Toga switch button?

[rapid_ascent]

You can just use 2 diodes. Tie the two cathodes together and connect the cathodes through the switch to ground. Each anode then goes to one of the inputs. They will independently be normally pulled up unless the switch is connected to ground. The two inputs will be isolated from each other with the reversed biased diode. Schottky diodes may be needed to get the lowest low state voltage. I?m not sure of the exact thresholds. You can look in the equipment manual. It?s a Better solution than using a mechanical relay that may wear out.

[Flyhud]

You could put it in your throttle lever, Paul Swearingen at 3D stick and rudder has some really nice products made specifically for this.

https://www.3dstickandrudder.com/

[Nihon_Ni]

I talked about it at length with the Garmin rep at OSH. He said that since both inputs are just ground, that I could combine them into a SPST switch without impacting either component. I haven't designed the solution yet, but I'll probably just use a single relay.

Thanks for reminding me that I have one more job to do!

[rbibb]

I?d go with the diode solution over relays. Suspect relay closure very noisy and would potentially create spikes on opening and closing. How much and whether it would cause a problem for the inputs on the avionics input is pure speculation. Pulling a diode to ground would be much cleaner electrically with faster transition time, etc. That said the fact you can simple connect those inputs to a mechanical switch per the manufacturers recommendations suggests the inputs are pretty robust intolerance of a momentary amount of noise on the input line. They are, as previously suggested, just detecting a threshold change. Normal ope state is ?pulled up? to some voltage level (5 volts?) and when pulled low to ground the internal logic circuitry detects this change of state when the input goes below a certain threshold voltage (what level is truly ground anyway?).

The diodes provide isolation of one input from another so that if one piece of avionics is powered down it won?t have a chance to affect the other box which, the Garmin rep alludes was possible as found during their internal testing. Easy to whip up a temporary circuit with a push button switch, two diodes, and some alligator clips. Connect the boxes and test the function. Diodes could likely be easily installed inline in the permanent wiring to the switch in question soldered in place and covered with shrink wrap for protection and mechanical stabilization.

Solid state diode good. Mechanical relay bad.

My two cents worth what you paid for it.

[Nihon_Ni]

Thanks Richard. I'll chat with you to get some specifics on what diodes to use and have you review my soon-to-be drafted schematic if you don't mind.

[g3xpert]

Hello,

We also agree that you can use diodes for input isolation in combination with a SPST button in lieu of using a DPST button.



To test your diode isolation, place your GTN (for example) on the discrete input test page and turn off your GEA 24 (for example). If doing so does not toggle the Go Around discrete on the GTN, your isolation is good.

You can repeat the test by powering off the GTN while viewing the AFCS TO/GA discrete input on the GEA 24.

Thanks,
Steve