Takling my first lighting project (AMT K-7 model)

[JMChladek]

Hi guys. The James Small K-7 lighting article on the Round 2 site has me inspired to tackle my first lighting project, the AMT reissue of the K-7 model. Rather then doing it exactly like the article though, with 7 super bright LEDs, I am adding a couple of warm white LEDs (for a total of 9) to illuminate the interior of the bottom mounted shuttle bay and also a segment on the main saucer (where the storage tank goes) for an external mounted drydock I am putting there (a Gamescience 1/3788 scale heavy cruiser under repair is going into the dock). When done, it will become Federation Starbase 17.

Anyway, I picked up all the supplies I think I need and I mocked up the circuit tonight with a breadboard to test the battery tray and using all the LEDs in parallel. With 9 LEDs in use instead of 7, I wanted to make sure the batteries could handle the additional milliamp load (20 mA per LED, meaning 1.8 amps instead of 1.4 for the total load). It seems to work as I left the circuit on for about 30 minutes. I detected no signs of overheating to the LEDs or the central wires.

Anyway, the reason I am posting this is although it seems to work, I want to make sure I don't end up with any unforseen problems. I am not using any resistors in this circuit as the power source is 2 AAA batteries (3 volts total, if anything slightly below the nominal operating voltage of the LEDs), just like what was used in the article. I figure I might need to beef up the main hub wiring though, since the full amp level will be pumping through those wires (given that parallel circuits are current dividers, not voltage dividers). Although it has been said that different LED colors have different resistance, I don't seem to notice any problems hooking warm white LEDs in the same circuit with super bright ones (all have the same voltage specs and seem to draw the same current).

So, am I kosher with this setup? I figure the slightly higher amp draw might drain the batteries quicker, but probably not by much. And it will have a power switch. Depending on how well this project works out, there are others I want to try.

[en'til Zog]

You might try 2 AA size battries for longer "run" time.

[Ziz]

http://www.flickr.com/photos/ziz62266/4733414049/

Radio Shack part @ 274-246. Rig that into your build (or the base) to use a power adapter.

[JMChladek]

Maybe on the next model, but not for this one. I am following the KISS principle here (You wanted the best, you got the best).

If this doesn't last too long with the AAA batteries, I'll go AA on the next build.

[en'til Zog]

Cool.

[Tchail]

Although it has been said that different LED colors have different resistance,

Check out the LEDs on this site:

http://www.uniqueleds.com

This should give you a good idea of the voltages of different colors of LEDs.

(Although your actual mileage may vary depending on the manufacturer's specifications...)

As long as your input current is below that of the maximum rated current for the LED, you actually don't need a resistor.

-Tchail

[Sparky]

Maybe on the next model, but not for this one. I am following the KISS principle here (You wanted the best, you got the best).

If this doesn't last too long with the AAA batteries, I'll go AA on the next build.

AAA have a very small current rating! If you are really drawing 1.8 Amps of power those batteries will not last. Even AA cant give you 1.8 of continuous power for very long.

If the LEDs will be flashing you can save a lot of power , duty cycle relieves the continuous load on the battery and extends run times significantly.

Typical 9V batteries have a 500mA capacity with a typical drain of 15mA. Go much above that and the battery will start getting hot.

On the flip side, AA's have a capacity of 2000mA and a drain of 50mA. I've found that 4, wired in series (which most battery packs are wired to) gives me MORE than enough juice.

I ran 2 4060's with 5 LED's on each along with a 555 flasher with an LED and 10 static LED's for over a week until the 4060's started acting up due to lack of current.

[JMChladek]

I'll set up the wiring so I can change out the power supply and stand if I need to then with a quick disconnect of some type (probably a JST style two prong RC plug as I can get those cheap and they are very small). As I said, I plan to keep this somewhat rudimentary and a learning experience. As such I figure it will have some minor issues. But if the batteries don't last, I am not too worried anyway as I like to build my models to look good without lighting. As such, I am not planning to have the thing on all the time. But, I figure if I take it to Wonderfest, I'll want to have it stay lit while it is on display (an optional spring loaded push button I suppose might work for that as opposed to an on/off switch). So I will likely build a new stand in the future and set it up with larger capacity batteries for that purpose. Thanks for the feedback.

Tonight I finished punching pretty much all the holes I need for the LEDs into the K-7 pieces. I have to make some minor decisions about the drydock mounting on the central saucer core, but all the ports for the cone and edge lights have been cut. If all goes well, in a couple days I can get the three outer pods wired up and light blocked. Then it will be just the matter of finishing the shuttle bay interior, the drydock and setting up the lighting for those assemblies.