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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.

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.

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, the On/Off switch, the pushbutton Zero switch or the battery connector.

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.

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.



  1. Check F2
  2. Check F1
  3. Lower C
  4. Raise C
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Digital LC Meter from Midland Amateur Radio Club (original here)

schematic (pdf)
pcb layout (pdf)
hex file