Interfacing an Exhaust Gas Temperature Gauge

[Mike.Powell]

This project is an offshoot of the Tachometer interfacing project covered here: http://www.mycockpit.org/forums/show...727-eng-gauges

In the original thread Rob (aka 737NUT) wanted to interface a real aircraft tachometer using an OpenCockpits RC servo control board. We realized this desire by developing an adapter based on a $2.25 microcontroller, the Microchip PIC16F648A. The PIC measures the pulse width of the RC servo control pulse, does a little arithmetic, and generates a square wave output with a frequency somewhere between 0 and 77 Hertz, depending on the pulse width.

Rob also has an EGT (exhaust gas temperature) gauge that he’d also like to interface. He did some testing and determined that the gauge requires 40 millivolts at a negligible current to display full scale. He’d also like to use the OC RC servo card as the basis of the interface. Because the EGT gauge takes a different signal, we need a different adapter.

Because of the flexibility of microcontrollers, we don’t have to start this project with a blank sheet of paper (empty file?). We need only modify the tachometer adapter.

From the PIC firmware perspective we can keep the portion that measures the RC servo pulse width and replace the variable frequency square wave generator with a pulse width modulated output. Pulse width modulation varies the percentage of time that an output is high. The average voltage of the output varies in proportion to the high-time. Since the PIC is powered by 5 volts, an output that is high 50% of the time will have an average voltage of 2.5 volts. If it’s high 20% of the time, it will have an average of 1 volt.

From the hardware perspective we simply replace the output transistor used to drive the tachometer with a few resistors and capacitors. This scales the maximum PIC output of 5 volts to the maximum of 40 millivolts required by the EGT gauge. The capacitors provide filtering so the gauge sees the average voltage value rather than the pulsating pulse width modulated output the PIC generates.

Here’s the schematic of the EGT adapter:



And here’s what a circuit board might look like. This actually combines two tachometer adapters and a single EGT adapter.



These two projects are good examples of why I really, really like microcontrollers. You can do neat things without having to dig particularly deep into your wallet. The cost of electronic components is only several dollars for an adapter. The development tool for writing the firmware (MPLAB) is a free download from Microchip. The device programmer for loading the firmware into the microcontroller is a $50 “PICkit 2” also from Microchip. (Buying the PICkit also gets you some tutorial material and a little practice daughter board with a microcontroller on it). Microchip updates the software of the PICkit and MPLAB for free. The printed circuit board artwork was done using free software from ExpressPCB. Three 2.5” by 3.8” ExpressPCB “MiniBoards” cost $51 plus tax and shipping.

[737NUT]

To add to this, i am new myself to using PIC's so i will post a thread soon telling you all how an average sim guy can learn to use these powerful little chips! I have no programming expierence so this will be a learning and fun process. Stay tuned!

[Mike.Powell]

The firmware for the PIC-based EGT adapter is complete and functions in a proto board test set up.

I've sent a programmed PIC to Rob who will run it through its paces with an OC servo control card and a real EGT gauge.

[737NUT]

I will post back with an update with pics and hopfully a video. I also will try and take the gauge apart to show the inside and how i wired it up. Thanks Mike!

[737NUT]

I have about 20 hours now on the EGT gauge fully integrated and functioning perfect with FSX. It is really cool to see this project come together and now working. Can't wait to see all 3 engines with all the gauges working together! Should be done in about 2 weeks or so. Will post up some pics.

[mlscotti]

How did the gauge work out? I want to make either a digital, or led driven EGT gauge. What interface did you use, fsuipc? If so what offset? Are u using PM systems or flight deck?

[737NUT]

How did the gauge work out? I want to make either a digital, or led driven EGT gauge. What interface did you use, fsuipc? If so what offset? Are u using PM systems or flight deck?

EGT gauges work off of the same principle of a thermocouple and mV and uA. What Mike did was take the PWM output from the servo card and convert it to 0 to 5Vdc output scaled down through a series of resistors and caps. It would drive any EGT gauge that is built to mil-spec. I used the EGT offset in FSX thru fsuipc.

Rob

[mlscotti]

Sounds good, but i'm using all LED'in a sequence, what fsuipc offset did you use, I found each engine has a get but not one for the main apu temp, essentially the one that has to stabilize prior to starting the apu gen.

[frazer]

I managed to get an EGT instrument from a fighter aircraft to run directly off the OC servo card by using the signal output only. It supplies low voltage, which was in the range of the EGT. But I guess that depends on the specifications of the gauge.