Enhance This Inexpensive NCVT




About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.


  • 2,200 Ohm resistor
  • Solder
  • Paper


  • Soldering iron
  • Needle nose pliers
  • Thin knife

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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.

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.

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    28 Discussions


    6 days ago

    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

    1 reply
    Phil Bcri8tor

    Reply 6 days ago

    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.


    17 days ago

    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.

    1 reply
    Phil Braphan

    Reply 6 days ago

    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.


    26 days ago

    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.

    1 reply
    Phil Bemrude

    Reply 26 days ago

    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.


    5 weeks ago

    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.

    6 replies

    Reply 4 weeks ago

    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 BPyrojason

    Reply 4 weeks ago

    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 BChrisS53

    Reply 5 weeks ago

    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.

    ChrisS53Phil B

    Reply 5 weeks ago

    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.


    Reply 5 weeks ago

    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 Bbeqwaam

    Reply 5 weeks ago

    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.


    Reply 5 weeks ago

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


    5 weeks ago

    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.


    1 reply
    Phil BGTO3x2

    Reply 5 weeks ago

    Mine also has some surface mount components. It came from Jameco. They also have a list of kits and circuits for builders. One of them is a circuit for a voltage tester I have seen before. It is three cascading NPN transistors with resistors of different values at each stage. The version I first saw was the same circuit, but with a note that changing the value on any one of the resistors changes the sensitivity of the tester. The antenna in that tester is a simple piece of copper wire, not a resistor. Changing the value on the right resistor might change the sensitivity of your tester, but it might also cause problems. Before I experimented with changing the value of the resistor mine uses as an antenna, I was prepared simply to use the tool I made that resembles a small insulated screwdriver. One video of a homebuilt tester shows replacing the short straight wire antenna with a coiled wire antenna for more sensitivity.


    5 weeks ago on Step 1

    Thank youvery much for sharing. Any tips how to get the antenna all the way in the tip?