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Project Notes

#535 AC Line Detector

Playing around with a simple AC line current detector.


Here’s a quick demo..



There are a bunch of suggested circuits for simple AC line detection if you just google.

This is a quick build of a common approach - essentially using an antenna to stimulate a “triple Darlington” cascade of NPN transistors for extreme gain.


The most common circuit is perhaps this design. It is powered from 9V, with resistors sized for current-limiting the transistors and load (usually and LED, piezo or both):


The circuit I’ve built here is however sized for a lower voltage. I’ve seen examples running from 4.5V, but I’m pushing the extreme end at 3V. To achieve that, all resistors are eliminated, and the construction of the antenna starts to become more significant.

Since we’re detecting 50/60Hz hum, the resonant frequency of the antenna is not particularly important, it is more a case of having sufficient inductance to tap enough energy to trigger the detector (I think).



Breadboard Test

I initially tested the circuit on a breadboard:


A quick demo showing it actually working:


Final Build

Finalizing the circuit, attached to a 2xAAA battery holder:


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.