Pullup and digital monitoring

I'm trying to monitor a digital input (from an alarm) that goes to 4.5V when the alarm is turned on, but after one minute it enters surveillance mode and voltage bounces continuosly from 0.1-1.8v.
I can't use rising and falling edges as the low voltage is inferior to the high logic level (which i believe is about 2.6v)

What else can I do to know my alarm is on? Would a pullup resistor work?

If you’re working with a 5v Arduino and digital input, it should ignore the fluctuation at the 1.8v range. Digital high = 0.6 x VCC.

You need a pull down resistor too, to hold the input low until it sees the alarm voltage.
Don’t forget you need to common up the Ov lines.

As an alternative , you could use an opto isolator - has the benefit of isolating the alarm system from your Arduino. By adding a couple of diodes ( and resistor) to the input side, you could ensure it doesn’t turn on below say , 2v

Check your alarm handbook to ensure you are not looking at an open collector output ( pnp?)and that’s why you see “noise” when there is no alarm .

I’m working with a nodemcu Screenshot by Lightshot
I’ve made a little circuit with a voltage divider and a couple diodes which outputs the voltage to 3v (from 4.4)

What about tying it to a analog input and monitor a loop so that every say, 5s I must have at least 0.6/0.8 volts for a variable to stay high