Non-Contact Probe Works Better With A Little More Complexity

Non-contact voltage probes have been around a while and some test equipment now has them built-in. This is one of those things that you probably don’t think about much, but surely it isn’t that hard to detect AC voltage. Turns out there are a lot of circuits floating around that can do it and [nsievers51] tried a bunch. Many didn’t work very well, but the best used a 4069 CMOS hex inverter. A dollar store flashlight provided power, a case, and an LED and the result was a good-looking and effective probe.

The circuit came from the Electronics Library website and is fairly complex for this sort of device. The CMOS inverters have a high input impedance so they pick up the weak signal. Instead of directly driving an LED, two inverters form a ring oscillator that generate pulses around 1 kHz. At that frequency, the LED appears to be on, but battery consumption is less severe. A single 2N2222-style transistor drives the LED.

We’ve seen a number of variations on this tool in the past. Many of them only use transistors.

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Make A Non-Contact Voltage Probe

You’ve probably seen probes that detect live wires in, for example, home wiring, without having to actually probe the wire. These are sometimes used to test strings of Christmas lights, too. We’ve even seen the sensors built into a voltmeter. [Crazy Couple] has a few do-it-yourself versions that can do the job. You can see the circuits in the video below.

A contactless probe picks up the changing magnetic field around an unshielded wire with an AC voltage on it. Current doesn’t have to be flowing since it picks up the voltage (for example, you can detect voltage on a switch that is turned off or a Christmas tree light that is burned out. There are several different circuits using chips ranging from a CMOS IC to a 555. There’s also a version with three bipolar transistors.

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