LED/Small Motor Tester/Evaluator

I've found no reason to have any incandescent lamps on a layout anymore. LEDs are cheap and prevalent. The only challenge (and not much of one) is knowing how to power them, as in what voltage source and what value load resistor. Yes, it's trivial to calculate based on the LED specs, but by and large today's LEDs are much more brilliant than they need to be for most modeling applications. So, what do you do—guess?

No, you break out the handy-dandy LED/Small Motor Tester/Evaluator. This little gizmo lets you simply dial up the correct settings for just the right brightness you need. With 7 voltages and 22 resistor values, you can be precise. As the name implies, it's also perfect to determine the right supply for small motors, such as pager motors that might be used in animation applications.

The secret to the device is an eight-cell AA battery holder modified with multiple taps, three 12-position rotary switches, 22 1/4-watt resistors, and a pair of clip leads. Although most any non-shorting switches would work, I used miniature ones to keep the finished unit as compact as possible. Also, the resistors are all standard values, versus creating a more typical "decade box" with values that would need to be approximated.

Using the device is simplicity itself. I first choose the voltage that matches the supply I intend to use, and set the resistance to the highest value. After connecting the LED, I dial the resistance values downward until I achieve the desired brightness. The dial setting shows me the resistor to use. That's it! There's also a zero ohm setting for evaluating small motors.

Here's the wiring diagram.

I've never been able to memorize resistance value color codes, so I printed a chart on self-adhesive label stock and applied it to the back of the unit so I can quickly determine what resistors to use.

Since LEDs draw trivial amounts of current, the batteries should last quite a long time. I haven't had to change mine since I built the thing about eight years ago.

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