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I'm not experience in making electronics, but I do have a working knowledge of basic circuit laws and such, but we never covered something with transistors such as this BFO metal detector circuit I've been making as a weekend project:

BFO metal detector

This metal detector is relatively weak and I wanted to improve it a little, but I can't get a grasp on what this circuit in particular is doing. I see 2 oscillators on the left, but they both have transistors in them, and I'm not sure what that does to them. I put them in a simulator and they didn't do anything.

I understand the way this works is supposed to be based on heterodyning. So I'd like to make my reference oscillator, a detector oscillator, then multiply the signal somehow and feed it to my speaker. The speaker will only be able to pick up the lower frequency, difference signal, but that's the one I want. I have no idea how this circuit is doing that.

If this circuit is easy for you to figure out, is there a resource you could direct me to so I can learn how it works as well?

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    \$\begingroup\$ I'm going to +1 this just for your use of the word heterodyning :) Have you considered using spice to simulate it? \$\endgroup\$ – Some Hardware Guy Dec 23 '14 at 22:31
  • \$\begingroup\$ @SomeHardwareGuy I should learn how to use that program \$\endgroup\$ – Michael Flynn Dec 23 '14 at 23:36
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Yes, the left two transtors each form a oscillator with their particular coil. The two signals are then added. This will cause the amplitude of the signal to go up and down at the beat frequency, which is the frequency difference between the two oscillators.

The oscillator signal with amplitude variations of the beat frequency is "detected" by the circuit around Q3. This makes the signal that is roughly the amplitude envelope of the beating oscillator signal. This is then gained up, rather crudely, by the remaining transistors to eventually drive the speaker.

The system functions by the metal you are trying to detect changing the inductance of one of the coils but not the other. The absolute frequency change is small, but it is made much more obvious by causing a beat frequency between the disturbed coil and the reference coil.

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  • \$\begingroup\$ would you say this design could be improved upon? (That's what I'd like to do) \$\endgroup\$ – Michael Flynn Dec 24 '14 at 1:28
  • \$\begingroup\$ @Michael: That depends on what criteria you measure "improved" against. This circuit is very good at being cheap, simple, and being able to use common off the shelf parts. For someone that "never covered something with transistors", it would be better to build this as is first, then try to really understand its operation and the waveforms at each step of the way. Then maybe you're ready to try a little incremental redesign. \$\endgroup\$ – Olin Lathrop Dec 24 '14 at 14:25

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