This is one of the most accurate
and simplest LC inductance / capacitance Meters
that one can find, yet one that you can easily build
yourself. This LC Meter allows to measure incredibly
small inductances starting from 10nH to 1000nH,
1uH to 1000uH, 1mH to 100mH and capacitance from
0.1pF up to 900nF. LC Meter's circuit uses an auto
ranging system so that way you do not need to spend
time selecting ranges manually. Another neat function
is the "Zero Out" switch that will reset
the initial inductance / capacitance, making sure
that the final readings of the LC Meter are as accurate
as possible.
To be able to determine the
value of an unknown inductor / capacitor we can
use the frequency formula given below.
Note that there are three
variables that we can work with; f, L and C (f represents
a frequency, L inductance and C capacitance). If
we know the values of the two variables we may calculate
the value of the third variable.
Lets say we want to determine the value of an unknown
inductor with X inductance. We plug X inductance
into the formula and we also use value of a known
capacitor. Using this data we can calculate the
frequency. Once we know the frequency we can use
the power of the algebra and rewrite the above formula
to solve for L (inductance). This time we will use
the calculated frequency and a value of a known
capacitor to calculate the inductance.
Isn't this amazing? We just calculated the value
of unknown inductor, and we may use the same technique
to solve for the unknown capacitance and even frequency.
Applying the Theory to LC Meter's Hardware
Now let's use the above theory
and apply it to electronics. The LC Meter uses a
popular LM311 IC that that functions as a frequency
generator and this is exactly what we need. If we
want to calculate the value of an unknown inductor
we use a known Ccal 1000pF capacitor and the value
of an unknown inductor. LM311 will generate a frequency
that we can measure with a frequency meter. Once
we have this information we can use the frequency
formula to calculate the inductance.
The same thing can be done for calculating the value
of a unknown capacitor. This time we don't know
the value a capacitor so instead we use the value
of a known inductor to calculate the frequency.
Once we have that information we apply the formula
to determine the capacitance.
All this sounds great, however if we want to determine
the value of a lot of inductors / capacitors then
this may become a very time consuming process. Sure,
we can write a computer program to do all these
calculations, but what if we don't have an access
to a computer or a frequency meter?
That's were PIC16F84A microchip comes handy. PIC16F84A
is like a small computer that can execute HEX programs
that are written using an assembly language. PIC16F84A
is a very flexible microchip because it has PINs
which can be configured as inputs and outputs. Besides
that, PIC16F84A IC requires very minimal number
of external components like 4MHz crystal / resonator
and few resistors depending on what project we are
building. Before we can use PIC16F84A microchip
we have to program it with a HEX code which has
to be sent from the computer.
In the next step we use the frequency generated
by LM311 IC and pass it on to PIC16F84A's PIN 17.
We designate this PIN as an input, as well as all
other PINs that are directly connected to switches
and jumpers. User can use these inputs to tell the
microchip to execute specified set of instructions
or perform calculations.
Once the microchip will calculate the unknown inductance
or capacitance it will use PINs that are designated
as outputs and pass the results on to the 16 character
LCD display.
LC
Meter's Switches & Jumpers
SW1 - Zero out the readings.
SW2 - Capacitance / Inductance switch.
J3 - turns on the backlight on LCD displays that
have LED backlight.
J1 - used by 16x2 two line character LCD displays.
J2 - displays the initial frequency of the LM311
oscillator which should be around 550KHz.
Character
LCD Display Connections
Most of the character LCD displays have 14 or 16 PINs.
The displays that do have a backlight have 16 PINs
and displays that do not have a backlight have 14
PINs. The PINs that are highlighted in green
in the table below are the ones that PIC16F84A uses
to pass the output information
represented in bits (0/1).
PIN
Symbol
Function
States
1
VSS
GND
-
2
VDD
VCC +5V
+
3
VO
Contrast
Adjustment
+/-
4
RS
Register
Select
H/L
5
R/W
Read / Write
H/L
6
E
Enable Signal
H/L
7
DB0
Data Bit
0
H/L
8
DB1
Data Bit
1
H/L
9
DB2
Data Bit
2
H/L
10
DB3
Data Bit
3
H/L
11
DB4
Data Bit
4
H/L
12
DB5
Data Bit
5
H/L
13
DB6
Data Bit
6
H/L
14
DB7
Data Bit
7
H/L
15
LED Backlight
VCC +5V
+
16
LED Backlight
GND
-
CAUTION: The controller chip
that is visible on the back of the LCD display can
be damaged with the static energy. Make sure to ground
yourself every time you hold it, and handle it by
the edges and not the PINs.
16x1& 16x2 LCD modules
with backlight (front)
both LCDs can be used
interchangeably
LCD modules (back)
16x1 LCD with pcb standoffs
and header pins
LC
Meter's Enclosure (4"x2.5"x1")
LC
Meter's Construction
Measuring
2pF Capacitor
Measuring
Inductance
40nH - small piece of
magnet wire
80nH - 4 turns of magnet
wire
90nH coil used in FM
transmitter
280nH - 10 turns of
magnet wire
500nH wire through choke
1uH VK choke
small RF toroid, 5 turns
medium toroid
365uH
100uH choke
1uH inductor
100uH inductor
2.2mH inductor
18mH inductor
Final
Recommendations
1nF Ccal is used
as a calibration capacitor so make sure
that you use a good quality mylar capacitor.
Keep the cables between LM311 and the
terminals as short as possible to keep
the stray capacitance to minimum and ensure
the highest accuracy. Also, you have to
use a "5V reed relay" because
the current passed from PIC16F84A is very
small. Reed relays require very minimal
amount of current to be switched. I also
suggest that you use a 7805 IC to protect
the LC Meter, in case you accidentally
apply a higher voltage. PIC16F84A will
be damaged if you apply voltage higher
than 5.5V.
Where
to get LC Meter parts or kit
If you are building
the above LC Meter and have trouble finding
some of the components, we are distributing
the following components and a kit in our Electronic
Store.
