LC METER

LC Meter

Construction of a inductance and capacitance measuring instrument with a PIC and LCD. The original idea seems to have come from Bill Carver, W7AAZ, in Communications Quaterly, Winter, 1993, and is described in EMRFD. The proposed circuit was published in the June/96 Electronics Now magazine and is the brainchild of Neil Heckt (AADE), who owns software international copyrights; He kindly allowed the material posted here, sufficient to homebrewing a similar instrument. AADE produces the L/C Meter IIB commercially, assembled or as a kit.
The version now presented on this page, with a different PIC, is the resulting effort from the radioamateurs of the Turkish group ANTRAK, in Turkey, specially Barbaros Asuroglu, TA2CBA, and was brought to Brazil by Carlos, PY2CSU



 

 
 

 

How it works 

What makes the circuit interesting is the method used to measure capacitors and inductors. A comparator IC LM311 is used as an LC oscillator. L is the 68 uH XRF1 inductor and C is C2, the 680 pF capacitor. Hence the frequency is f1 = 740 kHz but there are infinity pairs of LC values producing the same frequency resonance. A capacitor with a precise value, C3 1000 pF, is put in parallel to C2 by means of a relay. The new equivalent capacity is 680 + 1000 = 1680 pF, and the new frequency is f2 = 468 kHz. It is not important the absolute value of these frequencies, what counts is the ratio between the two, which derives from the precision 1000 pF capacitor. The two frequencies are read by the PIC during self calibration at start-up. What matters is that if the 1000 pF capacitor is precise, then there is only one value for L (the 68 uH inductor) and only one value for C (the 680 pF capacitor) which can determine simultaneously these two frequencies, it doesn't matter their absolute values. In this way, the PIC "discovers" the real values of the inductor and 680 pF capacitor and keep them in memory, and now can measure an external L or C. The inductor under test is connected in series with the choke. The capacitor under test is connected in parallel to the 680 pF capacitor.
During self calibration, wiring stray inductance and capacitance are absorbed into the calculations done by the PIC. The switch CH3 "zeros" the display. This way, only the external added inductance or capacitance under test is shown.

 

Bulding the LC meter

Bulding the LC meter does not present any special difficulty. Some details are important though:
1 - The switch CH3 is a "normally closed" type.
2 - The capacitor C4 doesn't exist. The precision of the meter depends mostly on C3. I used a polystyrene ("styroflex") 1000 pF 10% unit, in parallel to one disc ceramic 18 pF NP0, and a 30 pF trimmer to get the correct value. To set the trimmer, just a known precision capacitor is needed. I got a 10nF 1% polystyrene capacitor and used it as an external dependable reference, but any value will do, try to find an exemplar of low tolerance, 1% or even better. The LC meter from AADE comes with factory selected capacitors with 0.5% tolerance (1020pF in their model).
For C2 I used a combination of parallel capacitors as I didn't have a 680 pF in my junk box. Here we don't need low tolerance but thermal stability, avoid small unknown disc ceramics, a NP0 ceramic or polystyrene is prefered.
3 - At first I used a small RF choke for XRF1. I noticed the measurings were not stable, the oscillator drifted. Then I tried a toroidal coil wound with 82 turns, wire 30 SWG over an iron-powder Micrometals T50-1 (sold by Amidon, Palomar, Toroid King, etc...). The oscillator was much better! Avoid ferrite, its thermal parameters are terrible! As the Q of the toroidal core was way better than the small choke, I also noted that the range of the LC meter was now more extended. I was able to measure reliably a 10 uF cap!

How to put the code into the PIC  

"Burning" a PIC is easy. You'll need hardware and software.

How to use the LC meter
The switches Lx and Cx must be open when turning the meter on, otherwise the display will show "switch error". Then the microcontroller will start auto-calibration, you will be able to hear the relay actuating. After that the display will show "ready".
To measure a capacitor: press Cx switch, the display will show some pF, that's stray wiring capacity. Press the CH3 ("calibrate" or "zero" switch) briefly. The display should show 0 pF. Now connect the capacitor you want to measure and read its value.
To measure an inductor: Press Lx switch. The display will show "not an inductor", that's because there's an open circuit at the test terminals. Short-circuit the test terminal and press CH3 to zero the display. Now connect the inductor and read its value.

Measuring range:
      1 nH to 150 mH
      .01 pF to 10 uF

Code file lcm.hex to burn the PIC

copy the code below

:080000008601E1306600850174
:08000800103065003830620081
:080010008B01BF2103308500C4
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:0807B000A80A2A08A70729087E
:0807B8000318290FA607280809
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:0807C800EC2B031CCB2AA50C4D
:0807D000A60CA70CA40ACB2A19
:0807D8000318AC2AA509A609CB
:0807E000A709A70A0319A60AE4
:0807E8000319A50A8030AF06D9
:0207F000AC2A31
:00000001FF

 

 

 

Printed Circuit Board 

 

 

 

List of material:

D1, D2 - 1N4001, (any one with PIV 50V or higher, 0.5A or more)

LM7805

LM311

PIC16C84 or PIC16F84

C1, C5, C6 - 10uF 10V tantalum

C2 - 680pF

C3+C4 - 1000pF

C7 - doesn't exist

C8, C9 - 22pF

C10 a C22 - don't exist

C23, C24 - 100nF (0.1uF)

C25, C26 - 220 or 470uF 16V

Cx1 - 100nF

Cx2 - 10 uF 16V

R1, R3, R6- 100k 1%

R2 - 1k 1%

R4 - 47k

R5 - 10k trim-pot

CH1, CH2 - 2 DPDT

CH3 - normally closed

Display - 1 line, 16 characters 

Xformer sec. 9+9V 200mA or more
XRF1 - 68 uH inductor
FU1 - fuse 0.25A
Relay - 12V SPDT