Title :
A Simple metal detector
Circuit:
Components:
Capacitor:
1000 uf x 1
4.7 nf x 1
10 nf x 2
Resistor:
10 k (variable resistor)
1 k
Oscillator: x2 40 Khz
Miscellaneous:
Switch
9v battery
Piezo sounder
Enammeled copper wire
Description:
This circuit uses a
single coil and nine components to make a particularly sensitive low-cost metal
locator. It works on the principle of a beat frequency oscillator (BFO). The
circuit incorporates two oscillators, both operating at about 40kHz. The first,
IC1a, is a standard CMOS oscillator with its frequency adjustable via VR1. The
frequency of the second, IC1b, is highly dependent on the inductance of coil
L1, so that its frequency shifts in the presence of metal. L1 is 70 turns of
0.315mm enamelled copper wire wound on a 120mm diameter former. The Faraday
shield is made of aluminium foil, which is wound around all but about 10mm of
the coil and connected to pin 4 of IC1b.
The two oscillator signals are
mixed through IC1c, to create a beat note. IC1d and IC1c drive the piezo
sounder in push-pull fashion, thereby boosting the output. Unlike many other
metal locators of its kind, this locator is particularly easy to tune. Around
the midpoint setting of VR1, there will be a loud beat frequency with a null
point in the middle. The locator needs to be tuned to a low frequency beat note
to one or the other side of this null point. Depending on which side is chosen,
it will be sensitive to either ferrous or non-ferrous metals. Besides detecting
objects under the ground, the circuit could serve well as a pipe locator.
Application:
- To detect metal objects present under ground.
- To detect metal objects present in the luggage bags near air ports.
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