Sci-Spot.com - Accelerometer

	THIS PROCEDURE WILL HELP YOU MAKE AN ACCELEROMETER WITH AN LED BARGRAPH DISPLAY. ALMOST ALL OF THE PARTS ARE CHOSEN SUCH THAT THIS CAN BE BUILT WITH THINGS AVAILABLE FROM A LOCAL RADIOSHACK.

DESCRIPTION AN ACCELEROMETER IS A DEVICE WHICH MEASURES ACCELERATION; HOW FAST IT'S SPEEDING UP. IN THE PAST AN ACCELEROMETER COULD BE A COMPLEX, BULKY PIECE OF EQUIPMENT, BUT IN TODAY'S WORLD, ACCELEROMETERS ARE AVAILABLE AS SINGLE IC'S AND USE COMPLEX MICROSCALE MACHINING TECHNOLOGY. ANALOG DEVICES (WWW.ANALOG.COM) MAKES A VERY SMALL, EASY TO USE ACCELEROMETER, THE ADXL103. THIS PROCEDURE WILL DEMONSTRATE HOW TO BUILD A VERY COMPACT ACCELEROMETER CAPABLE OF POWERING THE ADXL103 AND DISPLAYING IT'S OUTPUT ON AN LED BARGRAPH. WITH THE EXCEPTION OF THE ACCELEROMETER IC ITSELF, ALL THE PARTS CAN BE FOUND AT RADIOSHACK.
BUY A KIT IN THE STORE!
MATERIALS	-10 LED BAR DISPLAY -LM741 OPAMP -2N2222 NPN TRANSISTOR -LM7805 5V REGULATOR -9 1N4002 DIODES (OR EQUIVALENT) -ASSORTED RESISTORS (ALL AVAILABLE IN THE RADIOSHACK ASSORTMENT) -3000PF CERAMIC CAPACITOR -ADXL103 ACCELEROMETER
EQUIPMENT	-SOLDERING IRON -SUPERGLUE

PROCEDURE		WHY?
1. YOU'LL BE WORKING FROM THIS SCHEMATIC, SO FAMILIARIZE YOURSELF WITH IT.		NOTE THAT R23 AND R25 CAN BE PART OF THE SAME POTENTIOMETER. SIMPLY USE A 10K WITH A CENTER LEAD.
2. FIRST, PREPARE THE ACCELEROMETER BY LOCATING THE SOLDER PADS AND ORIENTING IT WITH THE 'DOT' AWAY FROM YOU. V+ WILL BE UP, xOUT TO THE LEFT, AND COM TO THE RIGHT.		YOU DON'T WANT TO MESS THIS UP. THESE ACCELEROMETERS ARE ROBUST, BUT WON'T SURVIVE 12V TO THE WRONG PINS.
3. SOLDER LEADS WITH A LOW TEMPERATURE IRON TO THE PADS SHOWN.		I'M ASSUMING YOU WON'T BE BUILDING THIS PROJECT WITH SURFACE MOUNT COMPONENTS.
4. BEND THE LEADS BEING CAREFUL NOT TO BREAK YOUR JOINTS.		NOW THIS ACCELEROMETER CAN BE ATTACHED EASILY AS WELL AS USED IN A BREADBOARD.
5. BEGIN BUILDING THE CIRCUIT ON THE BREADBOARD. NOTE THE RESISTOR VALUES I USED. IN SOME CASES, I USED TWO IDENTICAL RESISTORS IN SERIES.		YOU ABSOLUTELY MUST TEST THIS PROJECT ON A BREADBOARD BEFORE YOU START TO SOLDER. THERE ARE MANY UNLABELED COMPONENTS AND POINT TO POINT WIRING ONLY MAKES IT HARDER.
6. WHEN THE BREADBOARD IS DONE, IT'LL LOOK SOMETHING LIKE THIS. HOPEFULLY YOU'LL HAVE LAID THINGS OUT BETTER THAN I DID.		TEST THIS TO MAKE SURE IT WORKS BEFORE CONTINUING. IT MIGHT BE A GOOD IDEA TO SET THE POT TO 50% SO NOTHING BLOWS UP.
7. ONCE YOU'VE TESTED EVERYTHING YOU CAN BEGIN BUILDING THE FINAL DEVICE. REMOVE COMPONENTS ONE BY ONE FORM THE BREADBOARD AND SOLDER THEM IN PLACE. START WITH THE LEDS AND RESISTORS. FOLDING THE RESISTORS LEADS AS SHOWN WORKED WELL FOR ME. BE SURE YOU'VE MAINTAINED THE POLARITY OF THE LEDS.		POINT TO POINT WIRING IS MORE COMPACT THAN A PCB BOARD. CHOOSE FOR YOURSELF.
8. WHEN YOU'RE DONE, USE A CUT PIECE OF RESISTOR LEAD TO FORM A COMMON RAIL FOR THE RESISTORS.
9. REPEAT THIS PROCESS WITH THE DIODES. NOTE THE DIODES FORM A SERIES STRING, NOT A PARALLEL STRING LIKE THE RESISTORS. FOLDING THE DIODES AS SHOWN WORKED WELL FOR ME.		BE CAREFUL HERE; LOTS OF TIGHT SPACES MAKE FOR EASY SHORTS. BEWARE SOLDER BRIDGES.
10. THINGS SHOULD NOW LOOK LIKE THIS.
11. PREPARE THE OP AMP BY STRADDLING THE POTENTIOMETER (R23 AND R25) ON TOP OF IT. BE SURE YOU HAVE THE LEADS CORRECT. USE A LOW TEMP IRON.		DON'T GLUE THE POT DOWN; IT HAS MOVING PARTS UNDERNEATH.
12. ATTACH THE OP AMP TO THE SIDE OF THE LED BAR AS SHOWN. THEN ADD THE CAPACITOR AND TRANSISTOR TO THE UNDERSIDE OF THE OP AMP. AGAIN, A LOW TEMP IRON IS NEEDED.		YOU CAN USE A BIT OF GLUE TO HOLD IT IN PLACE.
13. NOW PREPARE THE ACCELEROMETER AND VOLTAGE REGULATOR.		BEWARE THAT THE 'TAB' AT THE TOP OF THE REGULATOR IS TIED TO GROUND. DON'T LET THE ACCELEROMETER'S WRONG PIN TOUCH IT.
14. ATTACH THE REGULATOR AND ACCELEROMETER OPPOSITE THE OP AMP AND YOU'RE DONE!		DONE!
15. THIS IS OPTIONAL, BUT I POTTED THE WHOLE THING IN EPOXY TO PREVENT THINGS FROM MOVING AROUND AND SHORTING OUT.		THIS WOULD ALSO MAKE IT WATER PROOF IF YOU WERE TO COVER THE POT AS WELL.

ADJUSTMENT

THE ACCELEROMETER OUTPUT BOTH POSITIVE AND NEGATIVE ACCELERATIONS, SO YOU'LL WANT TO ADJUST THE POT SUCH THAT, WHEN ITS FLAT ON A TABLE, HALF THE LEDS ARE LIT. THAT WAY YOU'LL BE ABLE TO SEE THE FULL RANGE OF ACCELERATION:

IMPROVEMENTS

THE GOAL OF THIS PROJECT WAS TO SUCCEED WITH ONLY RADIOSHACK COMPONENTS. THAT SAID, THIS DESIGN HAS A FEW LIMITATIONS THAT COULD BE ADDRESSED.

THE RESOLUTION OF THE DISPLAY IS LIMITED BY THE VOLTAGE DROP OF THE DIODES. USING SCHOTTKY DIODES WITH A VOLTAGE DROP OF ABOUT .4 WOULD ALMOST DOUBLE THE RESOLUTION, OR THE NUMBER OF LEDS. USING THE GIVEN DIODES, THE SUPPLY VOLTAGE WILL NEED TO BE INCREASED BEYOND 12V TO USE MORE LEDS.
PUTTING ANOTHER POT ON THE BASE OF THE TRANSISTOR WILL GIVE YOU A KIND OF LEVEL SHIFT ABILITY, SO THE ZERO POINT CAN BE MOVED ABOUT.
AN LED BAR DRIVER IC WOULD BE IDEAL, AND HAVE ALL THESE FUNCTIONS INTEGRATED.

FOR THE CURIOUS, HERE IS A GRAPH OF THE AMPLIFIED SIGNAL VOLTAGE, AS WELL AS THE INDIVIDUAL LED VOLTAGES VS SIGNAL VOLTAGE:

AND HERE IS A PEAK INSIDE ONE OF THESE MARVELOUS MEMS ACCELEROMETERS:

READER QUESTIONS

RYAN - Quick question for you: I saw your accelerometer on your website and thought you may be able to help me. I want to be able to do something similar in the sense that I would like to use an led bargraph as a meter, but rather than a meter controlled by acceleration, a meter controlled by a knob (sort of like a volume control). Is there any way you could offer advice and help me understand how to modify your accelerometer schematic so that it will meet my needs. Any help would be greatly appreciated. Thanks in advance!

ANSWER - Surely, that's no problem at all. Notice in the schematic the two large components at the left side. One is the LM3805 voltage regulator and the other is the ADXL103 accelerometer. Both should be omitted obviously. In their place, put a potentiometer, with a value around 10k Ohms. Connect one leg of the pot. to V+ and the other leg to ground. Connect the center leg, the wiper, to the noninverting input on the opamp (opamp pin 3 on the schematic). That's it!

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