imsolidstate

Ducati 749/999 Tail Light

I finished this project a while ago, but never documented it. This is a taillight I made for my 749 before I sold it. I currently have a 999 so maybe I will make another one someday, but the 999 rarely changes out of it’s track/race clothes.

I was doing a lot of street riding when I made this, so I wanted something with better visibility than the (IMO) poor aftermarket replacement taillights. Specifically I was wanting to add a few high brightness pulses when the brake lights were turned on, but with a high enough frequency it would barely be noticeable. It catches your attention but isn’t really that obvious if you weren’t looking right at it. I’ve noticed recently that some fire trucks do this.
I’ve also been interested in a tail light with a light sensor so that the brightness can be ramped up during day time. Motorcycles need all the visibility they can get, but if the tail light gets too bright it will have the opposite effect at night and reduce visibility for people behind you.

While looking for an appropriate LED controller I ran across the PCA9952, which is a really nice piece that allows for up to 16 channels of LED strings at up to 40V each, with 8 bit PWM resolution selectable via I2C. I used this with an ATtiny45 that I had in my parts bin to run twelve total LED strings, 8 for brake and 4 for signals. This allowed me to use 5 LEDs per string so I didn’t need a boost converter to drive the LED strings.
Because I wanted a very low profile tailight that could fit in between the tail fairing and the exhaust I went with SMD side emitter LEDs this time. The acrylic is machined to allow the PCB to sit flat and the LEDs are countersunk into the face of the acrylic which couples the LEDs into the guide.

I had a new and improved version started based on a few issues I ran in to with the first one, but decided I wanted to sell the bike and get a race bike instead so the version 2 was shelved.

**Update**
Found schematics as requested.
Original version based on MP3391, didn’t end up going with this controller/driver because it only has 8 string output. With 8 strings and 60 LEDs I would need more than the available voltage to run a string which means adding a boost power supply. Too complicated.

Then I found the PCA9952 which can control 16 strings and has lots of other lighting functions built in. I used 12 strings with 5 LEDs each for a string voltage of approximately 10.5V with a typical red LED Vf of 2.1V. With an expected input of 12-14V this gives the driver some overhead but doesn’t force it to waste power dropping voltage.

If it was really hot or I got stuck in traffic the PCA9952 would shut down occasionally. This would have been easy to fix with some heatsinking either on the PCB (the device has a thermal pad) or externally but at this point I was thinking about selling the bike for one I didn’t mind throwing down the track. So I abandoned my plans for brightness sensing and brake transition flashing and replaced the fancy controller with this simple version.

Transistors U1/U2 sense brake state and U3/U4 sense signal state. If U3/U4 are off, transistors U5+U6/U7+U8 are biased on and the brake segments function normally. If either of the signals are on, the base of the transistor for the corresponding side is pulled to ground which turns those brake segments off and the signal segments on. I wish I had a video of it but I don’t, it looked really good. I never had any heat issues with this either from the electronics or the acrylic.

Tags: ATtiny, CNC, Electronics, I2C