Commercially available Flicker Free (Supercaps or Stay Alive) circuits are good for driving a chain of LED's if you hard wire these or use the manufacturers PCB. These are all designed around an output voltage of 3V, since the supercap is rated for max. 5.5V at say 1F or 1.5F. The capacitor itself is a $1.50 min. on eBay
Unfortunately they cannot drive the Flexible LED strips which use SMD 3528 or 5050 LED's and are rated at 12V. These strips require min. 7.9VDC to turn on for any visible light output. A supercap will explode if subject to higher voltage than 5.5V so they have a 5.1V Zener to shunt them (across the capacitor) and limit the voltage to 5.1V max.int he commercial Flicker Free products.
Also driving Flexible LED strips at full brilliance with 12V is non-protypical and too bright. So I use warm white LED's (not the harsh bright white, good for a florescent lamp effect) run them at at 9V to give the soft effect you would normally see in a prototype coach. Use Bright white LED's if using in urban commuter trains for a more modern look.
I found that a 4700uF capacitor works best to remove the flicker due to dirty track contacts or going over switch points or trouble spots or insulated block sections, but if you want the cars to have a gradually dimming effect till the lights go out, I suggest using a 10000uF capacitor which will give a drain timing of just over a minute when the power is removed from a block.
The circuit itself is a no-brainer and very standard, bridge rectifier, capacitor, LED, but I add the 78L09 Voltage regulator for the "warm light" effect plus my variation drives a standard Flexible LED strip at the right prototypical brilliance. A 12 LED strip section normally is good for a standard passenger car and these can be cut to suit individual car or wagon lengths, in multiples of 3 LED's. My circuit and 12 LED's draw only 22mA current, so nothing gets heated or even warm (plastic safe) to touch.
I cut my own contacts from PB sheet, I acquired few years ago or wind 0.008" PB wire into coiled springs on the axles for pick up. I do not remove the wheel, as instructed by some manufacturers who sell these ready made coil spring contacts, as it is easy to wind the wire by just popping out the wheel set and winding 10 turns on it very tightly. The trick is to maximize the surface area for making the contact, more area, the better conductivity.
Though I designed the circuit primarily for DCC application, it works with traditional analog DC too. To make things clean, simple and compact, I simply scrap the LED strip to bare the copper and solder the SMD Bridge rectifier, voltage regulator directly on to it and run 30AWG wire from the wheel pick ups to the LED strip.
Dab a drop of Instant glue to hold the components (not absolutely needed). Connect the capacitor to the copper pads at the end of the strip (with wires based on where you hide it and watch the polarity) and mount it with double sided foam tape on the roof of the coach or behind something that can't be seen from the cars windows.
TIP: LED flicker is less prominent in DCC powered layouts, due to to high frequency square wave form of the output voltage, even without using a capacitor, unless you have very dirty track. It is more obvious in DC powered layouts or when using incandescent bulbs instead of LED's.
Bonus: No holes have been drilled in the car (lowering its value). Existing holes and spaces have been used to guide and conceal all wires from the pick ups to the LED strips.
Parts List:
(assuming you buy them on eBay like me in MOQ of 50, 75, 100 pcs and free shipping, good for a lot of coach lighting)
1. SMD MB6S 0.5A 600V Single Phases Diode Rectifier Bridge SOP-4 US (Cost $0.10)
2. L78L09 9V 100mA 0.1A Voltage Regulator Transistor TO-92 (Cost $0.05)
3. 10000uF 16V Radial Electrolytic Capacitor (Cost $0.98) or 4700 16V Nippon SMT Low ESR Long Life electrolytic capacitors (Cost $0.75)
4. 0.008" PB wire (Cost $0.20) or .01" PB strip (Cost $0.05) for pick up contacts
5. Warm White SMD 3528 Flexible Led Strip, cut to 12 LED length to suit length of coach. You can get 25 sections of 12 LED from a 300 LED strip (Cost $5.49 for full 5M reel or $0.22 for a 12 LED section)
Parts Cost: $1.55 (max) + solder + 30AWG wires ($0.20) = Total cost $1.75 + labor (invaluable)
Circuit schematic:
Photos:
Note: Capacitor not yet installed as I am still awaiting on my caps to arrive from China. It will be connected to the two bare copper contacts you see on the right side of the LED strip (opposite end of the voltage regulator).
