Load Cell Amplifier

INA 125 Instrument Amplifier
for rocket motor test stand

Board 2
A little better than the first one, I think.

Board 2, Small photo

This amp starts with a slightly larger PC board which is designed to use with integrated circuit packages.

It has three advantages over the last board:

1.		The 15-turn Cermet potentiometer is more precisely adjustable.
2.		It includes a resistor and terminals so that one can use a naked LED.
3.		The extra elbow room makes it a little easier for a klutz like me to work on.

Parts:

Most parts are from Radio Shack.

Those that aren't:

1. INA125P came from Digi-Key. To find it, do a search for INA125P-ND. They cost about $5.00 each.
You might want to check out Texas Instrument's 15-page Data Sheet on this device (.pdf file)

2. The A/D converter is the 194RS "Starter Kit" from Dataq - $24.95 plus shipping. It comes with software that is pretty good once you get used to it.

3. Computer. I assume you have one, or you would not be reading this. A laptop is almost a requirement, but I often test small motors by running a long cable the signal from the loadcell/amplifier to the Dataq/Desktop computer in the house.

I have occasionally set up a spare desktop computer out in the field near the test stand, running long 110v drop-cords to get power out there.
One day I will build an outhouse in the field and leave the computer there. It's a Compaq.

	The board and its components:	Number needed:	Radio Shack parts number	Price as of 10/04
1	9v Heavy-Duty battery snap connectors	2	270-324	$2.50/pack of 5
2	1k Cermet potentiometer, 15 turn	1	271-342	$2.59
3	Grid-Style PC Board	1	276-150	$1.29
4	16-pin IC Socket	1	276-1998	$1.29/pkg of 2
5	9v batteries	2	2300883	$9.99/pkg of 4
6	2-Position PC Board Terminals	6	276-1388	$2.29/pkg of 4 (get 2 pkgs)
7	Solid core hookup wire, 22ga or so	a foot or so, total	276-173	$6.29 for 3 rolls (a bit much!)
8	1000 ohm fixed resistor, 1/4 watt (or higher)	1	271-1321	$0.99/ pkg of 6
9	INA125P	1	Digi-key INA125P-ND	$5.10 each

	The case and its furnishings:
10	Stranded core hookup wire, 22ga or heavier	three or four feet		$6.29 (this is getting absurd)
11	LED - 1.8v (to let us know when the amp is turned on)	1	276-1622	$2.59/pkg of 20
12	DPST power switch, or DPDT switch, using only one throw.	1	275-666
13	9v Battery Holders (To keep the batteries from rattling around loose)	2	270-326	$0.99/pkg of 2
14	Project Box, 6x3x2 inches (to keep everything safe and together)	1	270-1805	$3.79
15	PC Board Standoffs (to hold the little board firmly in the box)	set of 4	276-195	$1.29
16	RCA Jack (for connecting to Dataq unit via audio cable)	1	274-346	$3.99 /pkg of 4
17	5-pin audio jack (you might use a different kind of connector)	1	Not a Radio Shack part	$1.79 at local electronics store

Notes on the parts:

1. Get the "Heavy-Duty Battery Connectors." Yeah, the standard ones are a little cheaper, but these are much more durable and easier to work with. They are well worth the extra $0.10 each.

2. 1k potentiometer. Since there is plenty of room on this board, I am using the Cermet potentiometer. It's adjustment range is 15 turns, making it much more precisely adjustable than the dial-type pots which cover the same range in less than one turn.

When mounting the board in its case, make sure to arrange it so that you can angle a screwdriver into the adjusting screw. That is so you don't have to take it out of the box to make adjustments. I mounted it wrong the first time, such that to make adjustments I had to take the board off its legs. That was a pain., so I re-mounted the board. An alternative I though of afterward - could have drilled a hole in the case at the right spot to let a screwdriver blade in.

3. Grid Style PC Board - This larger board is designed to accommodate IC - DIP packages, and is a blessing in other ways, as will be illustrated shortly. Strongly recommended.

11. Always a sucker for bright shiny things, I bought a package of 20 "Assorted LEDs" on one of my sprees to Radio Shack (276-1622, $2.59) . They are apparently surplus, being of various shapes, sizes and colors. No specs were given, but I noticed that most LEDs in separate packages were rated to run at 1.8 volts, so I assumed that value for these, and it has proven correct. The full 18volts of this system is thus way too much. I even burned out one by holding it to the terminals of a 9v battery for a second too long.

A drop-down resistor is used to reduce the voltage.
I found several websites that offer assistance in figuring the proper resistor to use with given LED and voltage.
The one I used is on Bill Bowden's web page: LED Series Resistor Calculator

So on this board I have added a sixth terminal block wired to ±18v with a 1k resistor in series with one of the leads. This drops the power to 1.75 volts. I can clamp an LED directly into the terminals for testing purposes and have tried every type of LED that came in that package with the amplifier. They all worked just fine, some bigger, some brighter than others, but they all worked. I'm still trying to figure out which one I like best.

Once I decide, it won't be difficult to solder longer leads on the LED, drill a little hole in the project box and glue it in with a drop of epoxy.
Now where to put it, where to put it...

The Larger Board

The little board worked OK, but this one is the ticket. The white spots indicate that the holes are electrically connected: White paint on one side mirrors copper film on the other.

There is an extra row of holes on the left side, offering a bit more room. I will put the terminals and most of the wiring on this side, so things will be a little less crowded.	Notice how the IC socket nicely straddles ground strips running up underneath it. This PC board layout offers lots of possibilities!
	Other side of the board. The socket is held in place digitally, no solder yet. Note that each pin has two adjacent holes connected with copper film. This makes it easy to run jumpers from any pin to anywhere else.

The two long connected strips in the middle do not contact any of the DIP socket pins. I will use this as ground - a couple of pins on the INA 125 (pins 5 and 12) will need to connect to ground. Since they are directly across from each other, I will simply run a strand of wire from 5 to 12 on the copper side and solder it down.

Yes, yes. I will remove the chip from the socket before soldering. It is here now for illustrative purposes.

Here the terminals and LED resistor are installed, but nothing is soldered yet.

Astute observers may note that I really need only 11 terminal connections.
But these things come in pairs, and I was not interested in cutting a set in half.

Aaah! The sweet Autumn scent of burning rosin!

The temperature is finally tolerable here, and the mosquitoes have gone to sleep, so I can solder out at the picnic table. Some really smart guy installed a power outlet on it awhile back. Not going to name any names...

I had intended to show you perfectly clean, beautiful solder joints, a delicate parabola of silvery solder sweeping up each lead and down to the copper trace in elegant symmetry.

It would be a bold and compelling demonstration of my consummate mastery of the manly soldering arts, propelling me to greater success with rocketry, wealth, and women.

Wishful thinking, bordering on delusional. Especially the "women" part.

You get a reality show instead.

I will have to settle for joints that work. Hopefully.

Something else I learned after making the board illustrated below:

These terminal blocks have a dovetail spline that allows them to attach to one another, forming neat little rows.

The bottom two terminals on my board are not so attached because I didn't know that at the time. I'd read it, but didn't quite realize it. Funny how that happens.

They will be much neater on the next board, I promise.

Here is a layout/wiring chart for the completed board. (Please accept my apologies if you have tried to make this amp from a previous version of this page - I had the labels for terminals 1 and 2 reversed. Terminal 1, Yellow, pins 4 and 15, is Load cell Excitation Positive Terminal 2, Red, Pins 5 and 12, (ground) is Load Cell Excitation Negative. Revised 3-7-05	If you build this amp, you should also have the schematic from Mike Bennett.
Click for larger image	Click for larger image
	Yeah, the photo is a little different from the drawing. I drew it for clarity Note that three of the black wires in the wiring chart are not necessary, and in fact are not included on the actual board. They are the wires going from pins 5 and 12 of the chip to ground, and from one side of the ground to the other. The extra wires are drawn-in to make it really clear what connects to what. In reality, I just soldered a jumper on the underside from pin 5 to pin 12 and to the ground strip as it went across.

This little amplifier doesn't do much by itself.
Yeah, the light comes on, but there are no funny sounds or stimulating conversations.
It needs to be hooked to some other stuff.

But first - like the smaller board, this one needs to go in a box.
I don't have explicit instructions on this, but most of the parts are included in the list above.
It is a mechanical thing that any good rockit scientist should be able to finger out.
The next page offers some photos which might help.

Next: The Box

Jimmy Yawn
jyawn@sfcc.net
rev. 3/7/05