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

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.

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.

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.

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.

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.

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.

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.

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.