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Simple LC meter
author: Hristo - xristostyahoo.com

This project is a simple LC meter based on PIC16F682A mcu.


Here is another piece of laboratory equipment - LC meter. This type of meter, especially L meter is hard to find in cheap commercial multimeters.

Schematic of this one came from this web page: https://sites.google.com/site/vk3bhr/home/index2-html.

It uses PIC microcontroller 16F628A, and because I recently acquired a PIC programmer, I decided to test it with this project. Following the above link you will find the original schematic, PCB, source and HEX files for programing the microcontroller and detailed description.

Here is my adaptation of the schematic:

I removed the 7805 regulator, because I decided to use a 5V adapter from Sony mobile phone.

In the schematic, trimmer-potentiometer is 5k, but actually I put 10k, after consulting with the datasheet of the LCD module I bought. All three 10uF capacitors are tantalum and C7 - 100uF actually is 1000uF. Two 1000pF capacitors are styroflex 1% and inductor is 82uH. Total consumption (with back light) of the device is 30mA.

R11 limits the back light current and must be calculated according to the actual LCD module used.

I used the original PCB as a starting point and modified it to suit better to my components.

Here is the result:

The last two pictures shows LC meter in action. In the first of them, there is 1nF/1% capacitor and in the second - 22uH/10% inductor. The device is very sensitive - when I put the test leads there is 3-5 pF reading on display, but it is eliminated with the calibrating button.


download schematic in PDF


download PCB top layer on PDF

download PCB bottom layer in PDF or mirrored




Update: 24 June 2012

One of my colleagues in a Bulgarian audio forum ask me to build him a copy of the LC meter, but this time with 9V battery power supply.  I made a new PCB with little rearrangement of the old one and adding a 78L05 voltage regulator.

I also added an automatic sleep mode, schematic of which I found here: http://www.marc.org.au/marc_proj_switch.html

The goal here was to make power consumption as low as possible. With increasing the value of R11 to 1.2kOhm which control the back light current, total current of the device was decreased to 11-12mA. Without back light at all the power consumption will be decreased even further, but the visibility suffers greatly.

After some tests, the value of the C10, which defines the ON time, was chosen to be 680nF. The ON time in this case is 10-11 min.  The MOSFET Q2 may be replaced with BS170, but bear in mind that the leads are in reverse order.

ON-OFF switch is momentary non lock type.



download schematic in PDF


download PCB top layer in PDF

download PCB bottom layer in PDF or mirrored




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