Request to Review LED Push-to-test Circuit...

Would you guys mind looking at the attached circuit for providing push to test functionality to a handful of Annunciator LED's (12v) and let me know if it is okay I need some peer review... thanks.

http://www.mattandmel.com/pictures/PTT_Circuit.png

Depending on type of LED devices you use, if they don?t have built in current limiting resistors you will need to add them externally. Size varies according to LED voltage and current but 1k resistors should be ok. Also, the line coming in from the Aux Alt Relay should probably go to the LED rather than the 4001 diode, should probably be like the Starter Contactor circuit.

Other than the previously mentioned current limiting resistors on the LEDs (if not integrated into the indicators), I'd also add a diode across the relay coil.

Thanks Joe,

The Aux alt relay going to the diode was a mistake, it should look like the starter contactor circuit. Also, all LED's have internal current limiting resistors.

What is your opinion on using a relay? I know they are more likely to fail then the LED. I am just trying to avoid spending lots of money on an annunciator controller for a handful of LED lights that are directly switched from power or ground. (All other sensor based annunciators are controlled by my EFIS).

Also, do you think I should put diodes on the line coming in from the input or is it not necessary?

Thanks.

- Matt

Personally, I detest relays. You could use a DPDT momentary pushbutton or toggle switch and remove one more potential point of failure (plus a little wiring, time and expense).

If you used a DPST pushbutton switch you could avoid the relay altogether. Otherwise a good quality low current sealed unit should be fine. If it failed, you would probably have at least two of the annunciator LEDs fail the lamp test so you would probably suspect something out of the ordinary. The failure mode of the relay (unless really weird) should not provide a condition that would hide a failure in your annunciator.

I don’t think you need diodes in the input lines. [Correction - Yes, as pointed out by 'roee' you should have diodes on the input lines also!]

I like the DPDT Momentary button solution much better. I will remove the relay and add that. Is the symbol for a DPDT momentary button the same as for a DPDT momentary switch?

idleup said:

Is the symbol for a DPDT momentary button the same as for a DPDT momentary switch?

Click to expand...

There are different symbols for pushbutton and toggle switches, but the function is the same. The DPDT toggle is similar to the top part of the relay. I can dig up an example of a pusbutton if you need it. If you can't find the right one I wouldn't sweat it -- just note on the diagram that it's a pushbutton or toggle.

Now having said that with zero experience with aircraft wiring diagrams, I'll probably find out it's a huge no-no. I have had a little experience with schematics, just usually not automotive or aeronautical.

You really don't need a DPDT, a DPST will be fine, less expensive and easier to find.

Also...

To do this right, for each LED circuit you need a pair of diodes, not just one, to "diode OR" the push-to-test circuit in with the other driving circuit. Otherwise, you have some undesired current paths, which could have some interesting consequences depending on the characteristics of the driving circuits that aren't shown on this schematic. For example, as drawn, your push-to-test circuit will attempt to power the starter contactor. Not good.

Roee,

Duh. Good catch.

Curious if you considered something like this in your panel development.

LED Press to test indicator with dimming function

The relay, switch, diodes would all go away. The only wiring would be related to the circuit being monitored and a constant +12 for the internal test function.

idleup said:

Would you guys mind looking at the attached circuit for providing push to test functionality to a handful of Annunciator LED's (12v) and let me know if it is okay I need some peer review... thanks.

http://www.mattandmel.com/pictures/PTT_Circuit.png

Click to expand...

Hi Matt, I think the links for this are now working. This is the best solution for controlling cockpit lamps: http://store.makerplane.org/annunciator-controller-il-4a-kitset/

It's been around for 10 years and has been proven in many applications, including the FLS Microjet.

Reconsidering... Please advise...

Guys, I am considering just doing away with the whole push to test circuit for these LEDs and maybe some of the indicators altogether. I originally had identified 4 indicators in my power distribution planning related to what I thought would be good information to know. However, I am not so sure anymore, what do you guys think? My original design included the following:

E-Bus Alternate Feed On - I was planning to leave this on during normal flight operations so having a green light showing it was on would prevent me from leaving it on after shutdown (and running down the battery) and also ensuring that power was in fact flowing from the Bat Bus to the E-Bus as intended. After further analysis though, I realized it is not good to fly with an indicator light on and maybe I should change this to a yellow light showing the E-Bus Alt Feed is offline or remove it altogether?

Aux Alternator Offline - Yellow indicator showing that the Aux Alternator is offline. However, this is something I would see from the Ammeter monitoring the Aux Alt in my EFIS.

Main Alternator Failure - I was going to use the indicator output built into the Plane Power Alternator for this LED but again, the Ammeter attached to my EFIS is an indicator of this Alternators failure already.

Starter Engaged - I was going to simply connect the "I" terminal from my starter contactor to an indicator to see when the contactor was engaged (it would light during starting and if the contactor ever was stuck)


Anyway, are any of these even worth while to have?

- Matt

I'd go with the dual diode approach myself. I consider it simpler.

Use two diodes to create an logical OR function. So it's the push to test OR the application function. Then you don't need all of the switching stuff, relay etc. The control function can work independently.

So if the indicator would normally be turned on with +12 then tie the two cathodes (end with the line) together and connect to the anode of the LED. Apply your PTT signal to one anode (no line) and your control signal to the other anode (no line). If the indicator turns on with a ground connection you do the opposite and tie the two anodes (no line) together and apply your control signals to the cathode. If you have a mix I would use a switch that can create an active high(12V) and an active low(GND) PTT.