Fig. 1: Circuit diagram of the FET dipper with LED display
Circuit diagram
The circuit comes out (1), operates with an indicator instrument with 50 µA final value and by me was already several times structured.
Over this dipper in a pharmacy box to accommodate a very
small indicator instrument must be available or however one replaces
it by another indicator. It offers itself an LED, in addition, a tone
generator that its frequency as a function of the RF voltage which can
be measured modifies is conceivable. First sceptically I decided for
the LED, because she nevertheless bring along in relation to other
solutions to few volumes. In the practical operation I was then
nevertheless surprised, as a Dip appears clearly. Also the question:
" have itself does the pointer moved or I tremble with my hand ? "
no more does not place itself with this inertialess display.
The circuit consists of an oscillator with the FET VT1
(BF245C), whose resonant circuit from the variable capacitor and a
plug coil is formed. From this oscillator over a condenser (8.2
pf) RF is uncoupled, brought with a silicon diode parallelly and over
a Darlington stage with the transistors VT2/VT3 (BC547C) and the LED
to the display. The more highly the RF voltage, the the bright LED.
Central point of the circuit is the Sub d socket. Over a
bridge at the plug coils the voltage for the display module is
switched on. The plug coils serve measuring range switching.
Practical structure
First into the pharmacist box the necessary recesses and holes are bored. In the center the drilling for the variable capacitor, something at the edge for the LED. At the page the recess for the Sub d socket is ripped out. Since this recess must be trapezoidal, one bores much small 1 mm of holes along the desired form and tries the material out carefully with small shears to rip. Then the holes for the switch (" active - passive ") and the adjuster for sensitivity follow.
The cover of the box is demoted to the rear side and must in the places, where it comes with the switch and the Sub d socket into contact something is left blank, so that it closes correctly.
Then a piece of universal printed circuit board (best been
suitable strip PCB) is made suitable, so that they provide the
soil of the box covered and with appropriate holes for variable capacitor
and LED.
This PCB is stuck upward with the conductive strips
into the box. This serves at the same time the mechanical
stabilization of the housing. Then the Sub d socket and the
variable capacitor
with the links are inserted toward Sub d socket. Afterwards all
remaining sections become, into which fastens drillings. Now the
wiring can begin on the basis of the Sub d socket, i.e. from their
lower series.
For the production of the coils connector sockets for cable carriers (PVC pipes) serve the size PG16 (German gauge, diameter 16 mm), which gives it on each building market. Into these PVC cases three small holes are bored for executing the winding wire. One begins the coil of the " cold end " ago to windings. The " cold end " of the coil is the section, which is on mass potential, during " is hot end " by a capacity with the gate of the FET is connected. To " is hot end " lies close in the active operation the largest RF voltage. If one achieved a third of the total number of turns, one determines this part of the turn with something nail polish, so that the coil does not slip again from the spool. Now the wire is formed to a loop by approx. 10 cm length, broken in the center of this loop and been by the middle hole and led out at the lower end of the spool. The loop is first easily twisted. Now the remaining two thirds of the coil are wound, the upper end by the third hole is led downward and the entire coil with something nail polish is determined. After the lacquer is dried, the tapping is twisted firmly, tinned and shortened in such a way that about 1.5 cm from the spool stand out. The two ends (" hot and coldly ") are tinned likewise and shortened. So that the spools fit on the Sub d plugs, the plugs with the file must be made suitable, as at each page about 0.5 mm are removed. The finished wound spool is soldered on to the Sub d socket, over which shank fastens shifted and with second adhesive.
The coil windings are indicated in the table (1), to coil 1 cover the wide-spread IF area, the others the KW area. Are higher frequencies to be achieved must a smaller variable capacitor be used.
Measuring
The calibration of the dipper takes place with a frequency
counter. In addition a calibration chart for the coils with the
scaling of the rotary knob and the frequencies is set up.
With the organization of the " front plate " a label is used.
For this a file label is printed accordingly, cut out with a laser
printer and stuck on the box.
Table (1): Coil values for FET dipper for spools
with 16 mm
