Microcontroladores

Constructional Project

PIC
LCF METER
JOHN BECKER
Simple monitoring of inductance,
capacitance and frequency values

Using this formula, if any two values are
known, the third can be readily calculated.
For instance, if C and F are known, then L
can be calculated using the formula:

T

HIS simple PIC-based unit was
designed to measure and display the
values of inductors andcapacitors.
As a by-product of the technique used, it
can also display the frequency of an external 0V/+5V signal source.
The ranges are approximately:
Capacitance:
1pF to 6500mF
Inductance:
1mH to 10H
Frequency:
0·05Hz to 5MHz

OSCILLATOR
CONCEPTS

The design is based upon the concept
that oscillators can be constructed from
CMOS NAND gates or inverters, and that
their oscillationfrequency depends on the
values of inductance, capacitance and
resistance in their feedback paths. The
principles were discussed by George
Hylton in his two-part series Logic Gate
Inverter Oscillators of Sept/Oct ’02.
Using a suitable microcontroller, such
as one from the PIC16F62x or PIC16F87x
families, software can read the frequency
of an oscillator and calculate the value of
an unknowncomponent if the values of the
other components are known. In this
design, a PIC16F628 is used and the
results are output to an alphanumeric liquid crystal display (l.c.d.).

1
( 2 × p × F)
L=

2

C

Fig.1. A basic inductance and capacitance (LC) oscillator.

Similarly, using the capacitance-resistance oscillator configuration shown in
Fig.2, the output frequency can be calculatedfor known values of R and C. Several
formulae exist for this calculation and the
one used in this application is:
F=

One technique for using an inductor in a
CMOS oscillator circuit is that shown in
Fig.1. Here the oscillation frequency is
determined by the formula:
1
2 × p × Ö(L × C)
where:
F=

1
p×R×C

from which the value for C can be calculated if R and F are known:
C=

1p×R×F

F = frequency
C=

C1 × C2
C1 + C2

L = inductance
p = 22/7

Fig.2. A basic capacitance and resistance (CR) oscillator.

FREQUENCY TO PIC

Referring to the full circuit diagram for
the PIC LCF Meter in Fig.3, two independent oscillators are used, one based on
inductance and capacitance (LC) values,
the other on capacitance and resistance
(CR) values.
The LC oscillator isformed around
NAND gate IC3a. The inductance is provided by inductor L1, used in series with
the external inductor whose value needs to
be measured. The external inductor is connected across probe clips P1 and P2 and
switched into circuit by rotary switch S2 in
position 1. The capacitance is provided by
capacitors C5a, C5b, C6a and C6b.
Two pairs of capacitors are used so that
the valueof C in the LC formula is simple
for the software to process. The values for
each of the four capacitors is set at 10nF.

92

Everyday Practical Electronics, February 2004

Fig.3. Complete circuit diagram for the PIC LCF Meter.
Therefore the parallel value of C5a/b =
C6a/b = 20nF (a value not obtainable in
standard capacitor ranges). In the formula,
as expanded above, the value for Cis thus
10nF.
A preset potentiometer, VR2, is also
included in the feedback path. Its principal purpose is to ensure that oscillation
starts reliably and then continues in a stable manner. The effect is due to its relationship with capacitors C6a/b, which
impose a more pronounced phase shift on
the signal being fed back through the
inductor than the output of the gate itself
can allow.Potentiometer VR2 also has the side
effect of providing a degree of frequency
control, although this is not important in
this application.
In use, the inductor whose value is to be
measured is placed in series with L1. The
latter provides a minimum inductance
value against which the circuit is “nulled”
prior to taking measurements. Its use minimises the effect of stray fields within the...