How to change a NC circuit to a NO one?!

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stuffed

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Master Geek
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Topic # 95120 28-Dec-2011 13:32

Hey you troops – have a alarm sensor that only has a NC (Normally closed) output. My DVR alarm inputs are very difficult to understand but have got the NO (Normally open) circuit to work ok.

What sort of relay (or whatever) can I get to take the NC circuit from the sensor to create at NO circuit?!

Thanks

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Uber Geek
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Reply # 562166 28-Dec-2011 15:44

what kind of sensor is it? and what is the output (if known)?

stuffed

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Master Geek
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Reply # 562173 28-Dec-2011 16:06

Bosch Passive Infrared. 125mA, 28VDC, 3W.
Hope that helps.
Many thanks

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gzt

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Reply # 562175 28-Dec-2011 16:57

got a model number?

gregmcc

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Reply # 562184 28-Dec-2011 17:31

stuffed: Hey you troops – have a alarm sensor that only has a NC (Normally closed) output. My DVR alarm inputs are very difficult to understand but have got the NO (Normally open) circuit to work ok.

What sort of relay (or whatever) can I get to take the NC circuit from the sensor to create at NO circuit?!

Thanks

You need an interposing relay, using the NC contacts from the sensor, use this to keep the replay powered via the power used to run the sensor, you will need to figure out what the voltage is, most alarm systems are 12VDC, but you do need to check this to make sure you get the right voltage relay. Now you have a set of contacts from the relay which are changeover, both NO and NC and you can use this on the input on the DVR

Greg

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Reply # 562188 28-Dec-2011 17:38

Not my area of expertise BUT it may be possible that the PIR can be made to give a NO contact.

The PIR's we have here at home emit a light clicking sound when activated, I presume that noise is caused by a relay being operated. Most relays have a change over contacts effectively giving a NC and a NO. If this is the case you just need to pick up the other contact and use that. On some relays they are labeled NO or NC

The other way is to use an external relay. In this situation the relay will be operated until the contact in the PIR operates so you will need to use the NC contact to achieve your aim. There are two downsides to using an external relay, one is the extra power (probably only a few mA) consumed by the external relay being operated for most of the time, the second is a failure of the coil - open circuit - (though unlikely to happen often, or at all) will trigger the alarm

If you choose to use an external relay you will need to choose the relay based on the amount of current that the PIR contacts can handle. i.e the coil resistance will be important.

I see Greg posted while I was replying. Good point also on the voltage though I thought I saw 28 v mentioned somewhere.

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gzt

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Reply # 562191 28-Dec-2011 17:51

Can you configure your DVR for NO/NC? Many have that option already.

Careful with that voltage difference though.

Consider getting some exact model numbers for your DVR/IR equipment, and posting links to some manuals if you can find them online..

stuffed

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Master Geek
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Reply # 562363 29-Dec-2011 08:48

Thanks for your help - attached are the details of the IR Detector

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Reply # 562380 29-Dec-2011 09:55

From the picture you've posted I'm guessing that NC is the industry standard or you buy the PIR's for NC or NO configuration.

Can you provide more info on your DVR inputs set up, I suspect there will be something there that can be configured to make it all work. What make/model is it? Perhaps there is some info on line that may help.

Failing all that, the external relay will need to have a coil of at least 100 ohms (96 to be exact), I would go for around 200 ohms, assuming you use a 12 volt relay since I guess the rest of the system is running on 12 volts.

The best solution will be to get the DVR configured to operate with NC.

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Master Geek
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Reply # 562388 29-Dec-2011 10:50

Attached are the DVR input instructions – find them as clear as mud but got the NO circuit working on terminals 1 and 10 (this DVR is a 4 channel one).
Thanks

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Reply # 562398 29-Dec-2011 11:26

Just reading bottom of that sheet. It says 3.7.2 Alarm input port. Please refer to the following sheet for more information. See figure 3.8. Normal open or normal closed type.

I think the next page with fig 3.8 might reveal some useful info. Can you post that?

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Reply # 562482 29-Dec-2011 14:48

Hope this completes the picture - thanks.

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Technofreak

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Reply # 562528 29-Dec-2011 16:46

Can you provide make and model of the DVR? Is there an on line manual for it?

From a quick look at what you have posted so far, more information is needed to ensure the PIR is connected correctly. It does appear that NO and NC connections are allowed for, it's just a matter of getting enough information to figure out how and so far it appears you haven't posted enough information to work out how to make the connections. Admittedly as I said I've only had a very quick look so far.

Would I be correct in saying the DVR is of Asian origin, just looking at some of the info on the pages you have posted would indicate there is some language translation issues.

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Master Geek
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Reply # 562541 29-Dec-2011 16:57

Correct – it Asian – Dahua – excellent product – this one is for home use and have a 32 channel one at a local enforcement agency.

This is their web site: http://www.dahuasecurity.com/index.aspx

Have been told that they have more than 1000 people in their R&D department!

Many thanks

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Reply # 562593 29-Dec-2011 18:19

What model is it?

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Reply # 562625 29-Dec-2011 19:40

Right, I think I have it sorted BUT theres no way of knowing until you try it.

I'm assuming negative earth here and that the PIR is powered from the same supply as the DVR.

You need to connect the -ve tag (which will be Earth) on the PIR to one of the relay tag, the other relay tag needs to be wired out to one of the alarm inputs on the DVR. The PIR of course will also have the +ve tag wire to the power supply.

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