor as part of the antenna, but is a simple stamped piece of metal sheet. If the plastic tip broke off, that metal antenna would be exposed and I see no blocking resistor.

I hope you will publish your first Instructable soon. Other people will comment on your work. Knowing that affects the comments you make on the work of others.

[ChrisS53]

The Klien is certainly as good as the Fluke - it meets all the same standards, Canadian, American and European, and is also rated Class IV, 1000 volts. My other issue is that you are modifying a piece of equipment in a way that makes it unsafe. Maybe it does have some approvals but you just blew those away. You should never modify anything like that that will be used at line voltage or recommend that others do so. In my jurisdiction it would be illegal. Moreover, YOU might notice the crack in the insulation but not everyone else might.

[beqwaam]

Hi Chris, the calcluation you made is incorrect. You made the current calc as if a body has 0 ohms resistance. My body has at this moment a resistance of 3 MOhm add the 0,022 MOhm Phill B proposes and you get 120 / 3022000 = 0,04 A thus 40 mA. That is more than 30mA wich is considered the safe maximum here in the EU.
I do agree that it might be lethal if anything is broken in the NCVT and in which case metal parts/wires get exposed. But if this thing is broken, you better not use it after all (or use it only to detect radiation thu wires).

What I miss is the comparison between the Fluke sensitivity and the low-end one sensitivity. Is there any difference Phill? And are the electronics the same?

[Phil B]

I have never had or used a FLUKE NCVT. From reading the description of the features on FLUKE testers it was clear to me the electronic circuitry is not the same. I do have a KLEIN NCVT-2 that had suffered from battery leakage and corrosion. (I finally got it working well again after using a home Waterpik dental gum and tooth cleaner on the highest setting, but had bought this tester in the meanwhile.) I really doubt I would use this NCVT in any way that would put me in the dangers Chris imagines. And, now it is sensitive enough that it will indicate the presence of a voltage if I simply touch it to the rubber sheath around a cable. I have a brain and I use it when working with electricity.

[dleland71]

An 1/8 watt resistor should work, yes?

[JuanA210]

yes it’s works fine

[InstructableSD]

There should be next to no current flowing, so the wattage of the resistor should be essentially irrelevant.

[Phil B]

Thank you. That is what I wanted to say.

[GTO3x2]

What a coincidence this came up. Mine just stopped working, and I have always wished it was more sensitive. It would not turn on or it would stay on.

I cannot modify mine as easily since its components are SMD. But after opening mine, I noticed the positive battery contact became loose from the circuit board from the force of battery installation; I resoldered it, and it seems to be normally working - I'll just accept its sensitivity.

Thanks.