I have a controller which operates a relay.
The relay is normally deenergised .
I wish to sense when it is energised.as simple as it sounds...
The + pin on the controller which goes to the relay is always energised.. The controller cuts/connects the ground pin to energise or deenergise the relay... Ho do i sense the state of the relay.
Monitor the -pin, when deenergized the pin will go high. What is the +pin voltage? Use a resistor voltage divider if more than 5V.
Does the relay have any spare output contacts you can use?
You could look at the base voltage on the transistor driving the relay. That voltage is coming from the controller and may be a logic-level signal.
What is the relay coil voltage? You may be able to measure the voltage at the negative side of the coil. That should read HIGH when the relay is not energized and LOW when the relay is energized.
if the voltage to the relay is always high, you can monitor that.
you will only know when there is voltage or when there is no voltage.
from that you can imply that the relay is #1) present #2) has voltage #3) energized.
if the relay has an un-used set of contacts, you can use those to monitor the state of the relay.
if you put one wire on, you can know that there is a completed circuit or an uncompleted circuit
and you can imply that the relay is in one state or the other.
if you add a resistor to one output and a voltage divider to the other, you can set them to get 5v when one is passing power and then 2.5 if the other is passing power. then you know :
#1) the relay is present
#2) the actual state of the relay.
alternatively you can tie into the control wire on the relay and when there is power, you can monitor that your device is getting power or not getting power. as was noted, you may need to use a voltage divider, or maybe an opto-isolator.
alternatively, if it is is a line voltage, you can add a current transformer and get motor amps on the line and then know that the device connected is operating.