Need A Little Help with Resistors

Thread Starter


Joined Sep 20, 2011
My circuits knowledge is limited at best so I need a little help with this.

I know the basics, unfortunately I don't know enough to do this. First, a backstory: Ever since our neighbors built a house above ours, delivery people constantly make mistakes and deliver stuff to them, it's getting annoying, even though I have numbers on my mailbox and my house.

The numbers on my house are just plastic numbers with no lights. I want to put some LED's in the numbers to light them up.

What I was thinking was 10 (3-4 per number) 5mm high-brightness 20mA, 3.2v LEDs running off one or 2 9v batteries. I know how to wire up a single LED but I don't know about multiple. I used a calculator which said for wiring it up in parallel I would need a 33 OHM, 2W resistor.

This is where I'm stuck. My local RadioShack only sells resistors up to 10, 100 and 1K OHM, 1W resistors.

Any help is appreciated. By the way, I also do have an outlet right above the numbers if I need to draw more power, but I would rather not get into using high voltage.


Joined Jul 31, 2011
You can run leds in series, but you can't parallel them directly. What you need to do is have 5 pairs of series leds, each pair with its own resistor.

If the forward voltage for the leds is about 3V, then, with a 9V battery you have 6V across each pair, leaving another 3V for the resistor to define operating current. For 20mA, about 150 ohms is needed. If you really only have access to 100 ohm resistors, you can make 150 ohms by paralleling a couple for 50 ohms, then can add another in series to give 150 ohms.

Multiply this 5 times, and you have 10 leds running at 20mA from a single 9V battery!


Joined Jun 30, 2011
Running 10x 3.2v LEDs at 20mA will draw 100mA from a single 9v battery, so if you have a 600mAh 9v battery you'd only get at best 6 hours illumination before the battery is exhausted.

Rich (BB code):
+----|>|----|>|---/\/\/----+  R = 150 ohms 
+----|>|----|>|---/\/\/----+  R = 150 ohms 
+----|>|----|>|---/\/\/----+  R = 150 ohms 
+----|>|----|>|---/\/\/----+  R = 150 ohms 
+----|>|----|>|---/\/\/----+  R = 150 ohms
Doubling up and running two batteries in series for 18v will actually drop the current draw to 40mA because you can string 5 LEDs together in series and have lower energy lost in the resistors. Using the two batteries you'd still only get 15 hours use out of them.

Rich (BB code):
+----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 100 ohms     

+----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 100 ohms

If I wanted to do this I'd consider running the LEDs at a far lower current or use a 12v plug in power brick to power them as changing batteries will soon become tiresome. In addition as the batteries use up their stored energy the voltage will drop down and the brightness will go with it.