Need help/advice to add a trim pot to a “roll ur own” led lighting module

The issue with adding a trim pot that will handle the current, let's guess about 40 ma, is that it will have to dissipate around a quarter to a half watt. Maybe around 500 ohms and reduce your fixed resistor to maybe 270. A potentiometer that size can be as big as what you have there. The advantage of using a regulator is that you can condense that size down to something like a fly spec and the regulator to a thin slab as the GRJ LED current regulator does. Often it just costs more to do it for the reasons stated. I looked on DigiKey and they cost around $3 each. Since the GRJ regulators are available for about $10 each, they are pretty competitive since they do the job so efficiently.

You could investigate the possibility of using a regulator chip and a tiny trimmer like the GRJ regulator. I think the instructions are in a thread on the forum, and I would look for it, but can't until later in the evening.

I do sympathize with your desire to do the building yourself. It's fun.

We could play a game of 20-questions to narrow it down to an "exact" solution, but life is short.  So with no additional information, here's my 2 cents (no refunds):

If you only need to reduce brightness relative to the existing design, then get a 1000 Ohm (1K Ohm) trimpot and insert it between the board and the LED strip.  In other words splice the trimpot into the D+ wire from the board.  The trimpot will have 3-terminals.  You will connect 2 of the 3 terminals.  Use the center terminal and one of the other terminals.  Which "other" terminal to use will determine whether you turn the trimpot CW or CCW to increase brightness.

If you need to increase and decrease brightness relative to the existing design, then in addition to the 1000 ohm trimpot per above, you need to change the on-board 470 resistor to, say, 220 ohm (which will about double the max brightness).

Trimpots are less than a nickel a piece on eBay (free shipping from Asia):

You could do a re-layout of the board and add the pot.

Wouldn't an adjustable voltage regulator chip like an LM317 be the simplest way to do this?

You mean something like this?  This is a constant current design so you don't have to worry about voltage, just set the pot for the intensity you want.

stan2004 posted:

 

Trimpots are less than a nickel a piece on eBay (free shipping from Asia):

I can't see the power capability on the ebay site...maybe reference the part number to Digikey or Mouser. Most of this size is about .15 watts or so. Adjustment of a regulator chip is a better idea.

Thanks guys for the help and discussion.  

Stan, I appreciate your sourcing the pot and help with explaining the necessary resistor change for up/down adjustment. I have some 220ohm resistors on hand for that option, and the explanation was excellent on where to install the pot.  

John, I sourced the board from Osh Park, it’s not my design, so I can’t do a layout change even if I knew how.  It would be convenient, to have the board include the holes for a pot.  I noticed a couple boards that you had there including the module shown above, but was flummoxed with looking up the parts listing necessary to assemble a working board.  Too many options for each part for me to handle.   🤔🤪

Thanks again for helping me expand the horizon a bit.  This small job is just one of the fun parts of the hobby.  

Ahh.  If this was not "your" design to begin with, then some consideration to power-handling (heat) is in order.

What cjack and others are referring to is the Wattage capability of whatever is dropping the higher track voltage to the lower LED voltage.  In your case it's the 470 Ohm resistor plus the added trimpot.  In GRJ's board it's the LM317 regulator IC chip with the metal tab (heatsink).

I figured you had a "working" design with power considerations already considered.  That is, your list shows a 1/4 Watt power rating for the 470 Ohm resistor.  Someone presumably chose that Wattage rating with some consideration.  Adding the trimpot to reduce brightness can only reduce power so not much to worry about in terms of power ratings.

But if you want to additionally increase brightness by lowering the 470 to 220 (or whatever), then you really need to run some numbers.  This is going to be a multi-step process to narrow into a design.  First, simply solder a 2nd 470 ohm resistor in parallel with one on the board.  This effectively creates a 470/2= 235 Ohm resistor that is now 1/2 Watt (instead of 1/4 Watt).

Now power up the board and confirm that the increased brightness is what you're after for max brightness. 

Then we can drill down into choosing a suitable trimpot wired up as suggested earlier...or whether you should re-group and consider using the GRJ method with an LM317 (which are about 20 cents or so).  In the latter case you still use a trimpot but when wired up for use with a LM317, the power demand on the trimpot is much less so a less expensive version (like the nickel type) can be used.

If you want one like this, just drop the attached Gerber files on OSH Park's PCB order form and you'll have it.  It uses very generic parts that are available cheap.  The lone resistor is changed in value to determine the current output.  It includes the choke for DCS compatibility as well.  You can probably do several hundred milliamps if you add a heatsink to the LM317, without it, I'd limit it to around 50ma.

Constant Current LED Driver Thru-Hole Gerber.zip

Schematic

Bill of Materials

I suppose I could have added the pot, but then it would be larger, and also nobody would have a reason to buy my lighting regulators.

John, very generous of you to add the file to upload to Osh Park.  I have purchased a half dozen of your kits in the past, which has led me to follow this and other forums where other modules were developed and made available.  I’m an end user only.  Thx.  BTW, your LM317T in the mockup photo does have the heatsink, right?

I don’t know Stan, we’re startin to talk real money now.  Before you know it, I’ll have a buck n a quarter in these things.   

EDIT:  I’ll start wiring up the modules to test per your instructions.  Other irons in the fire right now.

Ha...don’t forget your time and theirs 😀

cjack posted:

Ha...don’t forget your time and theirs 😀

My time?  Depending on who you talk to, not worth a pack of 50 1A diodes. 😏. Their time... priceless.  

Is there a way to tell the part number of D1 by looking at this?

Yeah, I too looked up the OSHPARK design and the description doesn't give an itemized part list like some of the other projects.

I'm sure GRJ will respond, but based on the hole spacing of the LM317 (U1) I'd say it's a "generic" DIP through-hole bridge rectifier (0.2" x 0.3" hole pattern) such as the wallet-busting 10 cent (free shipping from Asia) DB107:

And getting back to the original question, you can simply replace the 1-cent fixed-resistor with a 1-cent fixed-resistor plus a 5-cent trimpot to get brightness adjustment.  If this is something you would like to pursue we can drill down to specific values of the fixed resistor and the trimpot to meet your specific application(s).

I updated the schematic and added a BOM, that should show you representative parts.

BWRR posted:

Is there a way to tell the part number of D1 by looking at this?

A life time supply:

https://m.ebay.com/itm/100PCS-...6:g:9r8AAOSwznNbC8vm

LOL.  Now we're down to 5 cents for D1.  How loooow can you gooooo!  

Of course the single-diode method used in the original board is a whopping 1 cent!

I like the bridge for this task, it doesn't load the AC waveform unevenly.

So Stan, is this the proper parallel hookup of the additional 470 Ohm resistor for the experiment?  If so, I can show the differences in lighting.

Yes.  So rather than a 470 Ohm, 1/4 Watt resistor you now have a 235 Ohm 1/2 Watt resistor.

The strips are all 24 leds.  The control is GRJ’s module at full power on the left.  Center is the original board at 470 Ohm 1/4 watt.  The right is the mod at 235 Ohm, 1/2 watt.  CW80 for power.

Next is the control module(GRJ) at low power for comparison.

My opinion is not much difference in lighting in all three.  Perhaps slightly less bright in the center strip.   What’s yours?  And, to continue I think the brightness on the original module is sufficient at that level for my taste; I would probably go down from there.  But I like my lights down anyway with no glare.  My Menards buildings at 4.5vdc are too bright, so I power them at 3vdc.  Would welcome the opinions of others.  If somethng else needs to be tried, that’s fine too.  Let me know.

FWIW, I've always found my adjustable module to have plenty of range for any passenger car installation I've done.  I've never wanted the lights dimmer than the low setting, nor brighter than the high setting.  That's actually why I picked those ranges for the limits when I originally designed it.

gunrunnerjohn posted:

FWIW, I've always found my adjustable module to have plenty of range for any passenger car installation I've done.  I've never wanted the lights dimmer than the low setting, nor brighter than the high setting.  That's actually why I picked those ranges for the limits when I originally designed it.

Exactly, which is born out in the photos above, and, why I hooked it up as a control.  Did you think the brightness was similar in the three examples?  I will use these little modules inside some lemax building that I’m converting to a small strip of 3 leds. I anticipate wanting to dim those lights from normal.  Further, I have some two story buildings where I want to adjust the lighting in three or four locations.

It appeared to me that the GRJ strip has a greater range. A lot depends, however, on the angle when viewing LEDs.

cjack posted:

It appeared to me that the GRJ strip has a greater range. A lot depends, however, on the angle when viewing LEDs.

Yes, the other two boards are fixed, because at this point, they don’t have a pot.  Adding a pot is the goal, or if necessary a regulator and a pot.  But like you previously stated, at that point a commercial board may make more sense.

FWIW, it's very difficult in photos to capture relative brightness, seeing the results in person is the only accurate way to verify the results, at least IMO.

Allow me to digress.  There was a period of many years after fixed-track-voltage command-control but before practical/economical LED strip lights for passenger cars.  As I recall, the two common "solutions" to dimming bulbs were, 1) swap in higher voltage bulbs, or 2) insert a 10 cent diode to operate on 1/2 of AC track power.  I don't recall much if any discussion about a control (trimpot) to adjust brightness.  Trial and error.  

Fast forward to today.  So the question is do you really need after-the-fact adjustment...or is it a set-it-and-forget-it?  From what I can tell, very few (if any) guys mount GRJ's board in a way that you can access the brightness control trimpot through some hole in the chassis or whatever.  In other words you have to remove the shell which can be inconvenient.

To wit, I "thought" it would be important to have after-the-fact adjustability with one of those eBay regulator modules driving an LED strip.  So I mounted the module upside down with the blue trimpot screw accessible thru a hole drilled into the floor.

I can't count the number of times I've used this.  That's because the number of times has been zero, nada, zilch! 

As I see it, GRJ's module needs the brightness adjustment range because its meant to be a one-size-fits-all.  The range is almost 10:1 to let the end-user adjust to the specific application.

But in your case, if you like the tinkering and saving some bucks, why not choose the 1 cent resistor (brightness) that works for your specific passenger car and call it done...just like the good old days.  You probably even have a 1000 Ohm or 1K resistor lying around.  Try that.  Or, remove one leg of the 470 and insert a 2nd 470 "in series" which would make a 940 Ohm resistor.  This will roughly drop brightness in half.  

BTW, do you have a digital meter?  It doesn't have to be a fancy one - for  example the freebie (with coupon) at Harbor Freight would be fine.

I find a difference from type of car to car

Perhaps I have overstayed my welcome here.  I have re’cd a lot of help, with which I am grateful, and can probably go forward.  The passenger cars are not the only reason for the adjustment, so much as the the other applications I run into such as the variety of buildings, etc.  To Cjack’s point, the cars from one set to another can be different(see pic).  But, you’re right I can play around with various resistors, then re-install the wiring and modules until I get it to my liking.

So for your building lighting, what is your power source?  14-16V AC Accessory power from a train-transformer?

If you already have/use Menards buildings, then you know that they run on 4.5V (or less) DC.  And if you have some method of reducing DC voltage to 3V (an eBay 99 cent DC-to-DC adjustable module???), then it appears you already have DC available(?).

In which case why use the board under discussion?  If we're talking about layout DC lighting, you DON'T need the 22uH DCS inductor (which protects the track DCS signal), you DON'T need the diode (which converts AC-to-DC), and you DON'T need the large 330uF capacitor which relates to the AC to DC conversion and flicker reduction.

For layout lighting using LED strips (vs. AC-powered rolling stock), I suggest you're better off using DC to begin with.  You can get very inexpensive DC wall-warts and the like for about 10 cents per Watt or less.  Train transformers with their Accessory AC outputs will run you maybe $1 per Watt or more!  It's much cheaper/easier/efficient to manipulate one DC voltage to another DC voltage than it is to mess with AC.  Again, that's just my 2 cents!

 I just happen to finally get back to my lighting projects. GRJ, Stan, CJACK, etc had helped me before understand the car's circuit and where to go. I ordered a bunch of stuff and some of it failed or didn't work as intended. I had some cheap boards that blew when the volts went too high for example. I also ordered 220uh caps that proved to be too small for what I expected. I hate flickering lights! So I upped them to 330uh, and then again upped them for my G scale cars.

 Anyways today I tear into the fifth car of the 20th Century LTD set and went a new path. I had previously gutted all the electronics out of the car. This time I studied the stock circuit board and just removed a few of the components. I basically just left the wires, rectifier, and the cap. It saved me some work and the stock caps are 470uh! Duh. Why didn't I do this before? At least I remembered to turn the output down to like 8 volts so the cars don't look like alien space ships are landing on my rails. I do like having the pots to adjust the brightness!