INSTALLATION STEPS

I. DRL CIRCUIT SCHEMATIC
Here's how the DRL circuit integrates into the factory headlights.

- Ground-switching relay is the critical component that makes the hi-beams grounded by default, and un-grounds them and grounds the lo-beams when you turn the light-switch to ON
- DRL relay is pretty standard, takes a power-source, runs it through a resistor to remove some of the power and sends it to the hi-beams. Then switches off power to hi-beams when you turn the light-switch to ON

II. BUILD DRL MODULE W/GROUND-SWITCHING
Here's the raw-materials for building the Corolla-specific DRL module:

I used a 5mm aluminium plate for making ballast-resistor assemblies for injectors. Add some space for two relays. All holes chamfered for you Titanic fans. Wired up the DRL trigger to ground-switching relay and power-output through the resistors.

From other posts, you'll know I take wiring seriously. Nothing sucks more than intermittent issues relating to corroded & fraying wiring. So I take extra precautions so my wiring last longer. All connectors are crimped and soldered (as done in aerospace, military and F1 wiring). One layer of heat-shrink wrap over the strain-relief and an additional wrap over the whole thing. This gives a tapered increase in stiffness as the wire approaches the connector. Less likely to crack and break right where it enters the connector due to the sharp stress-riser.

The overall wrap also insulates the connector so if a wire is dropped or flailed around aimlessly, it won't short-circuit against something else such as other wires or ground. The colour is also coordinated with whether the wire is carrying power or ground. Testing the DRL power-input to power-output shows 1-ohm. That's 0.5-ohm from the power-resistors and meter rounds up.

III. CONNECT GROUND-SWITCHING RELAY

IV. CONNECT DRL RELAY TO HI-BEAMS

 

DISCUSSION TIPS & NOTES

My first design of using a 1-ohm power-resistor in the DRL module gives this result:

Given these fixed parameters:
- 13-volt supply from alternator/battery
- 2.6-ohms per bulb from V=IR

We can come up with the rest:
- 2.3 ohms ResistanceTotal = 1 ohm resistor + 1.3 ohms from two bulbs in parallel
- 5.7 amps current draw from V=IR
- 5.7 voltage-drop across resistor from D=IR
- 32 watts dissipation by resistor

- 2.8 amps draw per bulb
- 7.3 voltage-drop across bulbs
- 21 watts dissipation per bulb from P=VI

Looks like the 1-ohm resistor won't work because it drops voltage too much and results in each bulb running @ 21w, less than half. Also the 32-watts dissipation across the resistor is higher than the 25w rating of the resistor. I will need to lower the resistance of the resistor.

The second design has less resistance in the ballast resistor by using two of them in parallel (2x1 ohm in parallel = 0.5 ohms):

Given same supply-voltage and bulb-resistance, we can work out the following:
- 1.8 ohms ResistanceTotal = 0.5 ohm from parallel resistors + 1.3 ohms from parallel bulbs
- 7.2 amps total current draw from V=IR
- 3.6 voltage-drop across resistors from D=IR
- 26 watts dissipation by resistors (13w each)

- 3.6 amps draw per bulb
- 9.4 voltage-drop across bulbs
- 34 watts dissipation per bulb from P=VI

So this configuration will work. It lowers the power-dissipation by spreading it across two power-resistors to fall below the 25-watt rating of each. It lessens the voltage-drop across the resistors to apply 9.4v to the bulbs. As a result, the bulbs will be running at 34w instead of the original 65w. Perfect!

 

I want to do something similar with lights that will not work at ~6 volts.

 

Are you referring to LEDs?

 

Saved for smart-alec comments...

Actually, that's a really good idea, saving those immediate next two posts for relevant discussion.

 

Yeah, LEDs don't have linear response to voltage changes. Over-driving them won't make them brighter, just hotter and they'll burn out sooner. Under-driving them won't dim them much. There's a narrow window where they'll dim somewhat, like down to 80%, then they go out completely.

You can get adjustable dimmer circuit that varies the duty-cycle for linear changes in brightness. Which bulbs are you trying to dim and what's their max power consumption?

Here's an adjustable PWM controller with remote pot. You can mount the knob inside and adjust the brightness of the high-beams from the driver's sear. To use as automatic DRL, you'll still need the ground-switching relay to select the hi-beam ground as default.

ebay - PWM controller 20A
You can get smaller 3A versions to dim LED dash lights.

 

Here's an adjustable PWM controller with remote pot. You can mount the knob inside and adjust the brightness of the high-beams from the driver's sear. ebay - PWM controller 20A
You can get smaller 3A versions to dim LED dash lights.

WOW that's cheap! Have you used one of them? If they work ok I may get one just to have it sitting around.

 

I know the window is pretty small compared to incandescent, but a hell of a lot larger than HID.

I am not sure about the brand of the bulbs as I bought them off my friend solely because I knew they would dim. He does not remember the Ebay seller.

I know they are meant for a 12-13 volt system. Not sure about nominal wattage.

 

Get a PWM, it's the only way to reliably dim an LED. Unless it has a fairly good constant current driver that can take 6V, which is obviously not the case. PWM is a very simple device which will get your brightness perfect.

 

I ordered one shortly after posting. I have no specific use in mind, but for $4.60 delivered I just can't lose if it's not doa.