Connecting a Circuit which is not powered by arduino.

I'm trying to connect a circuit which sends pulse to the arduino when triggered. The pin connected to the circuit is set as INPUT. I first connected the pin on the circuit, after which, I connected the ground of the circuit to the ground pin of the arduino. The arduino has no power supply connected yet, but all of a sudden, it kinda powered up the arduino after connecting the two ground, it's like the circuit is supplying voltage to the arduino. Is it safe? The circuit is run by 9v DC. I also tried to measure the voltage of the pulse that will trigger the pin in the arduino and the reading is 2v.

You could use an opto device like a 4N25: then no matter what the voltage in the "other" circuit, the 5V pullup on the "arduino side" will make sure the pin is always be 5 or 0.

(The logic will be reversed, you can fix that in code: a high on your "other" circuit will close the transistor so the Arduino pin will ground and show a low.)

There's a nice pic at the bottom of this page. That pullup could be the Arduino's internal one.

I don't think that connecting a powered circuit to an unpowered Arduino input is safe. I believe the phenomenon is called parasitic power and can damage an input. Also, measuring the voltage of a pulse train with a multimeter is not going to be meaningful as the meter reports an average. So the actual peak voltage can be much higher than read.

The arduino has no power supply connected yet, but all of a sudden, it kinda powered up the arduino after connecting the two ground, it's like the circuit is supplying voltage to the arduino. Is it safe?

Well it won't cause you any problem but it is not safe for the components.

Yes groundfungus is spot on with the name. It can not only damage the input but your whole Arduino.

You must never apply a signal to a chip that is not powered up.

JimboZA:
You could use an opto device like a 4N25: then no matter what the voltage in the "other" circuit, the 5V pullup on the "arduino side" will make sure the pin is always be 5 or 0.

(The logic will be reversed, you can fix that in code: a high on your "other" circuit will close the transistor so the Arduino pin will ground and show a low.)

There's a nice pic at the bottom of this page. That pullup could be the Arduino's internal one.

I'll go find this component. thanks for the advice sir.

Grumpy_Mike:

Well it won't cause you any problem but it is not safe for the components.

Yes groundfungus is spot on with the name. It can not only damage the input but your whole Arduino.

You must never apply a signal to a chip that is not powered up.

So if I powered up my arduino, and connected that "other" circuit, I wont have any problem? I'm suspecting maybe the arduino's source and the other circuit's voltage would add up and it might blow up my arduino.

So if I powered up my arduino, and connected that "other" circuit, I wont have any problem?

Yes that is correct.

I'm suspecting maybe the arduino's source and the other circuit's voltage would add up and it might blow up my arduino.

How is that going to happen?

You need to clarify though: are you sure the other side's pulse is less than 5V? You said it seems to be 2V, but as pointed out it's difficult to measure the size of a pulse with a meter. You'd need a 'scope to be sure.

If the pulse is more than 5V, you'll do damage unless you reduce it somehow and the 4N25 is a simple way of isolating higher than 5V voltages from the 5V world.