CatEye TL-LD500 Tail Light Modification

by Bill Paxton

Hi. This is inspired by Myra VanInwegen's Bike Articles , especially the ending statement on the LED Upgrades page. Myra, Simon... Thanks for such a great site.

Dissection 101

Actually no, the board is not solidly soldered into place. Upgrade away! The battery tabs have angled latchy bits (teeth?) that grip the plastic; They go in easily but remove with a bit more difficulty. To remove the circuit board, just stick a (strong) flat tip screwdriver under the battery contacts and apply some upward force. You can work between the plastic cracks and try to flatten the "teeth" down a bit to make removal easier, but in the two that I've modded they removed without too much effort... or fear of breaking them. As long as you know it's not glued down I'm sure you'll figure out how to remove it without breaking it. Just remember to do both ends at the same time; if you pop a trace off the board while flexing it, that's more repair work for you. :-)

Cateye dissected Battery tab teeth close-up
Dissected Cateye TL-LD500 Battery tab teeth close-up

Basic schematic

The bored, err, board

Don't you have more important things you should be doing? Since the point of this is to make the tail light more visible cheaply, we won't discuss the chip under the epoxy dot. I have no interest in reverse engineering a $1 blinkie and neither should you. The basic (partial) schematic is shown to the right.

Ok, ok, it didn't cost you $1... but to bother reverse-engineering this? Volume coupled with exchange rate and I'm guessing this thing doesn't cost half a dollar.

Front, left to right: capacitor, LED row, and the magic black dot. Back, left to right: PNP transistor, 3 resistors, and the switch.


1) Add two more LEDs

I mean, come on. It's the TL-LD500. 5. As in 5 LEDs I bet. Yeathere's those two extra sets of holes in plain sight, what else would have motivated starting this project in the first place? Since all LEDs are parallel, adding more will lower the brightness of the others. Not because they're parallel mind you, but because the transistor's resistor coupled with base current is limiting what can get there in the first place. More on that in a minute. The end result on brightness depends on what type, color, and efficiency LED you add. 

2) Swap out the stock LEDs
Diffused and clear

The stock ones are diffused and fairly wide angle. They're pretty good LEDs in my lights, actually. Fairly bright and ~625nm red color. But technology marches on; there's always better and brighter available, every few months. 

3) Combine 1) and 2)

You could take out the stock LEDs and move the two outer ones inward, while inserting more directional ones on the ends and center. 

There is one note about the addition of LEDs: they are all driven parallel and there's only one resistor, meaning current will flow according to the individual LEDs... try to match the Vfd or use all the same LEDs or you could end up with widely varying brightness across them.

4) Angle of incident light (first real modification)

The point is to be seen easier and from farther away. If you get LEDs that are too narrowly focused it will help greatly for spotting you on a flat road hundreds of yards away, but that's it. To compensate, experiment. On the two outside LEDs stick something under the inner-most lead when soldering to "kick it out" to the outside, a bit. A 22 gauge wire or small resistor or something, you don't want it shining directly sideways. 

Because of the curve of the plastic and the cut of the reflector cubes you can get a GREAT IMPROVEMENT over the stock light for off-axis viewing. I don't suggest angling any of the other LEDs to the sides in the slightest.

What I do suggest is angling the center LED up a few degrees. Yep. Think about it. If you mounted your light on your seatpost just under your ass, you have it in about as optimal a spot for a seated driver in a passenger car as you can get. But what about something sitting higher like a pickup, or Navigator-ish beast, or my favorite road buddy the Kenworth W900?

I highly suggest you blind them with one LED, anyway. Someone cussing your obnoxious blinkie is better than getting squished.

All replaced with edge ones slightly angled outwards. back showing shunted base resistor

5) Epileptic Fit

Ok, the 360k resistor has both leads going into the epoxy dot. It controls the flash rate in blink mode. It's still going to be the same pattern; flash-flash-flash, pause. But you can speed it up or slow it down by changing this. If you take it really low it gets to behave like a strobe light! If you go lower than that, it'll just stay on solid for a short while and proceed to fry your LEDs. See LEDs cheat when flashing, they use much more current then they can continuously sustain in these abbreviated duty cycles. (so do all your remote controls' infrared LEDs) 

The quickest mod is to parallel a 1M resistor with it. That is the same as desoldering that one and putting in a 270k, which makes it appear more "urgent" to my test subjects.  (about 1/3rd faster rate) 

I wouldn't make it any lower than 100k, and even that value I have problems with as a useful bike light. You really don't need to adjust this;  it's here nonetheless. 

6) So I rewired it...

You are now entering Tim Taylor territory. While what I'm about to tell you works, and probably (probably) won't catch fire or send you to the hospital or explode your taillight. I DID THIS, I AM NOT TELLING YOU TO DO THIS. 
If you do and it hurts... it's your fault. 

Ok since the LEDs are parallel through that one 2.4 ohm resistor, we have a limiting factor. But do the math for +3V and LEDs with an average 2.0Vfd and you'll see this is not the bottleneck. And because of the magic black dot on the other side of that 330 ohm resistor, we have an unknown Ib situation. (since transistors only do one thing...) 

Again I don't want to reverse engineer this thing but suffice to say that if you lower the base resistor then there's going to be (probably!) more Ib and therefore more Ic. Sure enough if you parallel another resistor here you will get a slight increase in brightness. SO I REWIRED IT. Bypass the resistor completely.  :-) 

Seriously I (nor you) have any idea what the magic black dot is providing, is capable of providing, nor is expecting to see as a load. You could measure and extrapolate. But oh well. It'll make your average current go to ~50mA (from 40mA) in continuous on mode if you shunt the 330 ohm resistor.  No other side effects like smoke or pain, yet. It's been awhile. 

How bright is it?

To compare accurately I devised a rig to make the shots as fair as possible - both mounted to a tension pole on their provided reflector brackets, same angle and same distance from the center of the camera lens.
(give or take a millimeter, sheesh!)

It is so bright that...

Well ok, it's brighter. How much so? It's alot, actually.  More than I expected. But only about 24 hours on constant, and I'm not sure on flashing but I assume the same percentage of reduction.

I went with current generation 2000mcd 640nm 30° 50mA AllnGaP waterclear LEDs which certainly qualify as "high brightness" staring at one at 3' will make you see color-negative spots for awhile. These are also "deeper" red than the stock ones. While the original light is rated at 6000mcd for 3 LEDs also, we all know that gets skewed for sales not specifications. They are certainly not 2000mcd each - being wider angle and diffused they'd be brighter than my replacements!

How much brighter you LED swap will be will depend on what LEDs you went with and if / how you modded the circuit. If you don't have LEDs already, remember that one man's "high out" is another man's "burnt out" - some companies will sell 200mcd as superbrights while another sells 1500mcd versions with the same angle. Don't get suckered by the whole angle issue - of course a narrower beam will have more millicandelas at a fixed distance target - but that doesn't mean it's brighter. Only compare apples to apples.

At a measured 100m, it's apparently 6x as bright according to test guinea pigs.. er, volunteer ocular technicians. BTW it makes a great pocket flashlight now too.

I wish the ol digicam took better pics; it cannot. Again, technology marches on. There are much better cameras than a Olympus D-320 today, but hey it's paid for and still works. The pictures don't do the lights justice; but both are crippled equally. The ratio is about the same; that is, the modified one swamps the stock one in all aspects.

this is 6m away on-axis

Total mod cost was under $3. Makes for a great bag blinkie. And a spare taillight or even primary taillight if you want to risk it. Certainly more visible than the stock TL-LD500. It's still not as noticable as a Vistalite Total Eclipse, but then the Vistalite has a special magnifying reflector and does NOT meet C.P.S.C. qualifications as a reflector.

It goes without saying (well not now) that you could modify a clear front version of the LD500 to have some seriously bright orange/amber LEDs (a very efficient wavelength)

Again, thanks to Myra for the original tweak idea.


both lights shining on a white wall from 1m