Electronic – Need to convert a 12V/300ma alarm output to a dry signal that will interface with the security DVR

You are going to need an interposing transistor driver between the microcontroller and the relay.

There are two reasons for this:

1) Your relay coil takes more current than the microcontroller can supply.

2) The output voltage of the microcontroller is either 0V or 5V. The bottom end of your relay coil has 12V on it when it is not pulled to ground.

You don't need an optocoupler unless there is a need to electrically isolate the controller power supply from the relay coil power supply.

^{simulate this circuit – Schematic created using CircuitLab}

The above assumes that you are using a relay with built-in flyback suppression diode and the top end of the relay coil is tied to your 12V power rail.

Note that the ground of the 12V rail is tied to the controller ground.

Just want to post the resolution in case anyone else ends up here with the same issue.

First, I have to admit I was being lazy because I didn't want to remove the camera from its mount to bring it to a place where I could do tests.

I checked the voltage across camera pins 2 and 4. I found 5VDC. I connected camera pins 2 and 4 to the relay pins DC+ and DC-. Resulted in green "power" LED illuminated.

Next, I connected camera pin 1 to relay pin "IN." To test, I went to the camera setup page and changed the digital output state from NO to NC. No effect. I then moved the relay jumper from low to high. At that point, I was able to change relay by changing the digital output state on the camera.

Calling it solved. Now I just need to decide where to mount the relay and how to make the connections.

Thanks for all the assistance. I love this site!