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Discussion Starter #1
I know that it's best to wire LEDs in series, but I have a special circumstance where - in N scale - it would be vastly easier for me to wire pairs of LEDs in parallel in the crossing signals I am building. In these alternating crossing flashers, the LEDs obviously are on only 50% of the time, and current will be limited by a series resistor to protect the LEDs and also to reduce the brightness. Modern LEDs are very bright, and I want to tone them down to be visible in ordinary light not obnoxiously bright like my test signal. I am using 3D printed targets and hoods so the LEDs are protected from direct light just like a prototype. I want a realistic brightness.

Given these circumstances - 50% on-off cycle, and a relatively large series resistor - can I get away with wiring the pairs of LEDs with 2 left side in parallel and 2 right side in parallel? How likely is it that one in a pair will hog the current and burn out? I certainly don't want to burn them out and have to do this again later. N scale signals are very tedious.

Has anyone done this?
Does anyone have an idea for a starting point for the series resistors?

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since LED current is very sensitivie to voltage, LEDs wired in parrallel are less likely to draw the same current and be equally bright. (there's no worry the one "hogs" the current). this may be less of a problem if the LEDS are from the same batch

i can't imagine why it is easier to wire them in parallel?
 

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Discussion Starter #5
Why parallel? For these signals, with 4 LEDs crammed back-to-back in a bi-directional N scale crossing flasher, space is tight, tight, tight. The easiest way to do it is to have the 4 anodes attached to the brass mast. That leaves two pairs of cathodes, and since the leads touch, the easy thing to do is solder them together with a wire to the flasher, leaving only two wires to deal with (L-) and (R-) with (+) for all four being the brass mast. I don't have a drawing on hand right now so the description will have to suffice.

I could use 4 SMDs with 8 separate wires attached, but they are much more difficult to use than four simple 1.8mm LEDs with leads, which I can do in parallel with only 2 wires as described above. I have designed 3D printed targets and hoods for the 1.8mm LEDs. Even with leads, the 1.8mm LED samples I ordered are tiny and hard to handle, but with a jig to hold them in pairs for soldering they are manageable. I ordered the same ones in a SMD package, and they are much more difficult to handle.

The LEDs are of fairly high quality, but I have no idea how much variation there is from one to another. I wired a pair in parallel to a 3V coin battery, and they seem to be equal brightness. My only concern is the life of the LEDs. As I said, I don't want to do this project twice.


It looks to me like NJI wires their four-LED crossbucks in parallel pairs, but without one in my hands, I can't say for sure.

Speaking of NJI, I ordered a pair of their N scale over-the-road cantilevers last year, and I returned them because the lights flashed backwards. Instead of left+left and right+right, they flashed inner+outer. I haven't ever seen a prototype flash like that. I decided to make my own cantilevers from brass instead. Very tedious with handmade stanchions and railings, etched walkways, and etched ladders. As I said the targets and hoods are 3D printed. I need to finish these complicated tiny little models before my eyes and hands get any less steady!

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Discussion Starter #6
These folks show LED pairs with parallel wiring for their flasher:


I'm using a different circuit, but that should't matter (I think).

LED flasher2.jpg

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Discussion Starter #8
I'm finally back. I have decided to use a simpler flasher circuit originally posted online by Rob Paisley. I want to combine it with the additional circuit that he posted to make LEDs turn on and off more slowly to resemble incandeascent lamps. A circuit diagram of the whole thing is attached.

According to an online circuit calculator, a 710k ohm resistor in place of his variable resistor will give a flash rate of 60.5 flashes per minute, which is what I want.

My thought is that I could eliminate the 820 ohm resistors and use R1, R2, and R3 instead. These three would give each LED its own resistor when it is on rather than simply wiring the three "on" LEDs in parallel as that practice seems to be discouraged. I have no idea what an appropriate value for R1, R2, and R3 would be.

The LEDs that I have tested are much brighter than I like. All are brand new, and all are identical. Having 1/3 of the current go through each "on" LED when wired as shown would help reduce the brightness, correct?

This is N scale, and wiring the LEDs in pairs as shown, with two of the leads soldered directly together, is the easiest way for me to keep them securely together and spaced correctly as I mount them in the 3D printed targets that I have created. The shared lead would go through R1, R2, or R3 back to the common from the flasher circuit.

So, if we have someone here who knows more about LEDs and circuits than I do, what are your thoughts and suggestions?

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I recognize that circuit from a Radio Shack LED circuit booklet they sold in the 90's.
 

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Discussion Starter #11
I recognize that circuit from a Radio Shack LED circuit booklet they sold in the 90's.
The 4011 circuit is from Engineer's Mini-Notebook: Digital Logic Circuits, Radio Shack, 1986. I still have three that I made using it around 1994, back when all the parts were obtainable in the store.

The one using the 555 IC and the extra circuit to resemble incandescent lamps came from Rod Paisley, who I have followed practically since he set up his original website. Thankfully, he is back online:


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