I have a relatively low power application that includes 1 uno, 1 serial shield, and 1 PIR. I'm guessing that I should be pushing no more than 150ma max. I really want to power this off the USB of my TV, since it is controlling the TV via it's serial port. The issue is that the PIR becomes completely unreliable off that power source. It works perfect when the Uno is connected to a 9V battery. Are there any options I should consider? I'm not sure if this is a noise problem, or related to the actual power. My PIR pulls off the 5v leg on the Uno.
Does it work properly when powered from the USB port on your PC?
If so it suggests that the TV USB port is underpowered.
If not, it suggests that you need a separate power source.
USB power is usually incredibly noisy - bad for any sensitive analog circuitry.
I was using a usb to 3.5mm jack adapter to power my Arduino, which was causing the problem. When I switched to using a standard USB male A-B, the unit works perfect. Is there some sort of software negotiation that happens before a USB port provides "full power"?
Where were you connecting the power into the Arduino? If using Vin you loose too much across the regulator.
USB power is 5v and should be fed into the 5v in.