How to read 12 V signal with arduino in the most easyest way in my circuits

Hello guys,

I have a circuit that is shown in the image bellow. I use 5 V reed relays to turn ON/OFF 12 V LAMP. I have to check if the circuits is connected propery - so the rules here are: after we turn on relay with Arduino, if the circuit is connected properly, tle lamp is ON, otherwise the lamp is OFF. It is not necessery that when we turn on Relay, the lamp is ON. It is also possible that some connectivity isues happened in the circuits and the lamp is them OFF. So, we have to read somehow if the lamp is ON or OFF when we turn on relay.

Since Arduino can only read 5 V signal, I have to ask any of you what is the easyest way to read that signal (that will tell us if the lamp was ON/OFF). I know that we can do it by voltage dividor bit this is too much to integrate in the existing circuit...

any advice very welcome

With a voltage divider on the relay side of the lightbulb you can drop 12volt to 5volt, and detect if the light is off and if there is 12volt supply. You can't detect if the light is on.
For that you need to measure current through the bulb, or see if there is light with a light sensor.

You could use a voltage divider like this, but as Wawa said, it will only tell you if the light is powered, not if it’s really on:

So like this?

Any other way to do this?

How much current does the lamp draw?

You must have a diode across the relay coil to protect output pin from inductive kickback, a 1N4148 will do with the cathode (end with stripe) toward +.