Author Topic: Turning a light off when another is switched on  (Read 282 times)

Offline Krudda

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Turning a light off when another is switched on
« on: September 27, 2016, 04:55:15 am »
Ok, here's something for you electrically minded specialists out there.
I have aftermarket DRL (daytime running lamps) in my car. By law, they meet ADR revision 3 and are classed E-1... They're legal.

Anyway, I had assumed this was the extent of things, but not quite. To be fully legal, they must also turn off automatically when I turn on the parklights or above.

Currently, they're just switched through a relay piggybacking my parklights (apparently also illegal because they don't dim)


So what would I need to have a 12v auxilary power that turns off when the headlights are powered?
I can solder, but I don't know which components would achieve this.

Offline Mistgun_Zero

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Re: Turning a light off when another is switched on
« Reply #1 on: September 27, 2016, 07:32:30 am »
Do you know Normally Open and Normally Closed contacts (NC and NO). A bit of read should help you a lot on that. It's really simple and will take 10 min max.

Just wire your DRL to an NC contact and the other lights to a NO contacts (which they mostly are). It's basically saying when you have not switched on any lights, your DLR will start, but when you have your parklight or anything else it will switch off.

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Offline Krudda

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Re: Turning a light off when another is switched on
« Reply #2 on: September 27, 2016, 07:40:18 am »
Haven't heard the term (or just forgot it in the many years since I did electrical engineering at school)
Thanks. I'll definitely look it up.

Offline boxer4

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Re: Turning a light off when another is switched on
« Reply #3 on: October 04, 2016, 06:35:19 pm »
Might be more complicated than that because the headlights are the "other" light, and your AMDRLs have its own switch.

Probably the simplest solution that allows BOTH lights to be shut off at the same time (instead of using a single norm open/norm closed toggle switch) is install a relay switch in series with the AMDRL switch, which sounds like that you already have another relay right now.  Hook up the relay's normally closed contacts in series with the AMDRL switch, and the coil of that relay in parallel to the headlights.

This way when the headlights are turned on normally, the relay will energize, which will cut off power to the AMDRLs.  The AMDRLs will still be off when the parking lights are off.

A more cute way of doing this is designing a logic circuit (Turn AMDRLs on iff headlights off and parklight on) but I think the relay solution requires less "cute" circuitry.

Online kitamesume

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Re: Turning a light off when another is switched on
« Reply #4 on: October 04, 2016, 07:16:29 pm »
technically a selector switch would be easier to hot-wire, plus they don't have any floating power consumption.

a 2-pole 4-way selector switch with "off / headlights / aDRL / both" would be possible.



this would shut-off either when one of the option is selected, but you still have the option of powering both.
« Last Edit: October 04, 2016, 07:41:33 pm by kitamesume »

Offline Krudda

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Re: Turning a light off when another is switched on
« Reply #5 on: October 04, 2016, 08:14:43 pm »
Since I am doing this on a car with an already functioning setup, I cannot use that as it isn't legal and thats the whole point.
To be legal, I cannot be able to turn them on/off manually, it must be automatic.

Thanks for the suggestion though.

Might be more complicated than that because the headlights are the "other" light, and your AMDRLs have its own switch.

Probably the simplest solution that allows BOTH lights to be shut off at the same time (instead of using a single norm open/norm closed toggle switch) is install a relay switch in series with the AMDRL switch, which sounds like that you already have another relay right now.  Hook up the relay's normally closed contacts in series with the AMDRL switch, and the coil of that relay in parallel to the headlights.

This way when the headlights are turned on normally, the relay will energize, which will cut off power to the AMDRLs.  The AMDRLs will still be off when the parking lights are off.

A more cute way of doing this is designing a logic circuit (Turn AMDRLs on iff headlights off and parklight on) but I think the relay solution requires less "cute" circuitry.

This sounds like what I'd need to do, but I don't fully understand it.

To better help you guys understand, I have a relay on the lights but I have no idea how relays work other than the basic function. I wouldn't know how it does what it does.

Current system is this.
Park light switch in car > park lights. Park lights > ground.
Piggybacking the +12v of the park lights a wire runs to a 40amp relay.
A +12v wire with a fuse runs from the battery, to a toggle switch inside the car, then to the relay.
The relay sends +12v to my DRLs when both the switch and parklights are on.
The DRLs then return to ground.

To comply with local law, I need to replace the switch with something that is constantly powered when I turn the ignition on and turns off automatically when the park lights are on. So I'd need to move the relay power source from the park lights, to the battery instead, but thats as far as I have worked out.
I can still turn it on/off via switch, but the turning off function when the parklights come on, must be automatic.


I understand wiring diagrams better than technical terms, so that may work better.

I'll try to create a wiring diagram of my car when I have time.
« Last Edit: October 04, 2016, 08:31:22 pm by Krudda »

Online kitamesume

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Re: Turning a light off when another is switched on
« Reply #6 on: October 04, 2016, 08:32:17 pm »
use a relay, piggyback the relay's 12V coil onto the parklights power, and simply wire the power-source and aDRL on the NC contacts.

with this, if the parklights are on the relay will open, thus turning off the aDRL.
« Last Edit: October 04, 2016, 08:40:46 pm by kitamesume »