Laptop charger with a three-pin output?

Hello, I have this power supply and would like to know where the "+" and the "-" output would be since the power supply has a three-pin output

With a little luck you will find the schematic on the power supply itself ;-)

That is on the power supply. Submit picture of power supply.

The middle pin is only used for communication between the power supply and the laptop.

If it is neither on the power supply nor on the device operated with it, the easiest way would be to snap up your multimeter and measure it.

Unless you don't have a multimeter (or at least a voltmeter). In this case, the exact model name (at least that of the power supply unit would be very helpful.

Otherwise, in many (or most, but by no means all) cases:

Outside: -
Inside: +
If three-pole, pin: sensor data, PWM, etc.

I still have a universal power supply which only has two outputs, would it still work without the middle pin?

You have to try that. But make sure that the polarity and voltage are correct.

I still have a universal power supply which only has two outputs, would it still work without the middle pin?

Perhaps. It depends on how the contacts are assigned on the device side and whether the third is mandatory or optional.

Outside -

Inside +

Center pin… Data pin

Could I run the laptop without a pencil?

No.