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
- 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.