Mini metal detecror (colpitts oscillator)

Posted on Dec 24, 2012

The circuit uses two CMOS IC’s IC1 uses inverters connected as a Colpitts oscillator of 100KHz; the LC frequency determining elements being the search coil and parallel resonating capacitor. An 80 turn close wound 30swg 100mm diameter coil will fit inside the CD case. I will leave the choice of the IC to the reader: this can be any of the well known CMOS inverters or NAND, NOR gates etc configured as an inverters. The choice is yours, use whatever you have on hand. IC2 can be either 74C00 or 4011 quad NAND gates.

Mini metal detecror (colpitts oscillator) - image 1
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When configured as shown the is circuit is a "D" type flip-flop (read your theory books!) but in fact acts as a digital mixer. To quote from Ref 2 : A "D" type flip-flop can be used as a digital mixer to produce the difference between two square waves of different frequencies. (Fig 2). Assume initially that F1 and F2 are identical in phase and frequency. The D input will be 1 the instant clocking occurs, and the output will be a 1. Now, let the frequency of F1 vary slowly "slip cycles" with respect to F2. The output will alternate at the difference F2 - F1, and we have the equivalent of a digital mixer or down-converter. The are several restrictions to using the circuit. First, both inputs must be fixed or slowly varying square waves; thus you cannot mix several frequencies at once as you can with an analogue circuit. Second, jitter in the output is inherent, for the Q output can drop only immediately after clocking, thus quantizing the output square wave. The worst case jitter is one half clock frequency. For instance, if both clock and D signals are near 1Khz, the worst case jitter can be around half a millisecond. If the frequency difference is less than 50Hz or so, this jitter is tolerable, but as the difference gets larger, the jitter becomes intolerable. Thus the circuit works best when it is taking a small difference between two nearly identical frequencies. Another limitation (sometimes a handy...

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