THIS PROCEDURE WILL HELP YOU TURN ANY ELECTRICAL PANEL METER INTO A SENSITIVE BALANCE CAPABLE OF RESOLUTION BELOW 1 MILLIGRAM.

 

 


D
ESCRIPTION

A DIGITAL BALANCE HAS USES IN ALMOST EVERY INDUSTRY, FROM CHEMISTRY, TO PHARMACEUTICALS, TO COMPOSITE MATERIALS.  DIGITAL BALANCES HAVE PLUMMETED IN COST IN RECENT YEARS, AND 10 OR 5 MILLIGRAM UNITS CAN BE HAD ON EBAY FOR LESS THAN 20 DOLLARS.  SCALES WITH MILLIGRAM OR LESS RESOLUTION, HOWEVER, ARE STILL VERY EXPENSIVE, WITH LARGE UNITS INTO THE THOUSANDS OF DOLLARS.

THIS PROCEDURE DESCRIBES A SIMPLE BALANCE CAPABLE OF RESOLUTION WELL BELOW 1 MILLIGRAM.  IN SHORT, A PHOTO DIODE ADJUSTS THE CURRENT THROUGH A GALVANOMETER COIL IN ORDER TO PROVIDE A CONSTANT FORCE TO LIFT THE SAMPLE TO BE WEIGHED.  GALVANOMETER HARDWARE IS AN EXCELLENT MATCH, SINCE THE COIL IS ALREADY WOUND, AND THE SAPPHIRE BEARINGS ARE EXTREMELY LOW FRICTION.

MATERIALS        

-PANEL METER
-SOME PLEXIGLASS
-IR EMITTER/PHOTOTRANSISTOR PAIR
-MISC POT.S
-100uH INDUCTOR
-2N2222 TRANSISTOR
-MISC RESISTORS, WIRE

EQUIPMENT        

-SOLDERING IRON
-SUPERGLUE
-DIGITAL MULTIMETER

NOTE:  SORRY ABOUT THIS, BUT I DESIGNED THIS BY TRIAL AND ERROR.  AS SUCH, SOME OF THE PICTURES BELOW WERE TAKEN WHILE TRYING OUT COMPONENTS, OR COMPONENT VALUES THAT DIDN'T MAKE IT TO MY FINAL DESIGN.  TRUST ONLY THE COMPONENTS AND VALUES IN THE SCHEMATIC.  USE THE PICTURES ONLY FOR MECHANICAL DETAILS.

SCHEMATIC


PROCEDURE

 

WHY?

1.   GET A PANEL METER.  TEAR ONE OUT OF AN OLD AMP OR STEREO IF YOU'RE CHEAP, BUT MAKE SURE THE NEEDLE STILL MOVES FREE.

PANEL METERS HAVE THE MAGNET AND COIL FOR PROVIDING A LIFTING FORCE, AS WELL AS HIGH QUALITY SAPPHIRE BEARINGS.
2.  IF YOU BUY A NEW METER, DITCH THAT RESISTOR IT COMES WITH. WHO CARES?
3.  START TAKING OFF THE VARIOUS SHIELDS AND PLASTIC CASES.  DON'T DAMAGE THE NEEDLE (TOO MUCH). BECAUSE.
4. AT LAST PRY OUT THE MOVING BITS. OBVIOUSLY.
5. STRIP A PIECE OF STRANDED WIRE; YOU'RE AFTER A SINGLE STRAND, ABOUT 2 INCHES LONG. YOU COULD USE ANY THIN WIRE.  JUST GET SOMETHING NICE AND LIGHT.
6. USE A TINY PIECE OF THE WIRE TO FORM AN EYELET, AND SUPER GLUE IT TO THE END OF THE METER ARM. YOU HAVE TO MAKE A CLOSED LOOP, SINCE THE METER WILL BOUNCE A BIT AND YOU THROW YOUR PLATFORM OFF.
7. BEND THE WIRE AS SHOWN, AND CUT A SUITABLE CIRCLE OUT OF LIGHT CARDBOARD OR PAPER.  THE LIGHTER, THE BETTER. THE WIRE IS GOING TO GET SUPERGLUED TO THE BOTTOM OF THE DISK.  THIS FORMS THE PLATFORM / TABLE / MAGICAL BASKET.
8. THREAD THE WIRE THROUGH YOUR EYELET. UM, IGNORE THE FACT THAT I JUMPED AHEAD IN THE CONSTRUCTION FOR THIS PHOTO...
9. GLUE THE PLATFORM IN PLACE.  
10. YOU'LL NEED TO MAKE SOME ROOM IN YOUR PLEXIGLASS BACKPLATE FOR THE ZERO ADJUSTMENT SCREW ON THE BACK OF YOUR METER.  I DID THIS WILL A DRILL BIT. THIS ISN'T THAT CRITICAL, BUT THE MORE PARALLEL YOU MOUNT THE METER, THE BETTER.
11. USE SOME EPOXY OR OTHER GOOD ADHESIVE TO GLUE THE METER TO THE PLASTIC.  
12. ADD THE RESISTOR TO YOUR PHOTOTRANSISTOR.  
13. GET YOUR 2N2222 READY.  
14. SOLDER THE TWO ROUGHLY AS SHOWN.  
15. ADD THE LIGHT SHIELD MADE OUT OF HEAT SHRINK TUBING.  
16. GLUE THE SENSOR/TRANSISTOR IN PLACE AS SHOWN.

REMEMBER TO IGNORE THE COMPONENTS IN THESE PICTURES.  LIKE THE CAPACITOR SHOWN HERE, FOR EXMAPLE.
17. MAKE A HOLE FOR THE WIRES THAT SUPPLY YOUR IR LED.  
18. MAKE A LITTLE STANDOFF AND ATTACH THE IR LED AND ITS SERIES RESISTORS IN PLACE. NOT THE RESISTORS SHOWN.
19. ATTACH THE LED AS SHOWN.  
20. ADD THE VOLTAGE REGULATOR AS SHOWN.  NOTE THE INDUCTOR ADDED ON THE OTHER SIDE OF THE PLASTIC.  
21. ATTACH BATTERY HOLDER, INDUCTOR, AND SENSITIVITY ADJUST.  
22. DONE.
 

ADJUSTMENT / USE

DEPENDING ON THE MATERIALS USED IN CONSTRUCTION, SOME ADJUSTMENT WILL BE NECESSARY.  FOLLOW THE FOLLOWING STEPS TO BEGIN USING YOUR SCALE:

1.  WITH NO WEIGHT ON THE PLATFORM, TURN ON THE BALANCE.  THE NEEDLE MAY SWING TO THE TOP OF ITS TRAVEL; PUSH IT DOWN TO THE SENSOR REGION. 

2.  ADJUST THE SENSITIVITY POT UNTIL THE NEEDLE BALANCES.  IT MAY BE JUST BARELY HELD IN POSITION AND WANT TO SWING UPWARDS, THAT'S OK.  YOU WANT THIS POT ADJUSTED SO THAT VIBRATIONS DIE DOWN QUICKLY, BUT THE NEEDLE STAYS IN POSITION.

3.  NOW ADJUST YOUR TARE POT UNTIL THE METER READS 0V.

4.  ADD A TEST MASS OF KNOWN WEIGHT (PIECES OF MAGNET WIRE, ALUMINUM FOIL, ETC) AND ADJUST THE CALIBRATION POT UNTIL THE METER READ THE APPROPRIATE VOLTAGE.

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