Thursday, August 11, 2011

Daytime Running Lights

Another speed bump in the road to our Canadian re-repatriation is the installation of daytime running lights in our vehicles.  This means that when the car is in motion, even in the daytime, lights must be on.  The argument is this is for safety, and yes it has some logic to it, especially at the witching hour, right before the sun goes down.


Checking around, I find these modules from the dealer are in the $300 range, which I found outrageous, so I set out to find a way to do it myself.  Canadian Tire, the quintessential hardware store of Canada, had a DIY DRL module for $40!  I bought it, installed it, and phhhht, it didn't work.  Reading the manual ;), I determined that for the Toyota Echo, I needed a different module.  Further research revealed that Toyota headlights are ground activated.  Ok, so don't use the headlights.  I've Googled it, and others have connected to fog lights or parking lights.  Apparently however, the inspector will not clear your vehicle unless the lens assembly actually has the characters DRL printed on it, and having a separate set of fog lights isn't allowed.  Groan!

Not wanting to order a module for twice the price from the manufacturer, and not wanting to pay $280 to Canadian Tire to install one, I set out to build one myself.

Reverse Engineering

Armed with a $3 automotive test light, I took a few readings.  There were 3 wires going to the headlight.  I pulled of the connector and probed each line with the common of the test light connected to the car's chassis. In the off position, each contact illuminated the test lamp, thus I recorded it as high. With low beams active, one of the connections was low.  With high beams, the low beam signal went high, and another one went low.

See truth table:

Headlights     1      2      3
Off               H      H     H
Low              L      H     H
High             H      L     H

With this logic, it seemed all I needed to do was get a relay to pull pin 1 to ground, activating the low beams.  When the low beams were actually on, it would just be a parallel to ground, so no foul.  However, if the high beams were on, I would be driving the headlight with L L H, which could have some ill effects.  I reasoned that in essence, this would allow for slightly brighter headlights, assuming that pin 1 and pin 2 were weak pull-ups and not actually driven high.  Might need a bigger fuse is all.


Well where I'm staying, there aren't any electrical tools, and the new job is mainly software where there is a healthy fear of hardware among the Doritos and Jolt Cola crowd, so I headed to the local Hackerspace.  ENTS, as it's called is located slightly NE of downtown Edmonton across from the Mustard Seed, and above a seedy joint called the Bohemian Cafe.  Perfect ambiance for a bunch of hackers to hang out at all hours working on projects as varied as polishing your beater's rotors, building a greenhouse control system, or perhaps a self replicating plastic printer.  Definitely a place to be for what I needed.

So after introductions, and an Asiago cheese encrusted Focaccia bread, Italian dry salami and swiss sandwich, I got right to work.  Oh, and I shared a bunch of bananas.  Monkeys are funny.  Rummaging around through a box of discarded photocopier circuit boards, staples of any hacker workshop, I found a 12V relay with a large coil and 30 amp contacts.  I knew this as it had a nifty clear shell.  Find!  A large coil is good because it means the current required to drive it will be low and thus not a tremendous load on the control signal I plan to use.  The contacts are perfect as the headlight fuse is rated for 10A, and I want to be at least 2X that to ensure I have longevity and don't need to use a hammer to unfuse contacts.  Why not just buy an automotive relay? Do you change a diaper when it isn't full?  No.

So I wired the common of the switch to one side of the coil and out to a black wire.  I ran the other side of the coil to a red wire.  Leaving the NO (normally open) contact where I attached a yellow wire.  Wrapped the whole mess up with some electrical tape, and took it outside for a test.

Proof of Concept

First I attach the black (gnd) wire to the chassis.  Next, I slide back the insulation on pin 1 of the headlight connector (low beam) , and connect the yellow wire.  To my amazement, attaching the red wire to the battery + contact, turns on the low beams!  Awesome!  Unfortunately the mosquitos were starting to carry me away, so I gave up for the night.

Next Day

First I stripped out the Crappy Tire DRL module.  Maybe it'll prove useful on the Nissan.  Next, I start probing around in the fuse box to find a signal that is only active when the key is in the Start or Drive position. Found it.  Connected the red wire to it.  Ok, all set.

Moment of Truth

Set the switch to start, and viola, the low beams were on, but unfortunately the drivers side one was very dim while the passenger side was bright as could be.  Further dissapointment came when switching to high beams and the passenger side was completely dark while the drivers side was full on.

Back to the Drawing Board?

Alright, so I take a break, eat some Strawberry Rhubarb pie, and think on it a spell.  There should be no reason it does't work.  Since the driver and passenger signals are slaved by design, the issue must be with the common feeding the drivers side.  With my handy dandy test light, I start checking the fuses.  Sure enough a fuse is blown, but not fully.  I then replace the 10A fuse with a 15A fuse, and the DRLs work, the low beams work, and the high beams work!!  Success!

Engineering is a process.

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