This project is based on the Surprisingly Accurate LC Meter article that appeared in the April 2004 issue of the WIA publication, Amateur Radio. However, Phil Rice, VK3BHR, has re-visited his project and allowed the Club to use his updated version.

Phil, a Bendigo amateur, got the motivation for his original project after producing a PIC-based frequency meter in the September 2002 issue of Amateur Radio. He considered combining an oscillator with a cut down version of the frequency meter to make a direct reading inductance/capacitance meter with an LCD display. A few calculations convinced him that, although challenging, it would be possible. A little bit of help from Microchip's 24-bit Floating Point routines and the baby was born.
Phil has changed the PIC to a 16F628A. This PIC contains 2 comparators and this means that the LM311 of the original circuit is no longer required. There are a few other minor changes and this time a circuit board pattern is also available.

You can download the schematic and parts overlay for comparison purposes as I get other details ready for Club members to assist in building.

Note - The 4.7k resistor attached to Pin 3 of the PIC should be connected to VCC and not ground as shown on the diagram. The board pattern is correct.

You can now also download the HEX program file (zipped). for programming the PIC. If it doesn't download, here is an alternative link to Phil Rice's site.

Due to the amazing interest in this project from all around the world, a PCB Pattern pdf file is now included. This is a complete page with a number of copies of the board for 'mass' production.

Parts are available from a variety of sources and there is nothing really special about any of them. The following parts list is what was used for initial builds. Almost all of the parts can be sourced from Jaycar. The LCD was part of a bulk buy.

Not included in this list are the two terminals and battery connector.

A word of warning - If you use a different LCD, make sure all the pins - especially the power pins - are compatible. Otherwise you may be using at least two of them.

Qty Description Source Code
1 16 x 2 LCD AMPIRE AC-162D. Exetel  
1 Project Box Jaycar HB-6090
1 PIC 16F628A-20/P Jaycar ZZ-8520
1 18 pin IC socket Jaycar PI-6503
1 100uH Choke Jaycar LF-1534
1 4MHz Crystal Jaycar RQ-5274
1 Reed Relay Jaycar SY-4030
1 7805/78L05 Jaycar ZV-1505
1 1N914 Jaycar ZR-1100
1 5k trimpot Jaycar RT-4358
1 Dual-row pin strip Jaycar HM-3250
4 1k resistor Jaycar RR-0572
2 4.7k resistor Jaycar RR-0588
2 47k resistor Jaycar RR-0612
3 100k resistor Jaycar RR-0620
2 33pF ceramic caps Jaycar RC-5318
2 0.001uF MKT caps Jaycar RM-7010
2 10uF/16V tantalum caps Jaycar RZ-6648
1 22uF/25V electro cap Jaycar RE-6090
1 10uF/16V electro cap Jaycar RE-6066
1 0.1uF mono cap Jaycar RC-5490
1 Switch - for L/C Selection Jaycar ST-0310
1 Switch - for Zero Jaycar SP-0710
1 Switch - for On/Off Jaycar ST-0335

General Assembly Advice

Although assembly should be pretty straight forward from the overlay above (or the download), here are some general things to keep in mind.

There are 7 wire links on the board (sorry 'bout that). Install these first as some of them are under the PIC or the relay. Two of the links (near the display) share a common hole.

Use a socket for the PIC. Jaycar's "cheap" ones are perfectly OK. Use the expensive machined pin ones only if you want to make it hard to remove the PIC and maybe break the pins off.

Two of the capacitors on the board (in the oscillator) are tantalum electro's. Check 'em twice.

The two 1000pF capacitors should be MKT types. Polystyrene are better, but the board may be a bit cramped for 'em.

The recommended display has a backlight available. A limiting resistor has not been specified. You will need to select one to limit the current to maybe 90mA. A backlight is probably more than a little overkill in a project like this - especially if it is battery powered.

The connections between the pc board and display are up to you. Use a plug and socket if you have them. It is perfectly OK to use wires. Just don't flex them too much. An alternative is a SIL Header Terminal strip but you may wish to solder to the board first. The specified LCD has solder pads on both board sides, the pcb has not. Check your box arrangement first. If you use this method, the LCD and pcb become one module otherwise they will have to be mounted separately.

Keep the wiring around the L/C switch short, direct and reasonably rigid. The meter compensates for most stray capacitances and inductances EXCEPT for capacitance from the "cold" end of the inductor to ground.

The reed relay may be installed either way round.

The 4MHz crystal should be 4.000MHz, not 4.1, 4.3 or some nearby value. Any crystal marked as 4.0MHz (or sold as a 4MHz crystal) will be close enough, ie., accuracy of +/- 0.01% is OK and doesn't need trimming.

A suitable case to house the project is the Jaycar ABS Console Box - Cat: HB-6090. Details of mounting in this case will be presented here shortly.

A zero/reset, n/o, momentary contact pushbutton switch has not been included in the parts. Neither has an On/Off switch. Add them at your own discretion.

The label for the box, see notes below, was designed before anything was mounted in the box. Carefully position everything, considering all clearances, etc., design your label and once you have got it right, use the printed paper label as a mounting template. If you damage it, print another. If you muck up the design or positioning, it is easy to adjust and reprint it. If you mount everything in the box before designing the label it makes it more tedious to measure everything later to determine the label layout.
This image shows how the pcb and lcd were mounted inside the box. The final construction moved the two mounting screws on the edge of the pcb further towards the middle of the case so that they would be hidden by the label. The LCD bracket at the top is made from two right-angle plastic pieces with the LCD sandwiched between. The 9V battery will rest against the back of the LCD, wrapped in foam rubber to hold it in place.
This image shows the finished meter - except for the panel label and the soon-to-be-modified escutcheon plate. The text displayed is actually wider than the window in the escutcheon plate and it will have to be modified. As you can see, the two mounting screws would be difficult to hide with the label - the way the label was made at least. The switches are: Zero/Reset (left), Inductance/Capacitance Selection (centre) and On/Off (right).
This image shows the finished meter. You can click on the image for a larger view.

The escutcheon plate has been modified to provide a larger opening for the LCD and the pcb mounting screws have been moved and covered by the label. The label was designed with the aid of a simple CAD program to be a few millimetres smaller than the recess in the box top panel. A piece of clear plastic adhesive sheet was then placed on top. The label was then attached to the box and its edges trimmed with a very sharp knife. A download copy of the panel artwork is provided. Make sure you print it without any scaling. The complete artwork will just fit in the recess of the specified box to be used as a drilling template. I trimmed the edge off it, covered it with clear adhesive plastic and stuck it to the top of the box.

Now, Test & Calibration

1. Check that you have put all the components in the right places.

2. Check that you have soldered every lead.

3. Double check the PIC orientation, the diode and the 7805.

4. Don't forget - the PIC (as purchased) isn't programmed. You gotta load the LC Meter code into it before it will work. Phil VK3BHR is happy to help you do this.

5. Apply power carefully. If possible, use a variable regulated supply for the first try. Measure the supply current while gradually increasing the voltage. The current should be below 20mA. The prototype drew just 8mA. If you see nothing on the display and everything else checks out OK, try adjusting the Contrast trimpot. If it is set too far off, you will see nothing. The display should briefly show the word Calibrating, then C=0.0pF (or some other capacitance up to +/- 10pF ).

6. Allow several minutes "warm-up", then press the "zero" button to force a re-calibration. The display should now show C=0.0pF.

7. Connect your "standard" capacitor. The LC meter should read somewhere near its value (with up to +/- 10% error).

8. To raise the indicated capacitance, join the links marked "4" on the diagram below. To lower the indicated capacitance, join the links marked "3" on the diagram below. When the indicated value is "close enough" to the standard, remove the link. The PIC will remember the calibration. You can repeat this as many times as you like (up to 10,000,000 times I think before you wear out the PIC).

9. If the meter misbehaves, you can use the links "1" & "2" to check the oscillator frequency. Apply link "2" to check the free running frequency "F1" of the oscillator. This should be shown as 00050000 +/- 10%. If this reading is too high (near 00065535), the meter may go into "numerical overflow" and give you an error message. If the reading is too low (say below 00040000), you will lose some accuracy. Apply link "1" to check the "calibration" frequency "F2". This should be near 71% +/- 5% of the "F1" reading that you get by applying link "2".

10. Experts may like to adjust the inductor value to raise F1 to near 00060000 to obtain maximum resolution from the meter. An "L" value of 82uH is preferred instead of the specified 100uH (but you can't buy 82uH inductors in Bendigo).

11. If the meter shows near 00000000 for F1 and or F2, then recheck the wiring around the L/C switch, 'cos it sounds like your oscillator has stopped.

12. The Inductance measuring function is automatically calibrated when you calibrate the capacitance function. All the testing required is to check that the meter can be "zeroed" with the terminals shorted together.

If you have serious problems with your meter, then ask Phil, VK3BHR! He wants to see 'em all working properly.

Test Links
  1. Check F2

  2. Check F1

  3. Lower C

  4. Raise C


The LC Meter has been built all around the world and there have been many emails with questions, etc. It was even featured as a construction project in the Australian Silicon Chip magazine this year. This section is designed to limit the number of emails by answering your questions before you ask them. The following additional information has been provided by the designer, Phil Rice.

Claimed Accuracy - At best, +/- 1% or reading +/- "one least significant digit". More likely accuracy +/- 2% or reading +/- "one least significant digit".

The whole design is one big compromise.
(a) The oscillator is an approximation to a "perfect" one.
(b) The frequency measurement quantises the frequency into 16 bits.
(c) Every add, subtract, divide and multiply produces a result "one bit less accurate" than the least accurate number involved.

My estimate is that the numerical accuracy (frequency measurements plus all the maths) is at best 11 bits (+/1 1 part in 2048) and maybe more like 10 bits (about 0.1% error). I think it is a "well balanced" design - No one part of it is excessively bad (or good). To make the overall meter any better, would be a major re-design.

Claimed measuring range - (But see point 1 above as well - small measurements are inaccurate, because of quantising error).

Capacitors - 0.0pF to about 0.1uF. The upper limit is set by the "quality" of the comparators and by the "Q" of the capacitor being measured and by the inductor "Q". The amplitude of the oscillator gets pretty low for "bigCs". This leads to erratic oscillation.

Inductors - 0.0uH to somewhere over 10mH. The upper limit here seems to be set by stray capacitance in the inductor being measured. The meter cannot compensate for this.

What sort of PIC? - Anything where the part number contains 16F628. The "A" and "nonA", 4MHz & 20MHz should all work. The code is actually assembled for a "non A" variant but I have used it on a "A" chip with no problems at all. I checked the "hex" code "bit by bit" (for the configuration word) and, on paper, it is compatible with all types.

About The 10uF Tantalum capacitors - Accuracy here is not important. What is important is low impedance (mainly low series R & Xl & Xc) I think anything over 1uF is plenty big enough. After doing some tests on ordinary aluminium electrolytic caps, I reckon they would be good enough too. If in doubt, you can parallel a 0.1uF chip capacitor across each one. Neither capacitor form a direct part of the oscillator "LC" circuit, so errors in their values will have only a minor effect on the oscillator anyway. (Having said all that, my "old" LC meter failed 'cos I used old scrounged capacitors and one of them went open circuit.

Over Range - Don't be alarmed if you see this indication. It is normal for the meter to display this when switched to inductor and not having an inductor connected to the terminals.

One final comment. The backlight connections to the LCD are a matter of choice and need not be connected.

Email: General Information: vk3cmz@marc.org.au.
Created 2007 by Kevin Crockett. Comments and suggestions may be e-mailed to