Figure 9. Metal/pipe locator.
Figure 10. 9 V to 300 V DC-to-DC converter.
Figure 11. FM radio transmitter alarm.
low frequency beat or whistle note is
heard from the radio. This beat note
changes if L1 (the search head) is
placed near metal.
Figure 10 shows another application
of the Hartley oscillator. In this case, the
circuit functions as a DC-to-DC converter, which converts a 9 V battery supply
into a 300-V DC output. T1 is a 9V-0-9V
to 250 V transformer, with its primary
forming the L part of the oscillator. The
supply voltage is stepped up to about
350 V peak at T1
secondary, and is
by D1 and used to
charge C3. With no
permanent load on
C3, the capacitor
can deliver a powerful but non-lethal
belt. With a permanent load on the
output, the output
falls to about 300 V
at a load current of
a few mA.
Figure 12. FM microphone/bug transmitter.
when used in conjunction with a MW radio, it functions
as a simple metal/pipe locator. Oscillator coil L1 is
hand-wound and comprises 30 center-tapped turns of
wire, firmly wound over about a 25 mm (one-inch)
length of a 75 to 100 mm (three- to four-inch) diameter
non-metallic former or search head and connected to
the main circuit via a three-core cable. The search head
can be fixed to the end of a long non-metallic handle if
the circuit is to be used in the classic metal detector
mode, or can be hand-held if used to locate metal pipes
or wiring hidden behind plasterwork, etc. Circuit operation relies on the fact that L1's electromagnetic field is
disturbed by the presence of metal, causing the inductance of L1 and the frequency of the oscillator to alter.
This frequency shift can be detected on a portable MW
radio placed near L1 by tuning the radio to a local station and then adjusting VC1 so that a
NUTS & VOLTS
Figure 13. The frequency response of this
1 V AC meter is flat to above 150 kHz.
Figures 11 and 12 show a pair of low-power FM transmitters that generate signals that can be picked up at a
respectable range on any 88 to 108 MHz FM-band receiver.
The Figure 11 circuit uses IC1 as a 1 kHz squarewave generator that modulates the Q1 VHF oscillator, and produces a harsh 1 kHz tone signal in the receiver; this circuit thus acts as a simple alarm-signal transmitter.
The Figure 12 circuit uses a two-wire electret microphone insert to pick up voice sounds, etc., which are
amplified by Q1 and used to modulate the Q2 VHF oscillator; this circuit thus acts as an FM microphone or bug.
In both circuits, the VHF oscillator is a Colpitts type, but
with the transistor used in the common-base mode and
C7 giving feedback from the tank output back to the
These two circuits have been
designed to conform to
American FCC regulations, and
they thus produce a radiated field
strength of less than 50 µV/m at
a range of 15 meters ( 15 yards),
and can be freely used in the
USA. It should be noted, however,
that their use is quite illegal in
many countries, including the
To set up these circuits, set
the coil slug at its middle position, connect the battery, and
Figure 14. Pseudo full-wave version of
the 1 V AC meter.