This is a digital test stand with a 50 Lb capacity for static testing rocket motors.

Contents 1 Design 2 Base 3 Load Cell 4 Amplifier & A to D Converter 5 Data

Design

I decided to take the easy way with this one and instead of providing regulated + and - 5v to just run it all at +5v. Precision and linearity may suffer a little, but it seems to get .1lb precision over the 50lb range (More accurate data forthcoming).

Gallery of all images

Base

The base is just a few chunks of steel C-channel. This was the most expensive component after the load cell.

Load Cell

The load cell is a Chinese made 30kg aluminum number I ordered from Ebay for $20 plus shipping. While the seller was, as far as I could tell, American the part was shipped directly from the factory in China, marked as an engineering sample for customs and included no documentation or invoice / packing slip. I am satisfied.

Designed to be excited at +/-10v I opted to try it at 0v / +5v.

Amplifier & A to D Converter

The amplifier is an AD620 low power instrument amplifier. The reference voltage is set with a 10k 15 turn resistor and the gain is adjusted with a 1k 15 turn resistor.

A Microchip PIC 16F688 that samples its A/D converter and outputs the value over RS-232 serial as human readable text as fast as it can. The sample rate is about 2.5khz raw, but I may decide to oversample and average a number of inputs to reduce data size.

The first version I made was a bit of a rat's nest of wire so I etched a PCB for it. Some of the other IO ports of the PIC are exposed on the board in case I decide to add any other features.

Data

Following is a graph of 0-50 lbs in 5lb increments. The very first step, from zero to two pounds or so, is the addition of the weight carrier, which is then filled to five pounds.