Transforming voltage from 12v to 3-5v

ericthompson

I am trying to accept an IO from a doorbell in order to activate some cameras. Right now I have 12V at the controller, and I understand I need to reduce this to 3-5V. Does anyone have any experience doing this? My electrician doesn't have a clue. I figured I would need some kind of in-line resistor, which I found at Radio Shack...but which one?

ericmedley

ericthompson wrote:

I am trying to accept an IO from a doorbell in order to activate some cameras. Right now I have 12V at the controller, and I understand I need to reduce this to 3-5V. Does anyone have any experience doing this? My electrician doesn't have a clue. I figured I would need some kind of in-line resistor, which I found at Radio Shack...but which one?

Here's what I'd do. I've never had much luck with sensing a voltage with the IO's. It has more to do with the fact that the voltages coming in can vary quite a bit. You have to deal with the length of the wire and gauge and whatnot already. Your best bet is to install a variable resistor at the AMX side and dial it in to the appropriate voltage. This is going to affect the doorbell side of things too. (not as much energy to activate the solenoids)

However, I'd do this instead. Put a relay on the same line as the doorbell. so in other words. pushing the doorbell activates the relay. Then use the open/closed circuit setting on the AMX I/O instead of voltage. Much more reliable. (as long as the relay lasts.) Most heavy duty relays will last a long time without problems. In addition, this isolates your AMX system from the power of the doorbell system, thus advoiding possible ground loops, DC offset and a whole host of other possible problems.

All this is assuming you're talking about DC and not AC...

Here's a diagram of both ideas. (drawing not to scale) : )



Just a thought.
ejm

JohnMichnr

Yeah - a relay would be quick and easy - just get one that is the same 12 volts as you doorbell, and take ground and the I/O conection to the contact side of the relay. You want as much isolation as possible in something like this. If you want to sense voltage from teh doorbell then you have to connect the gound of the doorbell to teh ground of the AMX system. And that is the same ground point as any Axlink connection you might have, so any issues in the doorbell ground can effect your system throughout...

DHawthorne

I would always use a relay in these kinds of situations. Besides it being easier than trying to figure what resistor you need for the proper voltage drop, you save yourself an awful lot of grief by isolating your equipment. Power surges and ground faults won't cross an open relay, and often the fact that your two devices run off different power sources will cause all manner of subtle problems.

I had a case once when we had an audio zone sub-zoned into two areas that could be turned on and off independantly, but ran off the same switcher output. The device used to mute the zone not in use used a 12 volt trigger, and we used an Axcent3 relay output to toggle the trigger. However, the installer, considering it a "better" way to do it, was breaking the ground with the relay closure instead of the hot lead. In theory, it should have been fine, but the actual audio connections completed the ground no matter the state of the relay, so the zone never shut off. Now, I understand this example is not quite the same thing as we are discussing, but the concept is there was a signal path we were not aware of because of the way the devices were interconnected (in this case, I considered it a design fault - the trigger mechanism should have been isolated from the audio paths internally). If you trigger an IO directly with voltage from your monitored device, you run a similar risk. There may be another signal path due to ground differentilal, or what-have-you. Isolate it with a relay, and you are far safer.