Monitoring circuit continuity / interrupt on break

FredrikNyman

I have a passive infrared (PIR) detector I'd like to interface with a MSP430. The PIR has two wires that normally form a closed circuit. When the PIR is triggered, the circuit opens.

What I would like to do is to have the MSP430 monitor the circuit while asleep, and have an interrupt fire when the circuit opens. Any suggestions about how to implement that?

Thanks!

Fredrik

over 15 years ago

0 ox over 15 years ago

Prodigy 230 points

have a look at this:

http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slaa283a

0 old_cow_yellow over 15 years ago

Guru 58965 points

I think the PIR you are using is much simpler and slaa283a does not apply.

You could connect one of the two wires to a pull-up resistor and the other wire to a very weak pull-down resistor and one of the P1 or P2 pins. When the PIR loop is closed, the pull-up will over-power the very weak pull-down and your port pin will see a high. When the loop opens, the very weak pull-down alone will make your port pin go low. P1 and P2 port pins can be programmed to generate an interrupt to detect high to low edge.

0 Fredrik Nyman over 15 years ago in reply to old_cow_yellow

Prodigy 50 points

Thank you so much! Yes, you're absolutely right. The slaa283a appnote would have been perfect if I'd been interfacing with a naked PIR sensor. But I am working with a retail sensor from GE that contains a MCU and which presumably does the task described in the appnote.

Your solution is very elegant, and exactly what I was looking for. Now I'm embarrassed I didn't think of that myself! :-)

Much appreciated!

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Monitoring circuit continuity / interrupt on break