Are mA proportional to Volts?

So here’s the deal. I’m just starting out with electronics and Arduino. I bought a 12VDC Siren at RadioShack. It says the Voltage rating is 6-14VDC and the current consumption is 150mA max at 12VDC. I know the Arduino has a max output of 40mA on a pin. I hooked the siren up and it worked on 5V. (Hopefully I didn’t damage something). But when I try with any resistors it doesn’t seem to work. I tried 100, 150, and 10 ohms. So is mA proportional to Volts and when I drive it through 5V the mA are low enough to work? Or is it working anyways even though its drawing more than 40mA and I hopefully didn’t break it? Any way to make it work or do I need a different siren to be safe?

Thank you all in advance.

Try here:

Look up ohms law... I = E/R -- just to start you off -- make sure the units are consistent.

You're getting some kind of marginal operation. I'm sure there's some active circuitry in the siren module. What the current draw is cannot be deduced simply by 12/5 = 150/x You should connect the siren to a transistor and turn the transistor on/off with an i/o pin. I'm sure that it'll be a lot louder that way, too.

Thanks both of you! That helps

Ohm's law doesn't apply to active circuitry, only to conductors. If it works at 5V OK then you'll need a switch (MOSFET or relay) to control it from the Arduino (as its an active device the current at switch-on could be a lot higher than the steady-state current anyway). However its rated as 6V..14V so 5V may be borderline. If you power the Arduino from Vin then that supply is the one to use.