What I have is a set of USB powered woot.com Woot-Off rotating lights that I bought to use as an attention-getting indicator for projects. They plug in to a single USB port and there are two rotating beacons. The data lines are not used, only power.
I broke out a USB extension cable’s wires to measure that this device draws 103 mA. So, that’s too much for an output pin to power directly.
I’m trying out a “Common BJT Transistor - NPN 2N3904” from here: http://www.sparkfun.com/commerce/product_info.php?products_id=521
I have the transistor wired like this:
C - 5V pin on Arduino
B - Arduino pin 5.
E - positive pin on my USB power jack* the lights are plugged into
*USB power jack is the female end of a usb extension cable soldered to header pins so I can use it with a breadboard. Negative pin is wired to ground. The light device is plugged into the jack.
When I set pin 5 HIGH and the device disconnected, I’m only seeing 4.5 V at the emitter, instead of 5 V which I kind of expected. (I measured 4.0 V when the lights are connected.) As a result, the rotating lights work somewhat inconsistently and I’m sure they aren’t happy being slightly underpowered.
Am I using this transistor correctly?
From what I’ve read I thought the transistor would essentially act as a switch. What can I do differently to supply this output device with 5V, 103 mA? As a test, running my Arduino from external power vs. USB power doesn’t improve the situation.
Thanks in advance!