I searched the Web and found a very cute project named LCM3 on this Hungarian site: hobbielektronika.hu . I love Hungarian rock since my school days, but I don't know a word in Hungarian :( . So, I searched the Web again, this time for this specific project and found a Russian forum where the project was discussed in details and I got more useful information about parts, settings and so on.
Specifications of the LCM3 are (according to authors of the project):
from 1pF to 1nF - resolution: 0.1pF, accuracy: 1%
from 1nF to 100nF - resolution: 1pF, accuracy: 1%
from 100nF to 1uF - resolution 1nF, accuracy: 2.5%
from 100nF to 100 000uF - resolution 1nF, accuracy: 5%
from 10nH to 20H - resolution 10nH, accuracy: 5%
from 1mOhm to 0.5Ohm - resolution 1mOhm, accuracy: 5%
So, I put the schematic in Eagle and this is the result:
As usual I designed a new PCB. The PCB is designed in such a way, that it is possible to mount the LCD display on top of the board. The multiturn trimmer for adjusting the contrast is bellow the display, so it must be of this type :
Тhere is also an option for mounting a 3-pin terminal connector, so it's possible to use the whole device without an enclosure.
All resistors are 1% metal film. Two 1nF capacitors are 1% styroflex. CX1 - 33nF is also critical - this must be polypropylene high voltage capacitor. I tried with two types: 10% X2 275Vac and Panasonic 3% 800V - worked fine with both. The inductor must be with low Rdc. There is a connector for a separate adapter, which bypasses the ON/OFF button. Maybe on next revision of PCB I would add an onboard power connector.
If the device is powered with external power adapter, you may increase back light current by decreasing the value of the resistor R11. You must consult with the datasheet of the display to select a proper value of the resistor.
I must warn you - capacitors MUST be discharged before measurement or there is a danger of destroying the chip.
Here is the final result. This is the second unit I built:
As you can see on the last photo, it is possible to measure small value resistors,
which is a nice bonus :)
Link for downloading project files in PDF: LCM 3.rar
Use them on your own responsibility.
Link for downloading hex file (on the bottom of the page).
Update: 08 June 2013
Recently I found in one of the local web stores an enclosure which I thought would be perfect for this device. As it turned out this wasn't the case :(
So I bought it and when it arrived I found that the internal width of the enclosure was insufficient for normal LCD 16x2 displays. I started looking for smaller displays and found out that it is hard to find such devices and they are very expensive.
Anyway, finally I bought the only one available (but three times more expensive) display and made a new PCB for this particular case and display. The schematic is the same as above.
Here are some pictures of my new LC meter:
|Expensive styroflex 1nF/1% capacitors.|
|For bigger inductance the meter shows parasitic capacitance.|