Isolating digital inputs

08 Jul 2011

I am trying to connect my mbed to my home alarm so that I can monitor the status of the system. The alarm unit provides 8 12v outputs indicating it's state. What is the best way to connect these inputs to the mbed. I assume that I need to keep the systems isolated and was looking at some kind of optoisolator. Is this a good way to go and if so can anyone provide any examples of circuits and/or components.


08 Jul 2011

Hi Nick,

An optocoupler is one possible solution but it's a really good one. If you Google “optocoupler circuits” you are going to find what you need. An alternative method is to select a general purpose device (you don’t need any special and expensive part at least from what you describe) and then look at the final pages of the datasheet or even for any available application note.

Have a look here as well:



For now don’t care about parasitic resistors and capacitance that you will find in the pdf’s above. You don’t have fast transients and time critical signals so the standard formulas with the basic peripheral components will work perfectly!

08 Jul 2011

Why do you need isolation? They are both low voltage systems. If you have a common ground you could just use a resistive divider 1:4.

08 Jul 2011

Sparkfun has an opto-isolation breakout board


If you have long wires and when you cycle two different power supplies on and off, if might not be a bad idea.