logic level for Arduino

I use a opto coupler to sense the AC voltage and provide a voltage 4.5VDC for high and 1.2VDC for low to Pin 8 of arduino nano for noAV and AV exit

but i have a problem that the high voltage 4.5VDC have no problem but the 1.2VDC seem not a low level and the indication of nano output will be fluctating Low or High

can someone talk me the logic level of nano digit pin 8 for low and high??

i try to use opto coupler to detect the input AV voltage and send the 4.5VDC for high and 1.2VD for Low to Arduino pin 8 but i have the problem that the Arduino sense 4.5VDC as logic high is no problem but it sense the 1.2VDC logic not low and it is fluctating wrong singal thru my program to indicate the AC voltage absense or on

i suppose the 1.2VDC will be a low level for digitial but it is not ok for Arduino

can someone help me this????

The 328P datasheet says VIL “Input Low Voltage, except XTAL1 and RESET pin” is 0.2VCC for VCC = 1.8V - 2.4V and 0.3VCC for VCC = 2.4V - 5.5V. If you are running at 5.0V, then VIL is 1.5V so it should be OK. Did you test this with a multimeter? Are you sure that signal is solid? What does it look like on a scope? Set a trigger level at 1.5V and see if it jumps up for some reason.

Please, do not cross-post : http://forum.arduino.cc/index.php?topic=262581.0

Please do not cross-post. This wastes time and resources as people attempt to answer your question on multiple threads.

Threads merged.

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Thanks JoeN

I do not check the voltage with scope but i measure this with multimeter and the voltage is stable i use PS2505-1 for AV voltage sensing

i can not find any problem inside the electronic circuit, it show low level already

Would a pulldown resistor help?

How to use this forum

You need to show your code and show your circuit, not just talk about them.

People are not replying because the whole thing is so vague.

See JoelN's reply above for the highest you can have for LOW on a digital input running at 5V (namely 1.5V).

i suppose the 1.2VDC will be a low level for digitial but it is not ok for Arduino

Yes it is OK for the Arduino. Post the details I requested.

A meter cleans up a lot of the noise. That could be a fairly noisy 1.2V, and still show up as "stable" on a DMM.