define(`TITLE', `Very simple metal detector') define(`DATE', `2014-12-15') define(`CONTENT', ` The design of the Theremin let my to the idea, that varying the value of the inductivity by approaching some metal should also detune one of two oscillators. Here, I reuse the design of the oscillator and the mixer from my Theremin and fed the mixed signal into a low pass filter, calculated with Analog's excellent Filter Wizard. This was btw the first deployment of my new Rigol power supply: The both identical 30V-Max-channels, put in tracking mode, can easily be used as a symmetric power supply, as it is required for the mixer and the filter. The reference frequency is provided using a generator: This is the full measurement setup: I use one oscilloscope to show the varying (yellow) and the reference (blue) frequency, here in the tuned state: When detuning the test oscillator by approaching some metal to the coil, there varying frequency starts wandering on the screen: And here are the results: [gallery columns="2" ids="546,545,544,525,543,528,541,540,535,534"]]]> ')