metal detectors
Simple BFO Metal Detector Schematic Diagram
only a few of components and an evening
's work. The two oscillators
are simple Colpitts designs using BJT
transistors. The reference oscillator's
frequency is approximately 370kHz,
slightly tunable with the help of a silicon
varactor diode. The outputs of the two
oscillators are fed to a mixer made
with Q3 and Q4. The signal then goes
through a low-pass filter (R13, C13)
and a JFET preamp. The LM386
audio amplifier has a gain of 20, more than enough for most headphones. If you need more gain, you can
add a 10uFcapacitor between pins 1 and 8.
Fig. 1: Simple BFO metal detector schematic diagram
a nonmetallic form to prevent microphonics.
After assembly, connect the headphones and slowly turn P1. The pitch will get lower until it disappears.
Continuing to rotate P1 in the same direction will cause the pitch to rise again. The point at witch the pitch
is the lowest and disappears is called "zero beat". If you can not get this zero beat frequency for the entire
turn of P1 you may have to increase or decrease the value of L2.
Turn P1 close to the zero beat position (a tone of 50Hz-200Hz), then move the search coil near a metallic
object.
The tone should change, depending on the size and distance of the metal.
Note: this simple circuit will only detect relative large metallic objects at a short distance. Coins and other
small objects will be much harder to find! If you want to build a detector with a performance comparable to
commercial products, try a PI or VLF design.
Parts list:
Resistors:
Other parts:
| Capacitors:
Active components:
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