Safe to read 9v signal from 5v pin, or transistor needed?

Other Hardware General Electronics
1

I'm powering a Trinket pro via the batt pin with a 9v wall wart.
The 9v supply is also powering another device. That device has a master on/off switch. To read the state of the device, you have to monitor the positive current running to it.
It seems like a bad idea to me to hook the 9v positive signal carrying a ton of current to a digital pin on the Arduino.
Is this something that should be solved by a transistor? Should I use a regulator?

Sorry for the simple question. Just looking for feedback before I potentially set my house on fire.

2

You can use a voltage divider to sense voltages above the microcontroller's supply voltage. 9V will destroy an
Arduino without some protection. Divide by 2 using 2 10k resistors would work for instance.

3

Your 9V wall wart is likely unregulated and actually supplies 12-15V with little to no load.

4

I'm not clear about what you're measuring. Is it the current flowing to the other device? How are you going to measure that?

5

MarkT:
You can use a voltage divider to sense voltages above the microcontroller's supply voltage. 9V will destroy an
Arduino without some protection. Divide by 2 using 2 10k resistors would work for instance.

Voltage divider - Wikipedia

Thank you very much. I will look into this.

6

MK1888:
Your 9V wall wart is likely unregulated and actually supplies 12-15V with little to no load.

No, it's regulated and supplies 9.5 volts.

7

Dr-Zee:
No, it's regulated and supplies 9.5 volts.

What is the voltage with no load?

8

SteveMann:
What is the voltage with no load?

9.5 volts