# Help Needed Wiring a Low-Volt switch for 110v circuit

NoTime
Homework Helper
berkeman said:
Also, you won't typically connect LEDs in series, even though that would save power in your application. The problem is that unless the LEDs are well-matched, they will not be the same brighness necessarily when being run at the same exact current.
Unless there is a significant difference in the led internal resistance(not likely) to match the difference in conversion efficiency. You are going to get the same brightness variation series or parallel.
What you are talking about is what happens when you use a single droping resistor to feed multiple parallel leds.

berkeman
Mentor
NoTime said:
Unless there is a significant difference in the led internal resistance(not likely) to match the difference in conversion efficiency. You are going to get the same brightness variation series or parallel.
What you are talking about is what happens when you use a single droping resistor to feed multiple parallel leds.
Interesting....I wonder if that's what happened. I just remember that a long time ago I tried to save on something (I could have sworn it was putting them in series, though) with LEDs, and one turned on and the other didn't. Guess I'll have to go run a quick experiment....when I have time

EDIT -- To the OP, it might make more sense to use 4xAA or 4xAAA batteries, since you wouldn't have to waste so much of the voltage. That might be too bulky for your intended app, though. You could always just do a buck down from the 9V battery to about 3-4V instead.

NoTime
Homework Helper
berkeman said:
Interesting....I wonder if that's what happened. I just remember that a long time ago I tried to save on something (I could have sworn it was putting them in series, though) with LEDs, and one turned on and the other didn't. Guess I'll have to go run a quick experiment....when I have time
You can work this out just by doing the math

LEDs are current mode devices. You need a change in current to get a change in brightness. Note that a small change in Von can make a big difference with a single drop resistor. Different Von means that the color is different as well. A blue led will have a Von of around 3.6v.

berkeman said:
EDIT -- To the OP, it might make more sense to use 4xAA or 4xAAA batteries, since you wouldn't have to waste so much of the voltage. That might be too bulky for your intended app, though. You could always just do a buck down from the 9V battery to about 3-4V instead.
If you are going to go through the trouble of designing active circuitry look into pulse mode.
Big increase in apparent brightness. Big decrease in power consumed.
Conceptually less complex than buck.
Just a 555 and a switch device.

berkeman
Mentor
NoTime said:
You can work this out just by doing the math
Yeah, I'll go back and figure out what had me spooked on the series connection. Could be I was working with jellybean LEDs mixed in a labstock drawer or something. No time at the moment to try it though (hey, that's funny!) -- we're in the middle of DVT on first Si. Busy times. (hey, that's funny too!)