understanding circuit operation

[amitaiwe]

Hi,
I have build up a metal detector circuit based on this implementation:
https://www.instructables.com/id/How-to-make-a-cheap-and-simple-metal-detector/
I am trying to figure out it's working operation.
This implementation appears in a lot of sites in the web but there's no real explanation of what's going on there and why are the specific components chosen.

In theory the LC's frequency is f=1/(2*pi*sqrt(LC). I used a 22uF cap and 33mH inductor which produced a 440hz frequency.
by theory it's supposed to be 186hz - that's more then times two. Now of course that the 555 is influencing this result via the (I think) astable frequency and I am trying to understand how.
Shortening up the two legs of the inductor results in a square wave with 2.6hz frequency and shortening up also the two legs of the capacitor results in a 4.54khz frequency. all frequencies I measured in the 555's output (pin 3).
Can anyone help me or direct me how to figure out the working operation?
Thanks,
Amitai

 

[Audioguru]

It is a horrible circuit like most Instructables who are designed by 10 years old little kids who know nothing about electronics.
A 555 is supposed to use a resistor that charges and discharges the capacitor to ground at pin 2 and pin 6. It is not designed to use an LC tuned circuit.
There should NEVER be a capacitor to ground at its output pin 3 that your circuit has, because nothing limits the very fast charge and discharge currents in it. The output is supposed to switch high and low very quickly, not ramp slowly up and down.

The Instructable has an 8 ohm speaker as its load but a 555 timer is not a high current power amplifier, its maximum allowed output current is only 200mA.
A current of 200mA into 8 ohms is produced by a peak voltage of 200mA x 8 ohms= 1.6V. But the 555 with a 12V supply will try to produce a peak voltage of (12V/2) - 2.5V= 3.5V which creates an output current of 3.5V/8 ohms= 437.5mA which might destroy the poor little 555 timer.

EDIT: The Instructable says it copied the horrible circuit from talkingelectronics.com but I couldn't find it there. There were many very good but more complicated metal detector circuits there.

 

[amitaiwe]

Thanks for the quick answer, I will make some measurements without the 0.33u cap.

I know that this isn't the best implementation but still I would like to use it,
and in order to know what are the ideal components values to use I want to understand
the circuit's working operation.

As seen in the last figure I am not using the 8 amp but checking visually the frequency using a scope and capturing the frequency using a uC.

Thanks again,
Amitai

 

[Audioguru]

The output of a 555 timer as an oscillator is supposed to be a rectangular wave. The output of an LC oscillator is usually a sinewave. The circuits are completely different.
I attach a 555 oscillator circuit and an opamp LC oscillator circuit.

If you want to make a metal detector then why not use a circuit that works properly??