Car headlights, wiring modification relay problem.

[JTP]

I put some aftermarket headlights in my car a while ago, and they use two different bulbs in each side, one for low beams and one for high beams. Normally, when you turn on the high beams, the low beams turn off. I found a wiring modification how-to that explains how to add a relay (simple 1P2T 12V 30A automotive relay) to the circuit so that the low beams stay on when the high beams are switched on. This really improves the light output of the car.

I did this and it works great.

My problem is that I live in Canada and my car has daytime running lights. DRLs run the high beams at half voltage whenever the low beams are off and the e-brake is off. When the DRLs are on, the relay gets confused by the half voltage and it buzzes like crazy, and all 4 lights flicker.

Is there any way to somehow send signal to the relay only when it's recieving a full 12V or more? Or any other way around this problem? Or should I just take the whole thing out...

Thanks for looking.

 

[Menticol]

You can do it using a voltage comparator

here is an example. Instead of driving a LED, you can connect it to your relay trough a transistor. So when voltage drops below the point set by R2 (see text and schematic), the led lights, and your relay turns off
________________________________________

Circuit 04: An Op-Amp based battery voltage monitor

An ECU must have a way to monitor battery voltage. Here is a simple op-amp based circuit which will illuminate the LED when the battery voltage drops to a certain level. The turn-on point is set with R2. You must be able to vary Vcc (usually with a good power supply) to set the circuit. Decide at what voltage you'd like the LED to illuminate, and apply this at Vcc. Adjust R2 until the light just illuminates. Use an LED which will light at the desired voltage level, and determine R3, using ohm's law (E=IR), to keep the current flowing through the LED and op-amp to around 10mA.

The LM324 will draw 0.8 mA with the LED out. With the LED on, current will increase to that determined by your selection of R3, with lower being better, of course.

 

[audioguru]

Like I said on the other website, cars in Canada that use the high beams for daytime running lights are blinding and are crazy.
Newer Chrysler and JEEP cars and trucks mostly.

On a cloudy day you don't want a JEEP truck coming at you with its huge bright high beams on.
My cars have used dimmed low beams and the bulbs last forever and don't blind anybody.

 

[NewUser]

Are you sure it's "half-voltage" that's the cause of the relay chatter/buzz, and not unintentional interlocking somewhere between it and another relay?

By "unintentional interlocking" I mean, for example, relay A's coil being fed through relay B's "normally-closed" contacts, but relay B's coil also being dependent on relay A's "normally-open" contacts. ie both relays "fight" each other.

 

[NewUser]

You can do it using a voltage comparator

here is an example. Instead of driving a LED, you can connect it to your relay trough a transistor. So when voltage drops below the point set by R2 (see text and schematic), the led lights, and your relay turns off
________________________________________

Circuit 04: An Op-Amp based battery voltage monitor

An ECU must have a way to monitor battery voltage. Here is a simple op-amp based circuit which will illuminate the LED when the battery voltage drops to a certain level. The turn-on point is set with R2. You must be able to vary Vcc (usually with a good power supply) to set the circuit. Decide at what voltage you'd like the LED to illuminate, and apply this at Vcc. Adjust R2 until the light just illuminates. Use an LED which will light at the desired voltage level, and determine R3, using ohm's law (E=IR), to keep the current flowing through the LED and op-amp to around 10mA.

The LM324 will draw 0.8 mA with the LED out. With the LED on, current will increase to that determined by your selection of R3, with lower being better, of course.

Would that circuit work with an LM741 in place of the LM324?

 

[audioguru]

Would that circuit work with an LM741 in place of the LM324?

Yes.
But the circuit does not have hysteresis so it will be very unstable when the sensing voltage is near its threshold voltage.
Add a 2.2M resistor from the output of the opamp to the slider of the pot to give it some hysteresis then the circuit will have a snap-action.

 

[Wings]

Why not use a 6v relay and place a 6v zener diode in series with the relay coil? The relay would energize only when you had 12v on the high beams, and is simple enough to do without building a circuit. Make sure your 6v zener is capable of handling the coil current (+50%).

 

[shokjok]

Most cars are wired so that high and low beams are fed separately. The only visible indicator of high beams is a blue light on the dash. Feeding both beams defeats the purpose of headlights and wastes battery power. You can parallel the relay across the feeds from the headlight switch to get the desired result.