OpenGauge Project

Introduction

The OpenGauge project is an aircraft EGT, CHT display intended for primary use in experimental aircraft.It features a very "sexy" ezLCD-301 display from  EarthLCD. Also, for fun, we have added a few other transducer measurements like oil pressure, oil temperature and fuel flow. We decided to build this gadget using shields for the Arduino to do all the concept testing. The first shield provides power for the for the ICs and the LCD. The second shield is dedicated to the thermocouples and the third shield was designed for all the transducers. They all utilize the DB-9 and DB-25 pin connectors to interface with all the transducers and thermocouples.

ezLCD-301 Display

We were very pleased with the simplicity of the display. It receives serial commands utilizing theEarthSEMPL (simple embedded macro programming language)to draw the information (bar graphs and numbers) and that will save a lot of processing power on the Arduino. We adapted the library used in the arLCD (another LCD product from Earth LCD) and it worked fairly well with the 301 display. Also, it is worth mentioning that this LCD is a touch screen! Earth LCD offers different models, with different sizes and features on their website.

Preliminary Design

Overall, the interface was very simple. It utilizes the analog read function on the Arduino board. A nice feature of this design is that it utilizes multiplexers to save the analog inputs on your board, as a bonous that helps to keep cost down by reducing the requirement of several thermocouple amplifier ICs ($17 bucks each!). Utilizing two AD595 ICs and two 74HC4051 multiplexers we can read 16 type K thermocouples. Since we only require 12 channels (6 Cylinder Head Temperature and 6 Exhaust Gas Temperature) that layout design worked well for us and perhaps an 8 cylinder aircraft can benefit from that too.

Since the AD595 amplifies the signal by a factor of (10mv/ ºC) exhaust gas temperatures nearing the 1500ºF will output signals close to 8 volts. Unfortunately, the Arduino analog read is limited to 5Volts. To solve that challenge we applied a simple voltage divider circuit that uses two 1K resistors. Given that we are only concerned with high temperature readings for the EGT, a voltage divider will do the job well for an output between 5 and 8 Volts.

During preliminary testing we decided to test two probes, one at room temperature and the other at water boiling temperature, to find out if the multiplexer was adding noise to the readings during the array or inducing any errors. From the readings we seen the multiplexer did a great job.It took a lot of experimentation with the oscilloscope and a few capacitors to smooth the thermocouple signals. We read a few papers written by Analog Devices regarding multiplexing on the AD595. (AN-274 and AN-369). The paper suggested using a differential multiplexer but we had trouble reading the signals. Finally we achieved a descent circuit design after lots of testing, trial and error and some luck. We are not 100% satisfied with the design and the noise signal but hopefully someone will be able to improve it or investigate other ways to achieve a solid design.(if such thing exists?). Maybe the cold junction compensation can be explored a little more on the circuit too. The plan right now is to calibrate temperatures on the software side when we calibrate the probes. It is worth mentioning that the circuit we created  for the AD595 ICs deviate from the suggested AD595 diagram, it gave a much cleaner signal and grounding of the thermocouples was not an issue.

Hardware Design

Another part of this project was to experiment with several DIY PCB etching processes. Before getting too "ballsy" and using a PCB manufacturer like OSHPARK.com we decided to NOT gamble and mature the project further with some trial and error. The method used was the press and peel paper. The results were acceptable but it required some retouching on the traces. We have not tried the photo resist method yet, but it seem to be very popular specially with double sided boards.

A messy circuit like this one...

With a few CNCed holes....

Can look pretty good when cleaned up

Hall Sensor RPM

For convenience the RPM is picked up right at the magneto using a latching hall sensor (Melexis US1881). The sensor is hooked as an interrupt in Arduino to make sure that the processor does not 'skip a beat' when measuring the period/frequency . Also it is important to keep in mind that the MAG in a six cylinder engine turns at 1.5 times the crank speed. So a ratio of 1/1.5 must be used when measuring the period from cycles per second to cycles per minute (1/f*40). We have used the proper fitting size for a Bendix Mag and great care must be used to keep clearance from the magnet. The hall sensor was secured with resin with some of the sensor exposed. Don't forget to include a hole to vent the mag. We included a notch in the thread seen in the bottom of the fitting.

Housing 3D Printing

The goal was to utilize the standard 3.125 inch hole in the aircraft panel.The housing would be built in 2 pieces to facilitate assembly and other required maintenance. The final design will be shared here, with a link to the STL files.

Software

The software still under construction and the goal is to keep adding more features to the gauge until we run out of memory!. The software is available under the github link.

Future Goals

• Increase communication speed (Refresh Rate) 
• Finish the Transducer PCB
• Test the Durability of the circuit
• Adding a Data logger (SD Card)
• Multiple screen Pages (Rotary Encoder and Buttons or screen touch)
• Dimming Screen (Touch) - Completed

DIY Thermocouple Testing Circuit

Since I needed to test a few electronic units that read K type thermocouples it was time to create a fake thermocouple, essentially a device that could would output between ~3mV to ~30mV, or ~100ºF to ~1500ºF.



The circuit is a voltage divider  with a 150Kohm resistor and a 1Kohm precision potentiomenter (trim pot) powered by a 9 Volt Battery. To add a little character to the board we added some capacitors, an On-Off switch, an LED, a 5V voltage regulator  and the famous LM 324 Op-Amp acting like a Buffer Amplifier.

Download ZIP file (PDFs, Schematics, Etching Traces)

Some History

To "ballpark" the circuit divider we used LTSpice to plot 1K Pot Vs. Volts. Fairly good slope and descent range.

A quick schematic of the circuit was drafted with Eagle to minimize some chaos on assembly.

The board was home etched. It was an easy board to make with one side. I used a jumper wire (blue trace) to ground the Op-Amp. Doing a top and bottom etch was just too much for this project.

Once completed the board was assembled with the components. Jumper wire is visible here.

 Testing an ALCOR EGT gauge. Even with a load on the circuit the Op-Amp keeps up with it fairly well.

Of course it needed a case, so we 3D printed one just for formality 

Download ZIP file (PDFs, Schematics, Etching Traces)

Gree CamSlide 1.1

About one year ago I designed my own camera slide. The goal was to be able to make a few time lapse videos and have a platform to shoot videos. While it worked, I was not happy with the results. The hardware was a little to bulky and there were a lot of parts. With the addition of a 3D printer to my arsenal I was able to unleash some creativity and bring some improvements to the design. So the version 2.0 is on the way!

My goal is to use the old software that I designed and programmed with an Arduino. Maybe in the future, I could add an LCD screen to the final project. (I just noticed that A4 and A5 on the Arudino are open, and and I2C can fit there). I proceeded to salvage the old PCB I  fabricated for the early project. 

The board is programmed with some presetting for time lapse photography and linear motion (video) at different time intervals and different linear speeds respectively. I ordered 3 extra boards. I will upload the BRD files and schematics once the revision is completed. 

Since I had some aluminum extrusions for the 3D printer project I decided to use a 2x2 inches square tube, 6 feet long for the slide platform.

2X2 Aluminum Extrusion (6 Feet)

Also I was able to use the custom pulleys fabricated in the lathe, one for each end.

Used some washers to set the height.

Original pulley. I will have to trim this one to maintain the height.

A sneak peek of the rail platform aluminum extrusion with the Urethane belt attached. Urethane belt is part number 6075K11 from McMaster-Carr.

Since I had the rollers from previous project I decided to put them to good use. I acquired them in the local hardware store. They were fairly cheap ( ~US$4.00) each. After some design for the platform I decided to use 3 of the rollers to take the weight of the camera and one roller to secure the camera from falling. The wheels are 1.25 inches in diameter and they measure 1 inch diameter from the lowest sport of the concave diameter.

 I tried to keep the design simple with some cool features to use the 3D printer to it's max capability.
The "S" slot secures the belt in place.

 The main cart is divided in 2 pieces. The vertical piece can be adjusted to keep the cart secure in the rail. Design of the cart is in my Thingiverse page

 Also had some fun designing the end cover of the extrusion to add a finished touch.

 I decided to print the support also. With a 3D printer it is just easier to print and watch the show.

The next goal is to design the cover box for the PCB. I had some fun with EagleCad and added the EagleUp extension to Sketchup. It works like a charm! With a 3D model I will be able to design the case/ box with a little more precision.