Hello. I'm a complete newbie at this forum & I though I might take a stab at getting some help here. I've a circuit for building a metal detector, but I'm somewhat lacking in completely understanding it.
The transmitter part has a relaxation oscillator which functions as an 800Hz modulator & then a Colpitt's oscillator which is a 130Hz oscillator. Could someone please explain the process that's going on in this section? Here's the circuit in pdf form.

 

I haven't a clue why they chose those frequencies, but a metal detector typically works on the principle that the presence of ferrous metal increases the inductance of a coil and non-ferrous metals (if conductive) slightly decrease it.

 

Well yes that much is a given. But that's not the concern here; the thing is I'm not too familiar with RF modulation & that's why I asked specifically as to how the relaxation & colpitt's oscillator work together. I'm digging around on the net for the time being.

 

I think the 130Hz is actually 130kHz - they seem to mention both in the article. 130kHz makes more sense.
One coil transmits this 130kHz modulated by the 800Hz and the other recieves and connects to the circuit that amplifies it with noise gates so that low levels aren't heard at all.

 

Yes that does make sense. I was kind of confused by that as well, but now it's rather clear as to what's what. Thank you both Markd77 & marshallf3 for your prompt replies.

 

They probably chose the best oscillator circuit for the frequency involved.

 

The magazine article explains the circuit's operation. Basically, the two oscillator coils are not coupled (much) because they are at right angles to each other. A bit of metal causes coupling to occur (either through direct magnetic coupling or via eddy currents). When the two oscillators couple, frequencies from both oscillators appear at the collector of Q3.

The output of Q3 is fed to detector Q4, where, among other things, it detects the beat between the two oscillators. This beat is amplified and applied to the meter and to the speaker.