Small electrical project

Old 09-16-14, 08:43 AM
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Small electrical project

This should probably be in "Hobby", but I didn't quickly see any category in which it fit.

I'm testing out a way to add Daytime Running Lights to my car that are on whenever the car is on (easy), and also turns on my taillights (more difficult). Small scale testing has provided me with an interesting observation, and I was wondering if anyone can explain it.

Using a single automotive relay with 87a connection (SPDT) and a 12v 23a battery;

Setup #1
30 - Positive
85 - Switch/ground
86 - Positive
87 - Led (then to ground)

Closing the switch triggers the relay (nice click), lights the Led, but then the led quickly dims. Opening the switch causes the led to flash bright and turn off.

Setup #2
Using two different 23a batteries;
30 - Positive #2
85 - Switch/ground #1
86 - Positive #1
87 - Led (then ground #2)

This setup works fine. Good click, bright light, lasted until I was bored enough to disassemble it. I have a bit of trouble believing that it works this way just because two batteries provide a deeper well of power (volts would be the same). But, I only just now realized that I could try setup #1 with two batteries in parallel to check on that.

Any ideas on why #1 would cause Leds to dim so quickly?
Old 09-16-14, 08:48 AM
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Either the battery is really low, or you didn't use the right resistor with your LED. (Unless it's a self-contained module)
Old 09-16-14, 08:50 AM
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The relay draws more than that battery should be delivering.
You need to be using one of those 12vdc wall warts. You probably have a spare one in your hobby drawer.
Old 09-16-14, 09:05 AM
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Just a note...DRLs are normally at a lower power than even low beams. If you turn the lights on manually, they come to full power based on the H/L position of the lever and the DRL circuit is bypassed.

Using your set-up, they will be at full (low power, depending on wiring connection) all the time.

I'm just wondering, why not train yourself to turn the lights on low when you start the car? Even if you forget to turn them off the first few times, you'll get the warning chime.
Old 09-16-14, 09:48 AM
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Relay draws 200ma. Maybe that's too much, but it doesn't seem like a lot. Again, this is only for testing, not production. I'll eventually be using the car's battery.

DRLs will be with LED halos. Total of 4 LEDs for both halos. That's less draw than the relay itself.

I have conditioned myself to turn on the headlights at all times. Been that way for 20 years (first two cars had no warning chimes (good times). I want to switch to automatically on because Nissan makes two different dashboard settings. When headlights are off, the dashboard can light up quite brightly; when headlights are on, the dash is dim (since they expect lights to be on at night). So, the dash can sometimes be hard to see in daytime with my "lights on for safety".

My car also has auto headlights that turn themselves on when it gets darker. Nice feature that I'd like to use, but I like the safety of DRL more than that. This will allow for much lower-powered (and cooler looking, imo) halos to serve as my DRL (and activate my LED taillights) for daytime, but resorting to normal function when the lights are actually turned on.

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