I wanted to make a rocket test stand so I could measure the thrust that is output from rocket motors. A thrust stand helps in the design of rockets by showing the characteristics of the rocket motor.
Step 1: Materials
#5-32 Screw & Tap | BoltDepot
PCB | Amazon
Arduino Nano | Amazon
Piezo Buzzer | Amazon
Loadcell HX711 | Amazon
Relay | Amazon
LED | Amazon
Resistors | Amazon
18-10 Cable | Home Depot
Mini USB Cable | Amazon
If you go to the home depot store you can get someone to cut you off about 6 meters for $18
$10 in nickles | I would get this from a bank or a store.
Total cost estimate $75
Soldering Iron | I recommend the ts-100 if you don't have one.
Step 2: Design
A load cell thrust stand is a rocket motor tester. The thrust stand measures the thrust over time. Arduino boards can send serial data to a computer or an SD card(which is really easy you just have to format every time you test.) I knew I could code the Arduino to output the weight measured by the scale and I could also program it to tell me the line number that is output in the serial monitor and the time measured in milliseconds which makes it easy to copy and paste into a graphing program and who doesn't like graphs?
Step 3: Print
Step 4: Make the Clamp
You will want to print out the sliding bracket and tap it with a 12mm
tap. After finishing up the bracket you will want to print the tightening thread. Once the tightening thread is about fifty percent completed you will want to pause the print(I use octoprint to do this) then move the extruded out of the way so that you can screw the bracket onto the thread. You will want to remember how far and which direction you moved the extruder so you can do it in reverse. Once you resume the print the extruder should have started where you stopped it and in an hour or so you will have the clamp portion of the stand.
Step 5: Cut the Bolts to 5mm
I did not have 5mm bolts so I had to cut some #5-32 bolts to 5mm long.
To cut the bolts I threaded a nut onto the bolt all the way to the head of the bolt. I used some calipers to mark 5mm on the bolt. Then I used my Dremel tool to cut the bolt. Finally, I unscrewed the nut once the bolt cooled down. The nut ensures the bolts threads are normal on the cut.
Step 6: Post Prossessing
Since I made the holes on the enclosures mounting points small you can use any bolt that is not larger than 6mm in diameter. and longer than 4mm.
I HIGHLY encourage you to skip doing what I did to tap the part. Either get a 5-32 tap or get heat set inserts for 5-32. Taping the part the way I did it makes a terrible thread and two of the holes I tapped are completely useless and in doing so the enclosure cover is not flush with the enclosure.
During this step since I did not have a tap, I used a self-tapping screw to make the hole a little larger and then screwed my #5-32 screw into the enlarged hole to make the threads, one of the perks of plastic parts.
Be careful if you do the same you can easily separate the layers on the part.
Step 7: Test and Code
I first started the testing phase by opening photoshop to design a simple circuit layout. The circuit is rather simple if you wish to make it simpler you can remove the led from the relay and just use one relay as an igniter, and you can remove the buzzer but the led and buzzer do act as a notification if the relay is hot or if the Arduino is connected to the computer.
Step 8: Build the PCB
To prep the PCB I used a Dremel and cut the corners of the PCB off so I can bolt it to the 3D printed part I did the same to the relay board as well.
When soldering the circuit together I used jumper wires to connect everything together.
Step 9: Cable
To connect everything I used an 18-10 cable.(x10 18 gauge insulated wire in one cable.) I soldered a USB mini port on the sending end and a regular USB on the receiving end. I need 5v intermittent for the LED indicator, 5v for the switch, and D6 Wire for the output of the switch, and a ground to wrap it all up.
As a side note, I recommend using some silicone alligator clips to connect the 9v relay to the igniter.
Step 10: Mount in the Enclosure
It's time to package all the hardware. Connect all the wires into the appropriate terminal blocks and bolt the boards down.
Step 11: Put It Together
To finish the hardware bolt all the parts together.
Step 12: Controller
The controller is rather simple if you would like to simplify it even further I recommend removing the led Without the LED you will need to be diligent to the sound of the relay closing.
You will also need a momentary switch to turn the relay on and off, and a 9v battery connector.
Step 13: Calibrate
Buying calibration weights cost money, so why not just use the money? Nickles weigh about five grams each. In order to calibrate the load cell you will need at minimum one known weight but it is better to have multiple. I went to the local store and asked for them to exchange a ten dollar bill into nickles. Once I got home I used my calibrated scale to measure how much it weighed and it was exactly one kilogram. If you have a scale and you know it is calibrated you can just weigh out something to around a kilogram and not worry about nickles, however, getting nickles allows you to have multiple calibration points and makes your scale more accurate.
I started by weighing out half a kilogram and changing the offset values to match, then one kilogram. after measuring and getting the offset value for both weights the cell is complete and everything is set up to test a rocket!
Instead of using my code and manually changing the values Sparkfun has an Arduino program that makes it easier.( Link)
Step 14: Closing Statement.
I did not like the outcome of the project I felt I could do a lot better and make a more badass version. I wanted to post the information about building a scale and my circuits for a simple scale and igniter for everyone to use. I will be working on a much more complicated and robust test stand in the future so keep checking in.