Two questions about panel lights at night/electrical engineers help?

[AndreiC]

These questions are pretty technical, and I hope someone here is more knowledgeable than me in electronics. My plane is a 1970 M20E, and I attach the schematic of the relevant dimmer circuit.

1) A while ago I replaced the lenses for the 6 indicator lights (gear up/down, hi voltage, vacuum hi/low, boost air) with nicer, new lenses. After this the lights were a bit dim, and the guy making the lenses recommended to me to replace the filament light bulbs with modern LEDs. I did this, and all was great. However, a couple of days ago I flew at night, and noticed to my surprise that even though the gear was up, both the "Gear UP" and "Gear DOWN" lights were on. The green "Gear DOWN" light was less bright, but certainly still clearly on. When I shone a light in the light sensor next to the dimmer switch only the Gear UP light stayed on. My guess is that there is a small residual current going to the Gear UP light, even though the gear is up, and only when the photoresistor is not illuminated. The original incandescent light bulb needed much more power to light up, so it was not visible, but the LED is more sensitive and senses this small current. However, I do not understand how this could be happening based on the circuitry. 

2) My second question is more standard. I guess the display on my KX155 is getting old; on this last flight, when it was pitch dark, the display was mostly unreadable. I understand that the issue is that the dimming circuit lowers the voltage to the display to a level where the older display cannot light up. I read that there is a relatively easy adjustment to up the voltage slightly to the display, so that even at its lowest setting it will be readable. Does anyone know how to do this? Or have a link to a tutorial?

Thanks!

[1967 427]

Retired ME but during my career I would say that I ended up getting a minor in EE.  As you stated the LED’s required millivolts to operate and after looking at the circuit, I highly suspect electrical leakage in the switch itself.  I would hit the switch with contact cleaner in an attempt to wash out the dust and dirt that maybe conducting enough current to lightly illuminate it.  Again just my first guess, and no help with the KX155.

[1967 427]

Forgot one more thing, GO Green Bay!

[MikeOH]

@AndreiC

Were you using Ram air?  If so, I suspect that you installed NON-polarized LEDs.  That is consistent with your symptoms if everything else is ok (e.g. no shorted diodes)

[AndreiC]

Just now, MikeOH said:

@AndreiC

Were you using Ram air?  If so, I suspect that you installed NON-polarized LEDs.  That is consistent with your symptoms if everything else is ok (e.g. no shorted diodes)

Yes, I *think* I was using Ram Air. I also suspect the LED’s used were non polarized. Do you know of a source for polarized ones?

[AndreiC]

@MikeOH But I still don’t understand how the power gets to the gear down circuit. If I understand the logic correctly, when the gear is up, no power comes from the gear up limit switch (U). Then the power boost line (P) is not energized, so even though the power boost is on, the return line PP is also not energized. 

This may work if, as @1967 427 said, there is some residual current coming from the gear down limit switch. 

[MikeOH]

34 minutes ago, AndreiC said:

Yes, I *think* I was using Ram Air. I also suspect the LED’s used were non polarized. Do you know of a source for polarized ones?

Try Mouser, or DigiKey

[ArtVandelay]

Hmm, never heard of a variable ground.

The transistors seem like an unnecessary complication.

The J models have a circuit that comes into play when navigation lights are on, used to dim the landing gear annunciator light, don’t know if older models have that.

[MikeOH]

7 minutes ago, AndreiC said:

@MikeOH But I still don’t understand how the power gets to the gear down circuit. If I understand the logic correctly, when the gear is up, no power comes from the gear up limit switch (U). Then the power boost line (P) is not energized, so even though the power boost is on, the return line PP is also not energized. 

This may work if, as @1967 427 said, there is some residual current coming from the gear down limit switch. 

If the LEDs are non-polarized there is a sneak path that exists through the Ram air switch. This will only happen if the 'variable ground' is actually above ground; this will be the case in 'dim' conditions.  Once you shine light on the detector the 'variable ground' voltage will drop too low to provide sufficient current.

[MikeOH]

3 minutes ago, ArtVandelay said:

Hmm, never heard of a variable ground.

The transistors seem like an unnecessary complication.

The J models have a circuit that comes into play when navigation lights are on, used to dim the landing gear annunciator light, don’t know if older models have that.

That's the methodology used to dim the lights.

Use heat sunk transistors or GIANT rheostats...I think the transistors were the better choice and not a 'complication'

[MikeOH]

@AndreiC

Before you buy new polarized LEDs, try turning the Ram air off; if my theory is correct the Gear Down light should go out.

[MikeOH]

3 hours ago, 1967 427 said:

Retired ME but during my career I would say that I ended up getting a minor in EE.  As you stated the LED’s required millivolts to operate and after looking at the circuit, I highly suspect electrical leakage in the switch itself.  I would hit the switch with contact cleaner in an attempt to wash out the dust and dirt that maybe conducting enough current to lightly illuminate it.  Again just my first guess, and no help with the KX155.

That was my first thought, too.  Thing is, when he shone the light on the sensor the Gear Down light should have become brighter, but it went out!

[AndreiC]

On 11/28/2024 at 8:14 PM, MikeOH said:

That was my first thought, too.  Thing is, when he shone the light on the sensor the Gear Down light should have become brighter, but it went out!

I tested this yesterday. The Power Boost makes no difference — the behavior is exactly the same with it open or closed. Any further ideas?

[MikeOH]

20 minutes ago, AndreiC said:

I tested this yesterday. The Power Boost makes no difference — the behavior is exactly the same with it open or closed. Any further ideas?

UGH!

I'll give it some more thought!  I agree with @1967 427 that there is leakage current, may well be through the down limit switch as he said.  I just can't reconcile that with your 'light shine' in the sensor causing the Down light to go out.

My troubleshooting is based on the circuit being 'as shown' in the schematic; may be there's a path other than leakage in the down limit switch.  I had an issue with my control box many years ago that turned out to be an internal short in the control box.  Different symptom, however.

[MikeOH]

@AndreiC

Ok, if D4 is shorted I can envision a scenario that would match your symptoms.  It also requires a leakage path through the down limit switch to exist to ground, not +14V.  If you can access and unplug the control box connector you can test D4 with an ohmmeter from pin U to pin V; should conduct in one direction and block when you reverse the leads.

At this point, if it were me, I'd start with cleaning the down limit switch!

You could also try an known polarized LED in the Gear Down light; that should also 'fix' the issue if my latest theory is valid

[1967 427]

I was originally going to state that it could be D3 or D4, but after looking at the circuit again, if D3 failed, D4 should keep the current from flowing back.