super basic help

Is the - (or GND) of your battery connected to the GND on your Arduino?

Check if you used the same GND with the arduino and the power-supply in the switched circuit.
Have you connected Drain, Source and Gate correctly?

Not a FET, an ordinary transistor

>>919589
What this guys says.

I once had my pin order backwards because the data sheet had the pinout from the "bottom view" while I assumed it was "top view"

>>919584
If you are switching an LED with an 80mA source, you don't need the BJT. Just use the current limiting resistor.

>>919584
I'll say what I did in your /b/ thread, make sure you're using the right kind of transistor. Seen people do that a lot.

>>919584
did you figure it out OP? I'm curious now.

>>919594
>Not a FET, an ordinary transistor
If its a bipolar transisator you need a resistor at the gate/basis. 1kOhm is fine.

>>919613
If it's being used as a switch you don't need to bother biasing the base (bipolars do not have a 'gate').

>>919925
I think he meant a limiting resistor between the arduino IO and the base, not a biasing resistor from the base to VCC. Without a limiting resistor the 4.5v from the IO pin connects almost directly to ground through the resistor; exceeding the maximum current output and causing a brownout or other problems.

>>919928
That's sensible, yes, especially if it's being driven with something that runs off a 5V rail.

>>919594
>Not a FET, an ordinary transistor
what transistor is it? (what is ALL the printing on it)...

when it should be turned on, do you get any voltage at all between the emitter and ground?

>>919584
base to o/i with a series resistor. (try 1k or something)
emitter to ground and collector to whatever load you want to pull down

As has already been said:
-needs a base resistor
(roughly: R<
( (4.5v-.7v)*DCgain)/collector_current)
-connect the grounds from the arduino to that circuit.
Also:
-The Voltage needs to be sufficiently high to cover the LED voltage and the drop across the transistor. (saturation voltage)

What transistor are you using?
What are you actually trying to power?

>>921487
> needs a base resistor
Or use an emitter-follower, i.e. put the LED and resistor between the emitter and the battery negative, and connect the collector directly to the battery positive.

That way, you don't need to determine a base current; it will be whatever it needs to be.