DIY Non-Contact AC Detector

You may have seen devices similar to this in the electrical aisle of your local home improvement store. They are handy gadgets that will detect if voltage is present in any section of your home wiring. This is especially useful if you are installing an electrical fixture and are unsure as to whether you have turned off the correct breaker.

I was curious to discover if I could home-brew one of these using simple components. Upon searching the Internet I found some circuits built around transistors, others utilizing 555 timer ICs, and several more ranging from very simple to somewhat complex designs.

I tested most of these ideas on a breadboard and was not impressed until I came across one that I found on the Electronics Library Website (https://www.electronicslibrary.org). This one was very sensitive, yet highly stable.

I had all the necessary components in my parts bin to build the circuit needing only something to house it in, which was provided by a flashlight purchased in a Dollar Tree store for, you guessed it, $1.

The circuit is built around the common CD 4069 CMOS hex inverter logic IC. This IC will operate with as little as 3.5 volts, so you only need two 1.5 Volt batteries to power it. My dollar store flashlight used three AAA - 1.5 volt batteries. I could have bypassed one of the battery compartments and use just two batteries, but decided to go with 4.5 volts. I also used the extra bright LED that came with the flashlight since a standard one was difficult to see at the front end of the device.

R1                          1MΩ

R2                          560KΩ

R3                           220KΩ

R4                           47Ω

C1                           2200pF (.0022 µF)

D1, D2, D3            1N4148 or any general purpose switching diodes

Q1                           General purpose NPN transistor, (see notes below)

LED1                       Just about any LED will do

IC-1                         CD4069 Hex Inverter IC

Probe                      About an inch length of heavy copper wire or other 

conductor wrapped in an insulator

NOTES

(1) The schematic calls for a BC547 transistor, which may not be available in the USA or some other countries, but you can use any small general purpose NPN bipolar transistor, such as a 2N2222 or 2N3904.

(2) Unused inputs (pin 1 and pin 3) should be tied to Vcc or ground, (pin 14 or pin 7)

Here is the schematic and the theory of operation that I adapted from the original article from electronicslibrary.org

I used a dollar store flashlight as the enclosure for the project. There is an on/off switch built into the back end that I retained. You could also possibly add an additional LED as an “on” indicator. 

I cut out part of a proto-PCB to fit in the flashlight housing. I used a prong that I salvaged from an outlet adapter so that it could be inserted into an outlet socket. It turned out that the device is sensitive enough without needing to insert the probe. I later swapped a green LED for the extra bright white LED that came with the flashlight.

I finished off the end using a bottle cap that I painted with a black Sharpie pen. I drilled a hole for the LED and cut a slot for the pickup probe after sheathing it in heat-shrink tubing. The cap was so snug that I didn’t even have to glue it.

Thanks to the Electronics Library for this circuit.

You can view their article here:

https://www.electronicslibrary.org/post/l82-invisible-broken-wire-detector