Arduino Input Isolation

[petez69]

Hi Guys

Just curious as to peoples thinking here, looking for better ideas.

I'm looking to connect a bunch (5+) PIRs to an arduino, the PIRs have N/C relay contact that go open in alarm state. I can use a 4n25 opto and a 330 ohm resistor and use the N/C relay on the PIR to turn the LED on and off( Relay jumpers the opto LED) ....

Anyone have a more elegant way to do this ?

Tnx Pete

[randy]

Hi Guys

Just curious as to peoples thinking here, looking for better ideas.

I'm looking to connect a bunch (5+) PIRs to an arduino, the PIRs have N/C relay contact that go open in alarm state. I can use a 4n25 opto and a 330 ohm resistor and use the N/C relay on the PIR to turn the LED on and off( Relay jumpers the opto LED) ....

Anyone have a more elegant way to do this ?

Tnx Pete

I had perfectly reliable results with the N.C. contacts directly between an input and ground, using only a 1uf N.P capacitor at the board. All of my hydronic valves (N.O.), yard moisture sensors (N.O.), and push buttons use the same. Never a false trigger or dinged input on the Arduinos. An opto-isolator would provide more protection, but if something works...

The PIRs were only connected to the Arduinos for a few months, before I went for a DSC security panel with Spud's Envisalink plug-in. My 2 driveway hybrid motion detectors are still connected via Arduino direct input connections.

[petez69]

Hi Randy

Thanks for the info. I wouldnt suspect there to be any issues with reliability. What I was thinking of unshielded cable running around the house that could be susceptible to static induced by low humidity or lightning.....

I may just put a cap across the input...

I just cant think of a more elegant way to isolate the input with a N/C input. If you do the maths, putting 20 resistors in adds 200ma current draw. As I ultimately want to use this as a pseudo alarm system it will need a battery backup so I'm trying to reduce current draw...A cap only would certainly not add to the current draw ;-)

Thanks...

Pete

[Raptor]

generally you would only opto isolate when dealing with higher voltages where you don't want them to mix. you can use a transistor to switch the input on the arduino if ever.

[randy]

Hi Randy

Thanks for the info. I wouldnt suspect there to be any issues with reliability. What I was thinking of unshielded cable running around the house that could be susceptible to static induced by low humidity or lightning.....

I may just put a cap across the input...

I just cant think of a more elegant way to isolate the input with a N/C input. If you do the maths, putting 20 resistors in adds 200ma current draw. As I ultimately want to use this as a pseudo alarm system it will need a battery backup so I'm trying to reduce current draw...A cap only would certainly not add to the current draw ;-)

Thanks...

Pete

We aren't Florida, but Colorado is in the top ten states for lightning strikes. I have unshielded runs of up to 50-75 feet connecting a N.O. contact to one of four Arduinos and the simple snubber cap with a 100ms debounce has prevented any false triggers. I also have not had a dinged input on an Arduino. It is my understanding that the digital pins on Arduinos are reasonably rugged when it comes to ESD spikes, etc. A 5V zener diode would give an order of magnitude more protection, especially if coupled with a series diode for isolation on the input.

For digital inputs I use the snubber cap, LEDs and diodes like the first image below from the Velleman shield, which gives very good protection coupled with the advantage of an LED to show the state of the input. The cap is from the cathode side of the diode to ground.

I use a 5V zener across analog inputs with a 4.7K resistor coupling the input since the analog inputs are such high impedance. It is the same as the second image below from the Velleman shield.

[petez69]

Thanks Randy...

Exactly why I started this thread, a flood of GOOD IDEAS !!!! I think I'm going to go down your track, its simple...

I'm having a ball with the arduinos, appears that all 6 of them are stable with no ethernet issues. Just waiting for endless supplies from China for each node...ie: case, terminal strips etc etc :-) I'll take a photo when it all shows up.

I'm waiting for a weigand reader to try with the API, should be fun !!!!

Pete