Daylight (DRL) circuit doubts

Hello, I've just registered on this forums. I hope to learn a lot here. I've coursed a few electronics and electricity subjects on the university, but it's not my specialty, so I would like to increase my knowledge of these fields.

Now I'll present you my doubts.

My idea is to design a simple circuit to control the daylights (DRL) of a car. The circuit should do these two things:

• Possibility of disconnecting totally the lights using a switch.
• When position lights are on, the brightness of the LED must decrease.

The circuit has been modelled with 5 LEDs, but it can change. Also it is important to keep in mind that the values of the resistors are provisional. The greater resistor is supposed to work when the position light is turned on in order to drop more voltage ergo the LEDs drop less voltage.

For me the circuit is correct, but I've seen other versions of similar circuits using a diode bridge and capacitors, so I would like to hear your opinions and suggestions.

Thanks a lot!

I assume that your car is a small toy car because the LEDs will be VERY dim.
You have 5 White(?) LEDs in parallel. A white LED has a forward voltage of about 3.2V so the current limiting resistor will have 12V - 3.2V= 8.8V across it when the battery is 12V. Then Ohm's Law calculates the total current in the 1k resistor and in all the LEDs at 8.8V/1k= 8.8mA. Then each LED has only 1.76mA which is fairly dim. An ordinary 5mm diameter LED has its brightness rated at 20mA. High current LEDs rated at a few thousand mA are used in real cars. My solar garden lights cannot be seen in daylight but at night use about 15mA in each LED.

When the 20k resistor is used then each LED will have a current of only 0.09mA which is extremely dim.

LEDs are not light bulbs (which are fairly accurate resistors) so LEDs should NEVER be connected in parallel unless they are tested and sorted so they all have the same forward voltage.

Thanks for the reply.

I knew that the values of the resistors were not correct, I didn't fit them yet. However I appreciate your clarification.

On the other hand I didn't know that LEDs shouldn't be connected in parallel. Firstly I disposed them in series, but I was trying to emulate one of these:



If a single LED burns, the rest of them still work. For this reason I thought they were in parallel. A burnt LED still works as a diode?

Anyway I'm not going to implement the LEDs by myself, the idea is to use one of these chinese kits.

cerdiablo said:

I didn't know that LEDs shouldn't be connected in parallel.
I'm not going to implement the LEDs by myself, the idea is to use one of these chinese kits.

Click to expand...

An LED with one part number has a range of forward voltage, maybe from 2.8V to 3.5V. If a 2.8V LED is in parallel with a few higher voltage LEDs then the 2.8V LED takes ALL the current that would have been shared with the other LEDs and the higher voltage LEDs would not light. Then the lower voltage LED quickly burns out. Their voltages MUST be matched for them to be in parallel so they equally share the current.

In China, a light manufacturer buys millions of LEDs and pays a kid a bowl of rice to measure and sort them into groups having exactly the same forward voltage. Then they connect a group of them in parallel safely.

Ok, so if we suppose that all of them have the same forward voltage and we change the value of the resistors, the circuit could work correctly, I guess. Or another option would be to connect the LEDs in series.

But my real question is if I need to add an element to adapt the circuit to the real behaviour of the electric system of a car. I mean, I really don't know if I can connect a wire to the fuse of the position light (the idea is to use an add-a-circuit) and use the signal to activate the relay. This will affect the brightness of the position light, but how much? Should I use a high value resistor and a low signal relay?

And the presence of capacitors as a filter would be recommended? I don't know how constant is a car battery.

Thanks and sorry for so many questions.

A white LED uses about 3.2V. With a 12V battery then you can connect only three in series (9.6V) leaving 12V - 9.6V= 2.4V for the current-limiting resistor.
The Chinese kit that you will use might already have current-limiting resistors inside for its LEDs.

I have never heard about "positioning Lights" before. In Google some car companies call them "parking lights" and Kia says they are additional lights around the headlights that can remain turned on when the headlights are turned on for a distinctive appearance.
Daytime Running Lights always turn off when the headlights are turned on but many cars use dimmed headlights or dimmed turn signal lights as DRL's. Except Chrysler and Jeep cars and trucks use high beams at full power as DRL's to blind everyone in the daytime, especially when it is not a sunny day.

I do not know why you will use a relay instead of a switch, transistor or Mosfet.
A car has a voltage regulator so a capacitor will not be needed.

I thought that a relay would be more adequate since the "position light" signal is 12V and I imagine a high current. I think that if I use a transistor I would need to adapt the signal and decrease it (?). Yeah the kit comes with resistors integrated, so the "low" resistor of the schematic would be obviated. But if I still want to decrease the brightness when I turn the lights on, it is necessary to keep the other resistor, am I wrong?

When I say position light I refer to the less bright lights of the front of the car, they aren't used to drive during the night. Here in Spain they are called "luces de posición" and I ignore how they're called in North America so I simply translated it directly. Sorry if it confused you.

I forgot to say that here in Europe all the new manufactured cars must come with DRL. I think that in North America it's not mandatory.

cerdiablo said:

I forgot to say that here in Europe all the new manufactured cars must come with DRL. I think that in North America it's not mandatory.

Click to expand...

In Canada, all new cars had daytime running lights since 1989. I think it came later to USA.

Your photo shows very dim lights that will be useless in the daytime and would be illegal here.
Many foreigners drive here at night with only their daytime running lights or parking lights turned on because I guess they cannot read the English manual. Then they have no rear lights and are a hazard. I do not know why ALL new cars do not have automatic headlights, my car is automatic and even my cheap solar garden lights are automatic.

For my, DRL are lights that turn on when I start the car. In my country I started to see them when some production cars used anti-fog lights at 30% of brightness; it was enough to see the car during the day. Lately, most production cars come with LED DRL. In the next pic I show you some DRL, but they didn't come in the car, they were installed later.


And what I called "position lights", for me is this:

In Spain the traffic regulation law says that you cannot drive using only these lights, you must use either or at least low beam headlight or nothing (during the night it is mandatory to use low beam headlight). Yeah, it sounds stupid but you cannot use "position lights" during the day, only if the car is parked in a specific type of way.

Browsing on the web I found this pic:



Okey in english you call them parking lights/DRL. Since in my country we call them "position light" I didn't consider them DRL. So from now I will call them simply parking lights. The idea is to install LED DRL apart from parking lights, although parking lights can be used as DRL.

:roll:It was a nomenclature problem...

Here in Canada I have never used and I have never seen "parking lights" because almost every street and parking lot has lights at night.
DRL lights are important. Old cars without them are difficult to see sometimes, CRASH! Especially when you are driving and an old car without DRL's is coming and also a Chrysler or Jeep car with full power high beams for DRL's is also coming and you are blinded by the bright lights.

It is daytime and cloudy. 50 cars are coming and it is obvious which ones are Chrysler. I wonder why the government doesn't force Chrysler to use dimmed low beams or dimmed turn signals for DRL's like most other car manufacturers.