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Molecular Frequency Discriminator (MFD) has become a popular term to describe electronically-enhanced dowsing rods. There are two components to the typical MFD: a signal transmitter, and "receiver" L-rods. Construction of the L-rods is covered in the Building LRLs report and will not be repeated here. We will focus on the signal generator.
The prevailing theory behind MFDs is that the signal generator transmits at a frequency that matches the "molecular resonant frequency" of a desired target. The most common frequencies used are 5KHz for gold and 8.7KHz for silver; other elements have different frequencies but we will focus only on gold and silver. Therefore we need a signal generator that can switch between two different frequencies.
There are many ways of generating signals, with tradeoffs in circuit complexity, accuracy, stability, and flexibility. Accuracy refers to the ability to tune to an exact frequency; stability describes how well the generator can maintain a certain frequency over time and with temperature variations; and flexibity means how easy it is to change the frequency or amplitude.
The simplest is probably a ring oscillator, the most complicated might be a crystal-controlled Direct Digital Synthesis (DDS) chip. The ring oscillator, while incredibly simple, has poor accuracy, poor stability, and moderate flexibility. DDS offers the precision and stability of a crystal oscillator while adding the flexibility of millihertz tuning. In between are a wide variety of methods, including a couple of fairly simple yet flexible function generator chips.
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| MFD1: Basic MFD |
One such chip is known as the "8038" (originally made by Intersil, and now produced by several companies) and provides a reasonably clean sinusoidal output as well as triangle and square waves. The output frequency is set by a few external components so it is easy to use, and it is inexpensive. Another similar chip is the Exar XR2206, which has a little better stability and cleaner waveform than the 8038, but is harder to find. We will use the 8038.
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 Fig. 1: Transmitter Schematic |
Figure 1 shows the 8038 signal generator circuit. This is a very standard configuration given in the Intersil data sheet. The output frequency is set by R1 and C1 (plus C2) to be approximately f=0.15/(R1*C1). The sine wave output at pin 2 is then fed through an R-C low-pass filter (R4-C3) to the LM386 audio amplifier. Figure 2 shows the PC board for this circuit. Note that the PC board includes some extra pads for optional circuitry that is not used in this project.
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 Fig. 2: Transmitter PC Board |
Images of the device after assembly
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