Cardboard ELSIE LC Meter

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Version:

August 20, 2016:
Revised: v1.0

The Cardboard ELSIE LC Meter

Shortly after arriving to start a new job and keen on keeping up my hobby, I needed to measure inductors and capacitors. I built this little device using the only parts I could find. It's based on the ELSIE LC meter designed by Steve KD1JV.

Introduction

When I got a new job in the Middle East, and being familiar with the situation, I recognized the importance of having something to do during my spare time. Despite evidence to the contrary, there is only so much time you can spend watching movies and shopping in malls.

One of the things I decided to do was to keep up my electronics hobby in some limited way. To do that, I decided to take just a handful of parts and a few small items of test equipment. That amounted to a small cardboard box containing those parts, a cheap soldering iron, a small homemade variable power supply, and a low cost multimeter.

After three months or so, I decided that I needed a few other items to make things a little easier: A transistor tester, a cheap digital oscilloscope, and an LC meter. Within a few months, I’d built the transistor tester, and I’d managed to bring a moderately priced scope back in my bag after a brief trip to the UK. However, the LC meter still presented a problem. (This was before all of those Chinese-made LC meters became available on the web….)

Almost every design I looked at required an LM311. This was used to in the LC meter's oscillator, its frequency being set by the inductor or capacitor being tested. A cheap microcontroller, most often a PIC16F48, was most often used to count that frequency, and then calculate and display the resulting value of the inductor or capacitor on an LCD. Problem was, my small parts collection did not include any LM311 chips, or a PIC, or an LCD.

While searching for an alternative approach, I realized that I could probably make a suitable oscillator by using one of several 74HC04 hex inverter SMD chips I had in my little box of parts. A few tests confirmed that approach was reasonably sound.

The LCD was the next problem. The discovery of a low cost design from Steve Weber KD1JV which used a Morse code and a beeper to report L or C values was timely. It used an ATtiny-compatible microprocessor which was no longer manufactured. However, I did have an ATtiny chip in my little parts box that was a recommended replacement.

Steve also described an upgraded version which added a small 7-segment LED display. I also had one of those in my small pile of parts, and even a 78L05 regulator. Actually, my LED display was a “common cathode” type rather than the common anode type specified in the Steve’s design, but the software changes required to sort that out were clearly going to be minimal.

Finally, the 14.318MHz crystal I had in my box of bits was on not the same frequency as the 8.192MHz crystal used in the original design, but I was again quite confident I could figure out the software changes required to sort that problem out. So, I was good to go.

Uh, almost good to go. I still had no way to actually build the design – I had no prototyping board nor any PCB scraps. Nothing. So how could I build the LC meter?

Scouting about, I stumbled across a scrap of cardboard. So, yes, my LC meter ended up being built on a scrap of thin cardboard. “Needs must”, as they say.

Here’s my revised schematic reflecting the parts I had available:

Construction

I built the 74HC04 oscillator section on top of the card, pushing a couple of holes through the card to anchor several of the leaded parts. The IC socket, crystal, the LED display and the other parts were all mounted on the card too, again, making holes with a pin and soldering the connections on the underside of the card.

The wires going to the 9V battery clip were anchored by passing them through an extra pair of holes.

After programming the ATtiny2313 and testing it, I glued the cardboard circuit to a backing piece of corregated card. This insulated the connections on the back of the card and added additional rigidity to the meter. You can see the result in the photo at the top of the page.

Operation

The resulting LC meter worked remarkably well, and I used it for about three years. It reports every result using six digits, sequentially displaying these digits in turn on the LED display and sounding out the digits in Morse, in picofarads for capacitance and in microhenries for inductors.

After all that time, I had managed to buy a few 16x2 LCD displays. I subsequently designed and built several LC meters which displayed the inductance or capacitance values more traditionally, directly on an LCD. Since they used I2C LCD displays which I happened to get reasonbly cheaply, they required some new software which I wrote from scratch. I'll describe these at some future date.

Conclusions

This cheap and basic design made it possible for me to continue my hobby in my limited space without much cost. I've described it here just to show how it is possible to build things when you only have a few parts, when necessary.

Download:

Cardboard_LC_Meter: This zip file includes the modified assembly source code (modified from Steve KD1JV's original software to suit the ATtiny2313, different crystal and changed display type), and the HEX file for those just wanting to program an ATtiny2313 directly.

The fuse settings for the ATtiny2313 are:

LOCK Byte: 0ffh (No locks)

EXTd Byte: 0ffh

HIGH Byte: 0deh
      DWEN 1 DW not enabled
EESAVE 1 EEPROM not preserved in erase
         SPIEN 0 SPI programming enabled
      WDTON 1 Watchdog timer off
  BODLEVEL2 1 Boot ROM size
BODLEVEL1 1
BODLEVEL0 1 No boot vector at startup
RSTDISBL 0 DISABLED

LOW Byte: 0ffh
     CKDIV8 1 NOT divided by 8
      CKOUT 1 Not enabled
       SUT1 1 Slow rising power
         SUT0 1
     CKSEL3 1 >8 MHz crystal clock
     CKSEL2 1
     CKSEL1 1
     CKSEL0 1

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