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  1. #1
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    Interfacing real 727 eng gauges

    This is a continuation of a discussion that started in this thread.
    http://www.mycockpit.org/forums/show...5114#post95114


    I was thinking about breadboarding a simple 30 Hz square wave oscillator and connecting it to the tach input. You could scope the signal to see if the tach is loading the oscillator too heavily. Based on what we see we revise and retest.

    We can build an oscillator with parts like a LM555 or almost any general purpose opamps. Have any parts floating around?

    Mike,
    I should have enough parts to do it, if not i can get them locally.

    Rob

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    Re: Interfacing real 727 eng gauges

    Okay, I will cobble something together, verify it works, and post a schematic.

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    Re: Interfacing real 727 eng gauges

    Quote Originally Posted by Mike.Powell View Post
    Okay, I will cobble something together, verify it works, and post a schematic.
    Ahhh my favorite word, cobble! lol Thanks!

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    Re: Interfacing real 727 eng gauges


    This is a basic 30HZ oscillator that produces a 0 to 5 volt square wave.

    In the best of cases, connecting A to the tach input will cause the tach to display 2100 RPM (+-). If so, interfacing is really easy. If not, we'll try removing the DC component from the square wave.

    In any case, the idea is to connect the powered-up tach to the A output and observe the waveform on the tach input with an o-scope, noting the voltages of the high and low parts. Also note any waveform distortions that occur when the connection is made.

    Repeat this with the B output and the C output.

    These measurement will let us calculate the loading the tach presents to the signal.


    Edit: the unlabeled ground pin on the 555 is pin #1.

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    Re: Interfacing real 727 eng gauges

    Thanks Mike, ill report back my findings asap.

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    Re: Interfacing real 727 eng gauges

    Hi Mike,
    Got the gauges and some testing done. When i hook the gauge up to the output the 0-5v output from the oscillator drops from 2.9 down to 2.6V and freq is locked in at 24Hz. O-scope showed no change in shape, just the drop in voltage that was verified with the Fluke. Needle did not move at all on the gauge. Tried both gauges. I also got an EGT gauge so i will e-mail the company and see if i can get the pin-out but i'm sure it will be the same other than the input which should be standard mil-spec for thermo-couple

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    Re: Interfacing real 727 eng gauges

    Ah, data!

    Your measurements were done on the "A" terminal?

    How about "B" and "C"?

    Your voltage measurements were done with the Fluke? What settings?

    Do you know if the gauge itself is actually functional?


    If the gauge is functional, its lack of display activity implies that we need a signal that swings around zero volts. Once we have the "B" and "C" measurements we can calculate an input resistance and determine how to modify the oscillator.

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    Re: Interfacing real 727 eng gauges

    Mike,
    I got it working!! Sort of, bear with me here. I downloaded this free software, free tone generator.

    http://www.nch.com.au/tonegen/index....FQjyDAoddhBIOQ

    I opened up a set of usb powered speakers, cut the lead off one of the speakers and ran it to the gauge. It worked! 70Hz put 99.9%on the gauge, i can change it 1% increments and the gauge responds everytime. It worked only in a sq wave pattern and it worked best at a setting of 1000ms and above pulse length. My O-scope to a crap on me so maybe if you DL that software and hook your o-scope to a pc speaker like i did it might help you. I did get these readings with my fluke,
    at 70Hz sound card output, the speaker leads were showing 163mV ac and 350Hz at
    17Hz output we had 81mV and 89Hz on the fluke. The gauge is working perfect right now so i hope this helps you? Let me know what else i can do.

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    Re: Interfacing real 727 eng gauges

    Let me add a little more, the software has the capability to output a sine, Sq, sawtooth, triangle wave. You can adj the freq. and the volume has to be all the way up.

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    Re: Interfacing real 727 eng gauges

    I have both the N1 and N2 working with the speaker output from the PC and the audio generator. I sent out e-maills requesting info on the TGT gauge. It has the same amount of pins and i am going to assume it will be the same pin-out just needing a different input signal. These gauges are SWEET! Analog and digital mix is cool!

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