Dirty Power Issue?

A friend of mine and I have been working on reading some sensors in his old Jeep CJ5 and ran into a snag.

I built a voltage regulator circuit with an LM1084-adj, outputting 9V that is powering over the barrel jack. The output hot-line is switched with a toggle switch. This powers an Arduino Uno, which in turn powers a 20x4 character lcd screen. The sensors we are measuring are the fuel tank potentiometers and the engine temperature sensor (thermistor). To measure the resistance I'm using a voltage divider off of the 5V header. The 5V has a 2.4K resistor going to each analog pin which then goes to the pot/thermistor to ground.

When the regulator power switch is off, and the arduino/screen are powered over USB connected to a laptop, the values are fine. However when the power is coming from the voltage regulator circuit, everything powers fine, the Uno and the screen communicate, but the analog pins read 0. If the barrel jack is disconnected completely, the value maxes, as would be expected with a disconnected ground.

I'm curious if people think this is an issue with the voltage regulator used, as there is no isolation. I've never used isolation that needed to measure referencing the power source's ground, as this would require. Is there such a thing?

One interesting/concerning thing that happened was: power and ground are coming off the keyswitch, (I'm pretty sure they're tied straight to the battery, but I'll verify next time I head over there) when the key was turned to engage the starter, the arduino and lcd screen both lit up. This was with the regulator switched to the off position, but ground is a straight through connection. Any thoughts?

What is the max resistance of the pot? What is the resistance (hot and cold) of the thermistor?

The values of the pots/thermistor range from 70-200 ohms. I was reading somewhere that if the 5V rail of the uno board drops too low, the analogRead() stops working. I'm thinking I'll change the resistors on the LM1084 to make it give 5V and power through the USB port to see if it's an issue with the on-board 5V regulator not being able to supply enough power.

Hi, telling us how its connected is probably not the best way, can you post a CAD in png jpg or pdf, or a pic of a hand drawn circuit please. Show all your power connections including gnds.

Hope to help.. Tom..... :)

I changed it a bit. Instead of being 9v through the barrel jack, its 5v through the USB jack. It’s still not giving the right values. Attached should be a diagram of the current setup.