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Very Accurate LC Meter based on PIC16F84A IC.

LC Meter - Inductance Meter / Capacitance Meter

  LC Meter Part's List:

2x 1K
2x 6.8K
1x 47K
3x 100K
1x 10K POT

2x 10pF
1x
100pF
2x 1nF
2x 10uF
1x 78LM05

1x 16x1 LCD Display
1x PIC16F84A IC
1x LM311 IC
1x 4MHz Crystal
1x 82uH Inductor
1x 3V Reed Relay

  LC Meter's Technical Specifications:

Voltage Supply: 7.5 - 15V
Accuracy: 1%
Zero Out Switch
Automatic Ranging


LC Meter's Inductance Measurement Ranges:       
- 10nH - 1000nH
- 1uH - 1000uH
- 1mH - 100mH

LC Meter's Capacitance Measurement Ranges:     
- 0.1pF - 1000pF
- 1nF - 900nF

LC Meter Inductance Meter Capacitance Meter


About LC Meter
 

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.


LC Meter Kit
LC Meter Kit with Green Backlight LCD Display
LC Meter PCB


How does LC Meter Work?


 

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.


LC Meter's LCD LC Meter's 16x2 LCD
16x1& 16x2 LCD modules with backlight (front)
both LCDs can be used interchangeably

LC Meter's LCD back LC Meter's LCD
LCD modules (back)
16x1 LCD with pcb standoffs and header pins



LC Meter's Enclosure (4"x2.5"x1")
 
LC Meter LC Meter



LC Meter's Construction
 
LC Meter LC Meter

LC Meter LC Meter

LC Meter prototype LC Meter inside

LC Meter PIC IC LC Meter

LC Meter LC Meter

LC Meter LC Meter



Measuring 2pF Capacitor
 
LC Meter accuracy LC Meter LCD resolution



Measuring Inductance
 

 

LC Meter inductance meter LC Meter inductance measurements
40nH - small piece of magnet wire
80nH - 4 turns of magnet wire

LC Meter inductance measurements LC Meter inductance measurements
90nH coil used in FM transmitter
280nH - 10 turns of magnet wire


LC Meter inductance measurements LC Meter inductance measurements
500nH wire through choke
1uH VK choke

LC Meter inductance measurements LC Meter inductance measurements
small RF toroid, 5 turns
medium toroid


LC Meter inductance measurements LC Meter inductance measurements
365uH
100uH choke

LC Meter inductance measurements LC Meter inductance measurements
1uH inductor
100uH inductor

LC Meter inductance measurements LC Meter inductance measurements
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.





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