Enhance This Inexpensive NCVT

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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.

Materials

  • 2,200 Ohm resistor
  • Solder
  • Paper

Tools

  • 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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    20 Discussions

    2
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    InstructableSDdleland71

    Reply 3 days ago

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

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    GTO3x2

    2 days 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.

    Thanks.

    1 reply
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    Phil BGTO3x2

    Reply 1 day 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.

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    pjdw2008

    2 days ago on Step 1

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

    2 replies
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    GTO3x2pjdw2008

    Reply 2 days ago

    Maybe hammer it thinner, also.

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    Phil Bpjdw2008

    Reply 2 days ago

    If I were doing it again, I would measure the distance from the solder point to the end of the resistor and use that to size the replacement resistor. Bend the resistor leads so the outer lead aligns with the pocket as much as possible. Do not let the lead be too thick, or it will not fit the pocket.

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    Fast.Eddie

    3 days ago

    I have a friend who was badly shocked when his "wiggy" failed due to a broken lead. For myself, I refuse to cheap out on something that my life might depend upon. YMMV and good luck.

    1 reply
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    Phil BFast.Eddie

    Reply 2 days ago

    You would almost need to push hard on the tip. I am not going to do that. Any of them could be a problem used that way.

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    rflournoy

    3 days ago

    Thank you Phil B. Great work! I, too, enjoy tinkering with things to see if I can make them better, and if I do something that works I like to tell others. My NVCT isn't sensitive enough to do what I want, but I don't think it can be improved. It's a Klein probably similar to yours. I wish it would detect voltage from about 6 inches so I can determine whether there is voltage to a junction box behind my sheetrock without making a hole. Any ideas to make that modification?

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    Phil Brflournoy

    Reply 2 days ago

    Someone at YouTube has an NCVT based on a 4017 chip he adapted from redcircuit (dot) com. I built it, but mine is not nearly as sensitive as his shown in use in his video. I need to experiment with the length of the antenna. Some people use a stud finder for what you want to do.

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    garg11

    3 days ago

    Just bring it near to a wire carrying between 90 and 1,000 volts.
    ----
    If it currying between 90V and 1000V, I think It should work without upgrade in 110V outlet.
    BTW, what is diapason of measuring now?

    1 reply
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    Phil Bgarg11

    Reply 3 days ago

    Since I changed the value of the resistor that serves as the antenna the tester does alert to a voltage by bringing the tip of the tester near to the outer surface of an extension cord. If that cord is the ubiquitous 18-2 flat rubber covered wire, the tester will distinguish between the hot side and the neutral side. It would not respond at all with the original resistor in the antenna, and one could have wrongly concluded a wire was not live when it actually was. I expect all of my use with this will be on circuits carrying 115 volts. Even if I am checking a welder outlet carrying 230 volts, I can check only one side st a time.

    The word diapason is familiar to me, but only as the voicing on a stop for a pipe organ. Please clarify your last sentence. If you mean, “What is the range to which the tester will respond?” I only know it responds well to 115 volts AC. It would be nice if it were to respond to about 12 volts AC so I could use it to troubleshoot low voltage Christmas tree lights, but I do not have a way of checking that now. I rather doibt it is that sensitive. Thanks.

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    ChrisS53

    3 days 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.

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    Phil BChrisS53

    Reply 3 days 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.

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    ChrisS53Phil B

    Reply 3 days 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.

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    beqwaamChrisS53

    Reply 3 days 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?

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    Phil Bbeqwaam

    Reply 3 days 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.