Using transistor as a switch for a 12V 1.5Amp Led strip from Arduino Uno

Projects General Guidance
1

Hi All,

I am a newbie working on my first project, which is to turn on a led strip when motion is detected. The LED strip is powered by a 12V 1.5A supply. I'm using a NPN ( TIP31Ag ) from radioshack, which does nothing when i connect it all up. The specifications on the transistor are:

hfe(min) : 10-50
Vceo : 40V
Vcbo : 40V
Vebo : 5V
Ic : 3 A
FT : 3 Mhz
Dissipation : 20W

I've connected the led strip positive to the 12V positive, the led strip gnd to collector,
Arduino pin 13 to Base,
Emitter to 12v gnd + arduino GND

Followed the following article and made the connections: http://bildr.org/2011/03/high-power-control-with-arduino-and-tip120/

Any help would be greatly appreciated.

Thanks

2

so, you selected the Radio Shack transistor based on your first post?

This information should be a continuation of that first post, for continuity. Forum Moderators prefer continuous posts.

Here is an approach. It is OK to ask, but you are expected to do some research first.

Ray

3

Hi,
Can you please post a copy of your sketch, using code tags?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?
NOT A FRITZ diagram.
If you are aiming at stairway lighting, us the "Search the Adruino Forum" in the top right hand of screen, this has been done many times before so I suggest before re-inventing the wheel you use this search facility.

Tom..... :slight_smile:

4

Sounds like you have it wired wrong. NPNs want to connect LED- to GND.

LEDwithResistor.jpg
LEDwithResistor.jpg960×720 37.7 KB

5

You're missing the base resistor at least.
Check your wiring, make sure you have NPN and not PNP

6

Thanks, @CrossRoads, I have attached a hand drawn diagram of my connections. The transistor is a NPN from radioshack. I did some research on finding the right transistor, and the formula used to determine which one to use was : Ic/Ib = Hfe

My understanding is that Ic would be 1.5A, Ib would be 50mA( based on arduino specs ), so, was looking for a Hfe of 30 ( 1.5A/50mA ). What i'm confused about is the variable Hfe on the radioshack datasheet, which says Hfe = 10 to 50. I tried finding more information about this, but could not.

Also, sorry, but i don't completely understand the diagram you attached. Are the Vf and resistance for the led strip approximations, or calculated?

IMG_1299.JPG
IMG_1299.JPG2448×3264 388 KB

7

Too big - can't pan around to make it all out. Resize to about 1000 wide.

8

From the datasheet:
Base−Emitter On Voltage
(IC = 3.0 Adc, VCE = 4.0 Vdc)
VBE(on): 1.8 Vdc

Suggest you measure Vbe and adjust the base resistor to allow more current flow.
(5V - 1.8 )/.035A = 91 ohm.
Drop down from 2K to 180, 150, 120, 100, keep an eye to not exceed 35mA to avoid damaging the Arduino IO pin.
You'll likely see the output voltage drop as current increases - spec'ed output is 4.2V at20mA, and voltage dropping as the mA increases.

You have a 12V to 5V converter - that needs to connect to 5V on the power header, not Vin. Vin feeds the 5V regulator, it needs to be >= ~ 6.5V for the regulator to work.