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Using OpAmps….

Started by AAnnillo, May 22, 2022, 08:44:36 AM

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Good day, all.

Can someone please provide direction/information on the use of opamps?

I'm attempting to operate glide slope needles, localizer needles and course deviation needles on my OEM FDIs and HSIs and understand that these needles are driven via DC voltage. 
I have read about the use of these opamps to drive the needles but am still unclear how to wire them up, let alone what type/kind of opamps I need to purchase.

I'm using X-Plane 11 with the interfacing via PJRC Teensy boards.

Any help is truly appreciated.

Thank you in advance.



May 22, 2022, 06:24:59 PM #1 Last Edit: May 22, 2022, 06:26:41 PM by XOrionFE
Rob Archer and Jorg Bliesener made an "outputs" board with opamps for driving these.   You could try reaching out to Rob.   Send him a PM.   He is 727737Nut on this forum.  I used their board for my -300 build.



I thank you very much for the suggestion.



May 28, 2022, 06:16:51 AM #3 Last Edit: May 28, 2022, 06:42:08 AM by RayS
You can drive those needles directly from the Teensy. No need for op amps, which require a symmetrical power source.

For simple needles like a fuel gauge, connect a resistor in series (start with 10k or use a 10k trim pot.)

Then connect the needle to any PWM channel and drive it that way, playing with the values sent to that channel.

For needles that are centered and deflect left or right, use 2 PWM channels, making one channel a mirror image of the other so when one channel goes low the other goes high.

With a 10k resistor or trim pot, connect the circuit to each PWM channel.

I can send you some code tonight if you want.
Ray Sotkiewicz


May 28, 2022, 04:25:29 PM #4 Last Edit: May 28, 2022, 04:39:12 PM by XOrionFE
Hi Ray

The needles he is referring to (glidesope, cdi, etc) in the HSI's and ADI's cannot be driven by simple pwm on the teensy.  They do require opamps.  They are also extremely sensitive. As I understand they are not driven by varying current but by varying micro voltages. Rob could explain better than I but I had to do this with mine as well and they built a special board for this purpose with Jorg Bleisener helping design and do custom coding. Board is pictured below.

Best regards,


I wonder if it has something to do with the range of values coming out of the sim....

I have an old N1 engine gauge out of a 737 that I'm driving with PWM and the N1 sim values are rather large (Or rather the distance between min/max is significant)

Trying to map that large range to the 0-255 PWM channel made the gauge jumpy and inaccurate.
Ray Sotkiewicz

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