Basic(?) Project Questions

[Rigolon]

I'm not sure how to name this topic, but i have some curiosity that perhaps are common knowledge.

First, I have seen in some circuits, that use a relay output, that the output is inverted. The NO of the relay is considered NC, because when the circuit is powered on the relay is also powered. Why is that?
I've seen this mostly on sensors, infrared ones, those used on automatic gates so the gate don't close on the car.
I thought that maybe is made like this to identify that the relay might not be working, but in this case there are a lot of things that can be happening other than the relay is broken.

Second question, when using a digital input of a MCU, is it better to project it as HIGH (with pull-up resistor) when nothing is happening or as LOW? Given the fact that I don't have anything that forces me to use one or another.

[james_s]

Generally it's done so that if the device fails or loses power, the contacts fail closed but it depends on the circuit.

Doesn't really matter, it depends on what you're driving it with and what makes the firmware easier to write. I generally use high = on for outputs but there can be reasons to use negative logic.

[ataradov]

The coil of the relay is often normally energized on internal doors with key card entry. This is done for fire safety. If electricity is cut off, all internal locks will default to the unlocked state.

[tpowell1830]

Generally speaking, but not always, when a circuit is wired to a relay contact to the NO side of the contact and provides power when the relay is energized, it is a fail safe setup. If power fails, nothing is energized. Sometimes systems have battery backups and are driven on DC through the relay. Other commenters have stated similar reasons.

As far as the digital input is concerned, this setup is so that you can utilize either, depending on your requirements or ease of use. If there are safety concerns, again use your logic (your brain) to best determine what would be the optimal fail safe situation. Otherwise, it is a function of the type of input that you are using. If you have to add extra components in order to achieve either a pullup or pulldown situation, it may be advantageous to use the opposite input logic (the one where you don't have to put extra components).

Poorly explained, but I hope you get the gist.

Hope this helps.