MGL Avionics User Forum

I would like to use the O5 pin on the Analog/Digital Inputs connector of my iBox to create a Master Caution button, as alluded to in the manual.

Perhaps its the sentence in the manual that says "This output is a open collector darlington transistor output that is switched to ground whenever an alarm is active and not acknowledged by the pilot" that is throwing me off. Maybe your average iEFIS owner immediately knows exactly what an open collector darlington transistor output is but I certainly don't. Not even Deepseek could explain that one to me.

What would be great is a join-the-dots explanation aimed at cretins like me, which details what type of switch (possibly a momentary illuminated pushbutton?) to use and how it should be wired. Anyone care to 'illuminate' me - please?

I take it you mean a master caution INDICATOR- a light or buzzer to let you know of a caution or alarm condition?
If so, you can use the output to drive one.
An open collector output is just a switch that is open circuit (no connection to anything) when OFF, and connects to ground when ON.
All you need to do is connect your indicator between +12V and the output.
When the output is OFF- there is no circuit to ground, so no current flow in the indicator bulb/buzzer..
When the output is ON- there is a circuit to ground, current flows through the indicator and it is energised.

Thanks Stuart,

I suspected that it might be simpler than the terminology suggests.

Yes I do mean an indicator.

I was hoping to use an illuminated pushbutton switch to indicate the caution, that I can push to acknowledge it. I can certainly do that by using the switching function of a pushbutton switch to interrupt the switch’s light circuit. However, that would need a latching button, which would need to be unlatched to restore the functionality - not ideal - sure to be overlooked.

If there isn’t a way of acknowledging the caution and turning it off with a momentary switch, then I suppose it would be better to leave it lighting until the alarm situation is resolved.

You could achieve what you want with an illuminated pushbutton switch- 2 parts to it, the indicator and the switch.
The indicator part (LED or lamp) would be connected as described above, controlled by the alarm output from the iBox.
The switch part would be connected to an input on the iBOX.
Assuming the switch closes to ground when pressed, you will need a resistor on the input pulling the input up to 12V when the switch is open.
The input would be configured as active low (button pressed) with its function set as alarm acknowledge.