Using voltage difference as trigger

Thread Starter

geselaw

Joined Dec 21, 2021
18
Hi,
I am trying to relocate my DRL(Day time running Light) in my truck to a custom LED light. Chevy use the low beam bulb as DRL/Low beam. When the low beam is acting as DRL, it gets lower voltage(around 9v) so it becomes dimmer. In the night time the low beam acts as low beam so it gets 12v.

I am trying to build a circuit that could detect the voltage so that when the voltage is between 12v-14v, initiate a relay or a mosfet to send power to the low beam bulb and when the voltage is around 9v, send power to my custom LED light. If anybody could send me to the right direction, I would appreciate it.
 

DickCappels

Joined Aug 21, 2008
8,686
It seems from your description that the signal already goes from 9V in the daytime to 12V at night. Are these two separate signals (Day and Night) that need to be combined?
 

Ya’akov

Joined Jan 27, 2019
5,513
It seems from your description that the signal already goes from 9V in the daytime to 12V at night. Are these two separate signals (Day and Night) that need to be combined?
Dick, my understanding is that the low beam lamp gets 9V during the daytime and 12V for night use. The TS wants to direct the 9V to an alternative LED daytime running light, but when the line goes to 12V he wants it directed to the original low beam lamp in the headlight.

The goal is the DRLs will be the LEDs when 9V is present and the OEM low beams will be the nighttime low beam lamps.
 

Tonyr1084

Joined Sep 24, 2015
6,866
A comparator circuit should suffice for deciding when the voltage is above a pre-determined setting. However, since the DRL "Dim" setting is probably PWM controlled, a comparator might see the full voltage long enough to turn it on and act like yet another headlight.

If I understand the request correctly, you want the DRL to go out but have another light acting as the DRL. My concern would be the law regulating the use of lights on a vehicle. I've heard of people being charged with "Unauthorized Use of Lighting". Not a serious charge, probably just a fix-it ticket making you restore the system to OEM settings. Would you be opposed to having the DRL on both the headlights AND your secondary lights?
 

Thread Starter

geselaw

Joined Dec 21, 2021
18
Thank you so much for all your quick replies. Yakoov got every thing right. That is exactly what I want to do. As Tony1084 said, the voltage is controlled by PWM. I am not that worried about the law because the custom LED light that I intend to use is not too bright and is in obscured place. The second reason why I don't want my DRL to be in the low beam is because I changed the OEM halogen bulb with expensive after market LED bulb. I don't want to run that bulb as DRL all the time because I want it to last.
 

Thread Starter

geselaw

Joined Dec 21, 2021
18
It seems from your description that the signal already goes from 9V in the daytime to 12V at night. Are these two separate signals (Day and Night) that need to be combined?
They are combined in one line. In the daytime when the engine run voltage will be around 9v and in the night the voltage will be automatically changed to 12v.
 

Thread Starter

geselaw

Joined Dec 21, 2021
18
A comparator circuit should suffice for deciding when the voltage is above a pre-determined setting. However, since the DRL "Dim" setting is probably PWM controlled, a comparator might see the full voltage long enough to turn it on and act like yet another headlight.

If I understand the request correctly, you want the DRL to go out but have another light acting as the DRL. My concern would be the law regulating the use of lights on a vehicle. I've heard of people being charged with "Unauthorized Use of Lighting". Not a serious charge, probably just a fix-it ticket making you restore the system to OEM settings. Would you be opposed to having the DRL on both the headlights AND your secondary lights?
Is it possible to filter/convert the PWM signal to analog?
 

crutschow

Joined Mar 14, 2008
29,508
Is it possible to filter/convert the PWM signal to analog?
Yes, a simple Low-pass RC filter ( R in series followed by a capacitor to ground).

It would seem that you just have to detect 12V from the running lights if you can power the detection circuit from the ignition accessory voltage.
That way your LED lights will be on and the low-beam off whenever the ignition accessory voltage is there and the low-beam voltage is below 12V.
At 12V, the relay will be switched to turn off your LEDs and turn on the low-beam.
Would that work?
 

JWHassler

Joined Sep 25, 2013
292
It MAY be the case that the DRL drive is PWM and the low-beam drive is uninterrupted DC.
If that's so, you could use a retriggerable one-shot to do the job.
 

Thread Starter

geselaw

Joined Dec 21, 2021
18
Yes, a simple Low-pass RC filter ( R in series followed by a capacitor to ground).

It would seem that you just have to detect 12V from the running lights if you can power the detection circuit from the ignition accessory voltage.
That way your LED lights will be on and the low-beam off whenever the ignition accessory voltage is there and the low-beam voltage is below 12V.
At 12V, the relay will be switched to turn off your LEDs and turn on the low-beam.
Would that work?
That would definitely work. I have already pulled a line from the fuse box that is controlled by ignition. I really appreciate what you are doing. So you still think retriggerable one-shot is the way to go?
 

crutschow

Joined Mar 14, 2008
29,508
So you still think retriggerable one-shot is the way to go?
Not sure.
The first is analog and the one-shot is more digital.

The analog would require a few resistors and capacitors, an LM339/393 comparator, a transistor, and a relay.
The one-shot would take a 555 timer, a few resistors and capacitors, and a relay (no transistor).

So not a big difference in parts count either way.

To design the one-shot circuit, it would be helpful to know the frequency of the PWM frequency.
 
Last edited:

Thread Starter

geselaw

Joined Dec 21, 2021
18
Not sure.
The first is analog and the one-shot is more digital.

The analog would require a few resistors and capacitors, an LM339/393 comparator, a transistor, and a relay.
The one-shot would take a 555 timer, a few resistors and capacitors, and a relay (no transistor).

So not a big difference in parts count either way.

To design the one-shot circuit, it would be helpful to know the frequency of the PWM frequency.
I ordered hand held oscilloscope. I will get it tomorrow. As soon as I do, I will post the frequency. Thank you so much for sticking with me.
 

AnalogKid

Joined Aug 1, 2013
9,956
As I understand it, you want the added DRL to turn off during night driving when the headlight is at full low beam. Yes / no - ?

If yes . . . Since it is not critical that the added DRL come on the instant the standard one does, I would go with an extra-low freqyency lowpass filter and accept the 1-second delay it takes to trip a comparator. A simple R-C filter into one-half of an LM358 as a comparator that drives an external MOSFET for the added DRL.

ak
 

Thread Starter

geselaw

Joined Dec 21, 2021
18
As I understand it, you want the added DRL to turn off during night driving when the headlight is at full low beam. Yes / no - ?

If yes . . . Since it is not critical that the added DRL come on the instant the standard one does, I would go with an extra-low freqyency lowpass filter and accept the 1-second delay it takes to trip a comparator. A simple R-C filter into one-half of an LM358 as a comparator that drives an external MOSFET for the added DRL.

ak
Hi, that is exactly what I need, I want the new DRL to stay on in the night time too.
 

crutschow

Joined Mar 14, 2008
29,508
I think I missed something. If the new DRL is on in the daytime and the nighttime, then why does it need a switch?
You did.
I don't want my DRL to be in the low beam is because I changed the OEM halogen bulb with expensive after market LED bulb. I don't want to run that bulb as DRL all the time because I want it to last.
The new DRL will only operate in the daytime (as it's acronym suggests).
 
Top