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I NGENUITY
UNLIMITED
Our regular round-up of readers' own circuits. We pay between
£10 and £50 for all material published, depending on length
and technical merit. We're looking for novel applications and
circuit designs, not simply mechanical, electrical or software
ideas. Ideas must be the reader's own work and must not
have been submitted for publication elsewhere. The
circuits shown have NOT been proven by us. Ingenuity
Unlimited is open to ALL abilities, but items for consideration in
this column should be typed or word-processed, with a brief
circuit description (between 100 and 500 words maximum) and
full circuit diagram showing all relevant component values.
Please draw all circuit schematics as clearly as possible.
Send your circuit ideas to: Alan Winstanley, Ingenuity
Unlimited, Wimborne Publishing Ltd., 408 Wimborne Road
East, Ferndown Dorset BH22 9ND. (We do not accept submissions for IU via E-mail.)
Your ideas could earn you some cash and a prize!
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OSCILLOSCOPE WORTH £586
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If you have a novel circuit idea which would be
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Fluid Finder – Relatively Simple
circuit diagram show in Fig.1. is capable of distinguishing between different
T
liquids, or, alternatively, of helping to deterHE
mine their purity. It may also be used to measure the level of highly corrosive liquids that
would devour many types of probe.
A critical element of the formula used to
determine the capacitance of a parallel-plate
capacitor is the relative permittivity of the
dielectric, or the dielectric constant (Ar ).
Relative permittivity is defined as “capacitance with a material as the dielectric” divided by “capacitance with a vacuum or air as
the dielectric”.
Although the circuit is relatively crude, it
will determine relative permittivity to within
a fraction of one percent. It will therefore distinguish unfailingly between e.g. distilled
water, tap water, and milk, and reveals vast
differences between fluids such as water,
alcohol, and oil.
Circuit Details
a quantity of distilled water is first required
to determine the value of x. It is available
from most garages for topping up car
batteries.
The digital filter IC2 goes either “high” or
“low” at output pin 9 until the desired frequency (about 100kHz) is presented at the
input. In this case a square wave at output pin
9 is detected by means of simple diode pump
D2-D3 and C4-C5. When C5 charges, the
input of IClc goes “high”, and l.e.d. D4 illuminates. The l.e.d. D1 is included for easier
adjustment, as this shows whether the output
is “high” or “low”. With the oscillator typically operating at around 100kHz, this would
be multiplied by 78 if distilled water were
replaced with air, which has a relative permittivity of about 1. Switch S1 is provided so
that IC1 and IC2 are not overdriven in the
absence of a liquid.
At the heart of the circuit is a custom-built
parallel-plate capacitor C1, the “plates” of
which are sealed in epoxy resin and separated
by an air space. It is clipped into a liquid to
determine its relative permittivity, which in
turn is deduced from the capacitance measured by means of potentiometer VR1.
The frequency of RC oscillator ICla-IC1b
is fed to a digital bandpass filter IC2a-IC2b,
then VR1 is adjusted until l.e.d. D4 illuminates. The relative permittivity of distilled
water (which can be designated as x) is then
measured as 100,000 –VR1 ohms, and then
the relative permittivity of any other fluid
maybe calculated as:
(100,000 – VR1) × (78/x).
This assumes that the relative permittivity of distilled water is 78. Needless to say,
S1
IC1a
IC1b
14
40106
1
2
3
40106
4
16
R4
220Ω
5
2
C2
100p
R1
10k
AIR SPACE
BETWEEN PLATES
VDD
4
7
1
3
TR1+
Q1
Q1
TR1
7
NC
6
COPPER SIDE
11
RST1
TR2+
Q2
Q2
TR2
14
b
15
13
VSS
8
D3
1N4148
a
k
9
10
IC1d
40106
5
40106
6 9
8
NC
C5
47p
R5
2k2
R6
2k2
k
D2
1N4148
RST2
IC1c
C4
150p
4098
C2
*SEE TEXT
CONNECTIONS
P.C.B. 30mm x 40mm
(2 PIECES)
NYLON SPACERS
(2 OFF)
RC2
C3
100p
*C1
7mm GAP
C1
IC2
12
EPOXY RESIN COATED
PAPER SLEEVE
a
RC1
VR1
100k
R3
100k
+12V
ON/OFF
R2
100k
D1 a
RED
L.E.D.
k
a
C6
1000µ
+
D4 a
RED
L.E.D.
k
0V
Fig.1. Circuit diagram for the Fluid Finder. The sensor “capacitor” assembly is shown inset.
862
Everyday Practical Electronics, December 2002