555 Metal Detector Circuit - Basic Theory

Thread Starter

breadboard noob

Joined Apr 19, 2023
Hi Everyone!

I need help understanding how the following circuit operates.

I understand that the 555 timer is used to generate an oscillating signal, and the inductor (an air coil) will somehow change the frequency of the signal with the introduction of an external metal object, due to the physics principle of electromagnetic field.

My question first of all, is how in this circuit, the frequency of the signal coming out of pin 3 is able to change as a result of a changing inductance. If this can be explained in relationship to the other components that have been selected, what function they serve, and why the rated values are as such, this should help give me a solid understanding of the circuit parameters from an electronics basis.



Joined Jun 22, 2012
L1 will have inductance which will change with metal in it's presence, so this and the Capacitor C1 form a tuned circuit which will have reactance and alter the frequency of the feedback loop on pins2&6 .
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Audioguru again

Joined Oct 21, 2019
Metal near the coil changes the frequency only slightly but the loudness of the tone changes a lot because the metal object becomes the core of the coil making it much more efficient because the series LC becomes a low impedance.


Joined Jan 29, 2010
Please tell me how you set those specific colors to the .step param plot lines?
Hi ES.
LTSpice/Tools/Colour preference/Waveforms.
Select the colours you want to to allocate to the plot traces.
When you Run the sim using the .step param it will use colour #1 for the 1st plot trace, then it will use colour #2 for the 2nd and so on....
I choose White for the plot background colour as it makes the traces easier to see on some PC screens, also if you do a print-out you save Black ink... ;)

Hope this helps.

If you want to see only a particular plot trace.
Right-click on the plotted waveform window, then click View/Select steps, you can then choose the trace of interest.
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