Ever since getting the Roadster, I've wanted a light on the dash to indicate when the brake lights come on as a result of the deceleration due to regen braking. Several of the vehicles converted to EVs by AC Propulsion have included this feature. I never got around to implementing this idea because of the lack of a vehicle shop manual to help with finding the brake light wire and figuring out how to run the wiring to the dash.
Recently I came up with a better idea: mount an LED indicator light in the housing of the center brake light inside the rear window so that the LED shines directly at the rear-view mirror and into the driver's line of sight. This is a very simple installation because the brake light circuit is right there already, and it is a better location for observation than my original dash idea since the times when I care about the brake light being on typically the same times when I'm checking the mirror to see if someone is close behind me.
A poor-man's version of this might be achieved by simply drilling a hole to let light from the brake light shine forward, but I don't think that would be bright enough. Maybe a light pipe would work. My choice was to install a separate narrow-beam LED so I could have good control over the direction and light level. In case others might want to replicate this idea, I'll provide some construction details. I have a 2008 Roadster 1.5, but I suspect this portion of the design did not change in later models.
I started by trying to remove the interior dome light from the brake light housing as in this post from S-2000 Roadster about replacing the bulb with an LED. However, that was a mistake because it is unnecessary and not useful for this project. Instead the whole center brake light housing needs to be removed. Before doing so I strapped a little box to the brake light housing so I could hold a yardstick from there to the mirror to guage the angle of the sight line. I determined that the angle is approximately 8 degrees downward from the brake light housing to the mirror, and also approximately 8 degrees toward the passenger side of the car.
It is pretty easy to remove the center brake light housing. There are four bolts with 10mm hex heads that hold the soft-covered interior roll-bar panel and the plastic brake light housing in place. Remove the four bolts, being very careful not to drop any parts or tools down behind the seats or they may be gone forever! The push in on the vertical plastic cover on the inside of the B pillar to let the loosened panel drop past. Be careful not to pry the front corner of the interior roll-bar panel because it is just made of molded foam that can easily be torn. In fact, I discovered that the panel in my car had been damaged by someone during original assembly or some previous repair work. I repaired it to some degree with hot glue as shown in this picture.
Two cables attach to the brake light housing, one for the brake lights and one for the dome light. Both will release pretty easily if you push in on little tabs. Once the brake light housing is disconnected, you can remove the strip of LEDs that form the brake light by pushing back the clips to release the circuit board.
The first step is to drill a hole in the housing for the LED. I chose a red LED with a 5mm diameter clear resin body and narrow viewing angle from my parts bin. The hole needs to be fairly close to the lower surface of the housing in order to not be coverd by the interior panel, and should be near the center portion of the housing that extends further to hold the dome light. The following picture shows the position. When drilling in plastic, it is best to drill the finished-size hole on the first pass rather than drilling a smaller pilot hole first because the larger bit is likely to grab in the pilot hole and tear the plastic. Remember to account for the downward and rightward 8 degree angles when drilling.
The next step is to mount the LED. I used hot glue for this. To hold the LED at the correct angle and position I used a little block of wood and a C-clamp, then glued the LED in place. I wanted to minimize the amount that the LED protruded from the front of the brake light housing.
Then I soldered a pair of wires onto the brake light circuit board where the cable attaches. These wires must be bent back sharply because the end of the circuit board fits tight against a plastic support in the housing.
A resistor is needed to limit the current and drop the 13.5 V supplied by the car down to 2 V across the LED. I was concerned that the LED might be too bright and therefore be distracting, especially at night, so I started out with a 1 K ohm resistor, which results in about 10 ma current. What I had not considered is that at night the mirror will be flipped to night mode to avoid the glare of headlights behind, so that makes the relative visibility of the LED about the same in daytime and nighttime. Furthermore, the LED was not bright enough to be easily disinguished from the red signal lights visible in the mirror. I reduced the resistor to 500 ohm, allowing about 20 ma current, and that works better. Keep in mind, though, that the LED I selected from my parts bin has probably been sitting there for 20 years, so it is not as bright as modern LEDs for the same current level. If you get one of the high-efficiency LEDs that puts out 10,000 mcd at 20 ma, it would be way too bright. This shot shows the resistor soldered in place; the glue job looks ugly because I had not considered the 8-degree rightward angle, so I needed to soften up the glue and try to re-orient the LED.
Fortunately, it is easy to select the right current level by trial and error. Just tack-solder in a resistor that you think will work, then mount the brake light housing back in the car without the interior roll-bar panel and give it a trial run in both daytime and nighttime. Once you find a resistor value that you like, then you can solder it in more securely and reassemble everthing. My finished result looks like the pictures below.
Recently I came up with a better idea: mount an LED indicator light in the housing of the center brake light inside the rear window so that the LED shines directly at the rear-view mirror and into the driver's line of sight. This is a very simple installation because the brake light circuit is right there already, and it is a better location for observation than my original dash idea since the times when I care about the brake light being on typically the same times when I'm checking the mirror to see if someone is close behind me.
A poor-man's version of this might be achieved by simply drilling a hole to let light from the brake light shine forward, but I don't think that would be bright enough. Maybe a light pipe would work. My choice was to install a separate narrow-beam LED so I could have good control over the direction and light level. In case others might want to replicate this idea, I'll provide some construction details. I have a 2008 Roadster 1.5, but I suspect this portion of the design did not change in later models.
I started by trying to remove the interior dome light from the brake light housing as in this post from S-2000 Roadster about replacing the bulb with an LED. However, that was a mistake because it is unnecessary and not useful for this project. Instead the whole center brake light housing needs to be removed. Before doing so I strapped a little box to the brake light housing so I could hold a yardstick from there to the mirror to guage the angle of the sight line. I determined that the angle is approximately 8 degrees downward from the brake light housing to the mirror, and also approximately 8 degrees toward the passenger side of the car.
It is pretty easy to remove the center brake light housing. There are four bolts with 10mm hex heads that hold the soft-covered interior roll-bar panel and the plastic brake light housing in place. Remove the four bolts, being very careful not to drop any parts or tools down behind the seats or they may be gone forever! The push in on the vertical plastic cover on the inside of the B pillar to let the loosened panel drop past. Be careful not to pry the front corner of the interior roll-bar panel because it is just made of molded foam that can easily be torn. In fact, I discovered that the panel in my car had been damaged by someone during original assembly or some previous repair work. I repaired it to some degree with hot glue as shown in this picture.
Two cables attach to the brake light housing, one for the brake lights and one for the dome light. Both will release pretty easily if you push in on little tabs. Once the brake light housing is disconnected, you can remove the strip of LEDs that form the brake light by pushing back the clips to release the circuit board.
The first step is to drill a hole in the housing for the LED. I chose a red LED with a 5mm diameter clear resin body and narrow viewing angle from my parts bin. The hole needs to be fairly close to the lower surface of the housing in order to not be coverd by the interior panel, and should be near the center portion of the housing that extends further to hold the dome light. The following picture shows the position. When drilling in plastic, it is best to drill the finished-size hole on the first pass rather than drilling a smaller pilot hole first because the larger bit is likely to grab in the pilot hole and tear the plastic. Remember to account for the downward and rightward 8 degree angles when drilling.
The next step is to mount the LED. I used hot glue for this. To hold the LED at the correct angle and position I used a little block of wood and a C-clamp, then glued the LED in place. I wanted to minimize the amount that the LED protruded from the front of the brake light housing.
Then I soldered a pair of wires onto the brake light circuit board where the cable attaches. These wires must be bent back sharply because the end of the circuit board fits tight against a plastic support in the housing.
A resistor is needed to limit the current and drop the 13.5 V supplied by the car down to 2 V across the LED. I was concerned that the LED might be too bright and therefore be distracting, especially at night, so I started out with a 1 K ohm resistor, which results in about 10 ma current. What I had not considered is that at night the mirror will be flipped to night mode to avoid the glare of headlights behind, so that makes the relative visibility of the LED about the same in daytime and nighttime. Furthermore, the LED was not bright enough to be easily disinguished from the red signal lights visible in the mirror. I reduced the resistor to 500 ohm, allowing about 20 ma current, and that works better. Keep in mind, though, that the LED I selected from my parts bin has probably been sitting there for 20 years, so it is not as bright as modern LEDs for the same current level. If you get one of the high-efficiency LEDs that puts out 10,000 mcd at 20 ma, it would be way too bright. This shot shows the resistor soldered in place; the glue job looks ugly because I had not considered the 8-degree rightward angle, so I needed to soften up the glue and try to re-orient the LED.
Fortunately, it is easy to select the right current level by trial and error. Just tack-solder in a resistor that you think will work, then mount the brake light housing back in the car without the interior roll-bar panel and give it a trial run in both daytime and nighttime. Once you find a resistor value that you like, then you can solder it in more securely and reassemble everthing. My finished result looks like the pictures below.
