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s_tones 03-04-2013 05:42 PM

Test yee circuitry skills!

This is a dimmer circuit in my plane. Combo on-off switch/potentiometer controlling panel lighting circuit via a transistor. Makes sense except for the switched circuit (via 3-4) in yellow.
I can't figure out why it's there. The (3-4) switch appears to connect "1" to ground in conjunction with movement of the pot. I cannot see what use this has.

Any thoughts?


ecdamm 03-04-2013 06:10 PM

Without knowing what is connected to connector pin 1 it's impossible to know its purpose. It appears to be a normally closed switch providing connection to gnd with the pot in the off position. Once the pot is turned on the closure to gnd opens but it's not connected anywhere in this schematic.

s_tones 03-04-2013 06:22 PM

Thanks. Yep, that's what I thought. Oddly I can find zero info on what might be connected to "1". But it appears that it has no bearing on the function of the panel light circuit. I will have to look again for some reference to that unlabelled pin.


dspender 03-04-2013 07:08 PM

Is this information I need to know to complete my 10

rleffler 03-04-2013 07:25 PM


Originally Posted by dspender (Post 749801)
Is this information I need to know to complete my 10

Nope, there are much easier options available.

s_tones 03-04-2013 08:02 PM

that's for sure

Bill.Peyton 03-04-2013 08:08 PM

If I had to guess, this is a piper arrow dimmer circuit. The connection to ground pulls the landing gear lights to ground, applying full power and thus full brightness to the lights. When the dimmer is turned on, the aforementioned lights brightness level is controlled thought the bipolar dimmer. The other pot serves to control the avionics dimmer circuit, and also turns on the nav lights by connecting 1&2.
Just a guess.....

rocketbob 03-05-2013 08:21 AM

Yes it is a Piper diagram, landing gear wiring is marked with the letter L.

PeteP 03-05-2013 09:39 AM

Circuit quiz
Looks exactly like the dimmer circuit in my old Arrow. Had to replace those NPN transistor several times along with a string of grounding issues. When the pot is at minimium R you get full bright on the lights and when the pot is adjusted you modify the bias on the base of Q1 or Q2 dimming the lights.

s_tones 03-05-2013 12:00 PM

Very good! Actually piper archer II.
There is no landing gear though so maybe that switch is just hanging out, unused? Odd that it's wired up at all though. Maybe they just setup the same switch and wiring and then use it on several planes in the series.

In my case it appears the transistor is OK (replaced) and the pot is shot.


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