Ok, I'm about to go bald pulling my hair out on this one, and it's driving me crazy too....
I've got an arduino pro mini, I'm making a temperature sensor for my truck. It consists of the temperature sensor and a 4 digit 7 segment LED display.
If I use a FTDI board and a six pin header on the arduino pro mini, everything is perfectly fine. Readings are around 72 to 74 degrees Fahrenheit which another thermometer in the room agrees with (saying 73).
Remove the six pin header and connect a 9 volt battery through a 5v voltage regulator to the VCC pin, and the ground to the GND pin, and now the sensor reads 130 to 140 degrees. I've tried putting the power into the RAW pin, as well as the ground on the BLK pin since that seems to be where the FTDI's ground actually is. And have tried all variations there in and it doesn't matter, it still reads 130 to 140 degrees.
I'm stumped on this one, have checked and made sure all connections are being made good and everything checks out ok. Even taking a direct reading using a voltage meter on the pin and converting it manually the numbers seem to come out right. So the arduino somehow either isn't reading something correctly, or has just lost it's mind lol. But if I put it back on the USB, everything works fine again.
I've verified that 5 volts is getting through the voltage regulator, and it's a 1 amp regulator which should be more than enough amperage for this circuit...
Can anyone offer any advice on this please before I pull all of my hair out.
Thanks for the reply, just figured it out, I guess. I removed the voltage regulator from the circuit and it works fine now. I don't understand why it causes it to not work when it's supplying the same amount of power as the usb cable, but maybe 1 amp isn't enough... I don't know seems like if 1 amp wasn't enough it would simply not work, not show reads 60 to 70 degrees higher than what it should be. Since the arduino pro mini 5v is supposed to be able to work on 5v to 12v of power I'm assuming it will be ok in my truck on an accessory circuit. I'm more worried about pushing to many amps through it than to many volts lol.
don't understand why it causes it to not work when it's supplying the same amount of power as the usb cable, but maybe 1 amp isn't enough...
Keep in mind that Voltage, Current, and Power are all very specific terms. They are very different (related) things, but you are interchanging their usage. Voltage is "Pressure", Current is "Flow", and Power is a function of both. Power = Voltage times Current
Current is "sourced" and "drawn." So a source rated for 1Amp means it is capable of suppling 1Amp. A 7805 (and similar linear regulator) can source 5V @ 1A if they are properly heat sinked. (Which would be 5V * 1A = 5Watts of Power.) Other wise their output capacity could be as low as 100mA.
So your comment "they provide the same power" isn't correct. USB and the Regulator both provide 5Volts. However, their Power capability is different since USB only sources 500mA and the regulator can (under the right conditions) source 1A.
It sounds like you were connecting your 5V regulated output to VIN and not the 5V node. So the on board regulator was trying to regulate 5V-in to 5V-out, which doesn't work.
Is you temperature sensor being read with a analog input pin? If so keep in mind that the actual chip voltage on it's Vcc pin is used as the analog reference voltage. More often then not the exact voltage from the USB and on-board regulator chip may be slightly different but still within specifications, but it can cause a slight calibration shift in the voltage readings you get with the analogRead() commands.