Magic Box

[DKS]

I finally got around to building something I'd been planning to do literally for years: what I call a "magic box," which is nothing more than a battery power supply/resistance substitution combination. It's simply a box full of batteries and resistors wired so I can select a supply voltage from 1.5 to 12 volts, and one of 23 resistance settings, from 0 to 3.3K. The purpose of the box is to allow me to determine the appropriate voltage and resistance combination to operate LEDs at the desired brightness, or to run animation devices at the desired speed. Calculating the voltage/resistance combination for LEDs is not always practical, since most LEDs operated at the specified current are too bright for modeling purposes. So, the box allows me to select the ideal supply and resistance for the best effect. It also lets me select the right supply for motorized animation devices.

Everything is crammed into one convenient, compact box I can carry anywhere.



Rotary switches select the voltage and resistance.



On the back of the box is a handy resistance color code guide, since I've never mastered the ability to "read" resistors directly.



The device is not rocket science, by any means. It's all just batteries (8 AAs), switches and resistors.



I wish I'd built it a long time ago, though, because it's a very handy device.

[davefoxx]

Is this little gem the self-contained power supply that you mentioned for your Trenton Transportation Company layout, which will allow you to run the layout at the CRHS Rail-B-Q?

DFF

[DKS]

Is this little gem the self-contained power supply that you mentioned for your Trenton Transportation Company layout, which will allow you to run the layout at the CRHS Rail-B-Q?

Nope. The layout has its own dedicated power supply/throttle/control panel all built into the layout. This box is for projects in development.

[C855B]

Very cool, and very useful. Integrating the stepped power source makes it quite handy. The practical aspect of using "bought" resistor values makes the job quick; however, I would hope that you used 5%- or even 1%-tolerance resistors so you have reliable references.

"Decade boxes" are a staple on the electronic tech's workbench.

[DKS]

They are 5% resistors. And I also have a decade box, which I've rarely used.

[Bob Bufkin]

My knowledge of electronics/electricity consists of an on/off switch. 

You should do an article on this build and submit it to one of the model rr mags. 

[C855B]

...I've never mastered the ability to "read" resistors directly.

Since I've been tinkering less and less through the years and rarely read components these days, I rely on a bit of a crutch: "2" starts a rainbow. Then black = 0 makes sense, so I only have to recall brown, gray and white (and red). The third band as a 10X multiplier just has to be remembered, I guess.

Also, the color code has become less important (for me, at least) since what little I've done in the past 10 years has been with surface-mount components, which are labelled with plain numbers. Granted, values like "221" can be slightly confusing since, like the color-stripe system, the third digit is a 10X, so it's 220Ω, not 221Ω.

Don't feel bad, however. In the olden days the color system extended to capacitors (mica type) and inductors. I never could grok either, but especially inductors, which had four or five stripes, where in the four-stripe version the fourth stripe wasn't silver or gold. Made my head explode.

[DKS]

You should do an article on this build and submit it to one of the model rr mags.

I suspect the type of person who might find this project appealing would be able to build it right from what I've already posted. And my days of submitting articles for print publishing are long past, sorry to say.

[peteski]

Very handy gadget DKS!

I have similar item bread-boarded (one of my long overdue projects).  Mine works on a slightly different principle, but it is for the same purpose (for determining LED brightness and for figuring out the value of the current limiting resistor).   Mine uses a variable constant-current source and it displays the forward voltage of the LED running under that current.  The rotary switch in mine will select current (from 0.5mA to 20mA).  I often hook up multiple LEDs in series, so to me the Vf at the desired brightness (current) is more desirable than directly knowing the resistor value.

[Philip H]

Neat box. Any chance it will accompany you to Newport this weekend?

[DKS]

I often hook up multiple LEDs in series, so to me the Vf at the desired brightness (current) is more desirable than directly knowing the resistor value.

Seriously? I do the same thing with this box (power a series of LEDs), but since the box "self-determines" the current required to drive the LEDs by virtue of the voltage and resistance I select, I literally don't need to figure out anything: when the LEDs are the right brightness, it tells me what I need. While I understand all of the principles underlying the process of powering LEDs (or whatever), sometimes the empirical approach is a bit more straightforward. IMO, of course.

Neat box. Any chance it will accompany you to Newport this weekend?

It certainly can. Lots of room in the back of the Subi.

[peteski]

DKS,  I wasn't saying that your way is harder to do or incorrect. It isn't. Both devices will provide the info we are seeking - just using different methods.

[John]

Since I've been tinkering less and less through the years and rarely read components these days, I rely on a bit of a crutch:

Me too .. 

[robert3985]

DKS.  Clever as Hell.  Love the idea of an easy way to look at the brightness of my chosen LED or the speed of a motor used for animation and objectively see what I need to have to duplicate either the speed or brightness I think looks best in a modeling environment (a totally subjective judgement).  And easy as Hell to build too.