Bruce,
The LED came in the mail yesterday. It was packed in a small clear hobby knife blade holder. There was also a letter from you, but no resistors. I don't know if you forgot to pack them or they somehow fell out of the envelope (the envelope did not appear to be damaged). But since I have no resistors, I can't tell you if they were correct for this application or not. If you still have them, you can let me know what their markings are to see if they were the correct value.
Anyway, back to the LED. It is a standard run-of-the-mill SMD (Surface Mount Device) 0402 size orangish-red LED. The leads are enameled magnet wire (which is often used as leads for SMD LEDs). I noticed that the wire's insulating enamel coating wasn't stripped well on the end of the leads. I wonder maybe if that was the reason you experienced strange problems when you were hooking the LED up to the batteries?
The first thing I did was to strip the enamel insulation from the wire ends. Looking back, I should have left it alone and tested it that way, but I just saw what looked like unstripped wire, and as almost a reflex reaction I stripped to make sure it was bare wire.
I strip the enamel by sticking the end of wire to be stripped in paste flux (non-acid paste of course), then I stick it in a ball of molten solder on the tip of my soldering iron. That burns off the enamel and nicely tins the bare copper wire. But (unless the wire uses a spacial low-temp enamel) the iron has to be quite hot to be able to burn off the enamel. I crank mine up to 800 degrees F (when I normally use 600-700 temperature for soldering).
I then went onto testing the LED. It worked fine. As I mentioned, it is a standard red-type of LED and it has a forward voltage of 1.6-1.9 (depending on the current passing through it). It is not a "3V LED".
The 0402 size LEDs are typically designed to operate at 5mA (but can handle short burst of higher current). That might explain why it survived being hooked up directly to 3V source of power (2 AA batteries in series) which would have resulted in excessive current passing through the LED. Maybe when you tested it, you just didn't keep it lit long enough for it to fry.
And no, I didn't try powering it directly from 3V without a resistor.
Next, I took one of my Rapido passenger cars I bought couple of years ago. The batteries included with it were never installed, so they were fresh (but old). I took the car apart and installed the batteries. I measured their voltage with the lights off and it was 3.04V (or 1.52V per battery). When I turned on the lights the voltage dropped to 2.87V and it continued slowly dropping further. After several minutes the voltage was down to 2.7V. If the lights are turned off for a while the batteries sort of rejuvenate and the voltage goes up, but you have to remember that these are tiny button cells, likely designed for watches and other circuits which only consume micro Amps of current.
The light board consumes 9mA (1.5mA per each white LED) with fresh batteries (supplying 3V) and as the battery voltage drops, the LED current will drop too (and so will their brightness).
Looking at the LR41 battery specs, their capacity is between 25-31mAh. That means theoretically they can supply 25mA of current for 1 hour before they are drained. But of course these tiny batteries aren't designed for such high current, and it would kill them much sooner.
Since the LEDs here consume 9mA, theoretically if the batteries were the low grade 25mAh ones, the battery would last for little bit over 2 hours. I couldn't find a spec which would give me the voltage the manufacturer considers as "discharged", so the battery longevity in this circuit is only an estimate.
If you find that you have to replace the batteries too often, try the LR41SW battery (the SW indicates a silver oxide battery (plain LR41 is an alkaline battery). The SW version has a capacity of 38-44mA (almost double of the alkaline), but they are also more expensive.
Next step was to add the red LED to the E-P (Easy Peasy) board. I calculated a resistor value which would supply the red LED with 2mA of current. The resistor value was 470 ohms. I hooked it up to the E-P board and it lit up nice red color and the white LEDs were also glowing brightly. When I connected and disconnected the red LEDs leads I could see a slight change in brightness of the white LEDs. But between the fact that the batteries were a bit old and also because these tiny batteries cannot provide a lot of current, I expected some change in brightness of the white LEDs when I hooked up the red LED.
I then tried using higher resistor value for the red LED, to reduce its current (and load on the circuit). After trying few values I decided that 680 ohm resistor still provides decent brightness of the red LED and its current was down to around 1.4mA. Connecting and disconnecting it from the circuit resulted in minimal change in brightness of the white LEDs.
Here is photo showing how I hooked it up.
And a closeup.
As you can see, to simplify the installation I installed the resistor directly on the E-P board by soldering one of its solder pads (near the "6" marking) to the positive voltage copper pad on the board. That provided electrical and mechanical connection, and eliminates a wire which would have to be used if the resistor was mounted off-board. Then the positive wire from the LED is soldered to the other pad of that resistor (near the "1" marking) . The negative wire from the LED is soldered to the negative output (on that resistor marked "101"). That's all that is needed for the LED to work.
For clarity, here is the photo of possible hookup points for the LED/resistor combo I posted earlier in the thread.
Here I show the red and white LEDs glowing brightly.
I stained the LED's negative wire using a black Sharpie to make it easier for you to identify. I'll return the LED along with the 680 ohm resistor shown in the photo back to you so you can install it on your E-P board. If you rather work with larger resistor with leads, I can also mail you one of those. But the install will not be as clean as the one using the small SMD resistor.
