How to combine a PIR (motion sensor) and high current (>15A) 12V DC switch

Thread Starter

DenverMark

Joined Sep 7, 2019
2
How would one combine a low voltage (12V DC) motion sensor, with a low voltage (12V DC, but high current (>15A)) switch? And is there a recommendation for a specific high current 12V switch to use? I find motion sensors for sale, but I am ignorant on the subject of relay switches and the combination therefore isn't obvious to me. I have searched and searched for a DC sensor/switch to buy, but do not find anything off the shelf. Many options exist for the 120V AC world, but the 12V DC world appears underserved by manufacturers. I would like to construct my own combination for this and future applications, since I have become an LED convert. Thank you for thinking about it.

Some background: After completing a large project with 12V led light strips (36 feet of them) for a walk-in closet, I had the idea to use a motion sensor as a switch, one switch for all lights, rather than a mechanical switch.

My problem is that, after a great deal of research, I cannot find a combination Passive Infra Red (PIR) sensor and switch which can handle the current I need to draw (12 Amps, 143 Watts) AND which is for low voltage DC (12V). There is a large array of very cheap 12V sensor/switch combinations available for sale on Ebay and Amazon from Chinese (I don't know why, but then why not) outfits.

There are a few of these which are advertised for high current/power, but actually (I have tried three) they quickly heat up and begin to melt (false advertising). One or two I have found for sale simply don't work well (about what would one expect from a less than $10 sensor-switch). Surely this is a simple problem for a sensor/switch expert, but that person is not me (yet). Thank you for thinking about it.
 

Reloadron

Joined Jan 15, 2015
4,517
I would get a 12 Volt sensor pack like this. Use one or both to drive a 12 Volt Automotive Relay to drive the heavy LED lighting load. Typical 12 volt operated relay coils are 35 Amp relays. Any automotive store will have the relays and sockets. You could also just let the PIR activate a MOSFET but I would just run with a PIR driving a contrl relay.

Ron
 

Tonyr1084

Joined Sep 24, 2015
3,584
@Reloadron Pretty nifty PIR. Note, it's rated for 10 to 99 watts. So at 12 volts, (99 ÷ 12 =) 8.25 amps. That's not quite the amperage you're asking for, so you'll have to use that PIR to trigger a higher amperage switch like a relay or an IGBT. For the moment I'm going to trust you know how to wire relays. Automotive relays are typically good for 40 amps on their contacts. That should fit your requirements, and is easy to wire up.

[edit] just read more of the sensor output capabilities. It clearly says it's rated for six amps. {end edit}

At 24 volts, 99 watts is the equivalent of 4.125 amps. Even at that low amperage an automotive relay will not draw that much current. HOWEVER, we're talking about 24 volts. It's likely you'll burn up the relay coil. A good source for automotive relays is your auto wrecking yard. You're getting used relays, but they're from automotive manufacturers. Not likely to have been made in China where they over-rate everything. Well, almost everything.

The output from the PIR is wired to the coil of the relay. The relay contacts are then used to switch whatever load you are wanting to switch. Just stay within the ratings of the relay. If it says 40 amps at 12 volts and 20 amps at 120 volts AC, don't exceed the contact ratings. (BTW, I made those numbers up - actual numbers will vary)
 

Thread Starter

DenverMark

Joined Sep 7, 2019
2
I would get a 12 Volt sensor pack like this. Use one or both to drive a 12 Volt Automotive Relay to drive the heavy LED lighting load. Typical 12 volt operated relay coils are 35 Amp relays. Any automotive store will have the relays and sockets. You could also just let the PIR activate a MOSFET but I would just run with a PIR driving a contrl relay.

Ron
Thanks a million Reloadron, for these two pragmatic solutions!!
After some homework on my own I can see that the automotive relay would be the easiest path to follow. However, after learning enough about the five pin relay standard to be dangerous - and just before I clicked "buy" on the purchase - I also learned about the "click". The automotive relays I find rely on an electromagnetic switch with comes with an audible click. Since my application is indoors I can already hear the complaints from the family members. (Personally I would find the click reassuring). Therefore I am taking the plunge: I am going to work on a MOSFET circuit. At the moment I am thinking of using the (IRFZ34N). I have already studied a great deal and I can see that I will need to be concerned about heating, but then God did not put me on His green Earth for pleasure alone, so I will work out a heat sink. I promise to report back once I have a circuit design and a working prototype, just to pay the Forum organizers back.

Regarding the PIR sensor: I have in fact tested the model you mentioned in the past, and it is not bad. However, I want to have (a) user programmable timing delay (potentially up to more than five minutes) before shutoff, and (b) option for daylight setting so that the lights don't turn on in the day time. For the moment I find this option. I caution that this is one of the modules I tried to load with 12 Amps/143 Watts and it became too hot to touch in five minutes. It may or may not prove trustworthy at low current.
 

MisterBill2

Joined Jan 23, 2018
3,884
The click of a relay is indeed reassuring, just be sure that you are not getting a horn relay, which is for short on times ONLY. They burn out if you use them to switch driving lights. And the click is not that loud, really. And since you can enclose the relay with the 12 volt power supply tit will not be noticed at all. And even the best semiconductor switch will need heat sinking.
 

Reloadron

Joined Jan 15, 2015
4,517
Actually I doubt you would hear the click since I never have noticed the relays on my truck clicking or any automotive relays I have used but then by all means just use a MOSFET. A N Channel general purpose MOSFET I have used pretty extensively is a FQP13N06 for LED strips which I have gotten from Spark Fun but they are generic and can be had anywhere. I use them for low side switching LED strips powered by 12 VDC.

The reason I like using an automotive type relay is they can be had at any parts store and come with easily adapted 1/4" spade lugs connectors. They are also easily mounted. Most will easily handle a 30 Amp DC load with low coil current making them easy to drive. The linked MOSFET will also handle 30 amps when heat sink mounted so either way, whatever works best for you.

Ron
 

MisterBill2

Joined Jan 23, 2018
3,884
Actually I doubt you would hear the click since I never have noticed the relays on my truck clicking or any automotive relays I have used but then by all means just use a MOSFET. A N Channel general purpose MOSFET I have used pretty extensively is a FQP13N06 for LED strips which I have gotten from Spark Fun but they are generic and can be had anywhere. I use them for low side switching LED strips powered by 12 VDC.

The reason I like using an automotive type relay is they can be had at any parts store and come with easily adapted 1/4" spade lugs connectors. They are also easily mounted. Most will easily handle a 30 Amp DC load with low coil current making them easy to drive. The linked MOSFET will also handle 30 amps when heat sink mounted so either way, whatever works best for you.

Ron
Relays are far more forgiving of temporary overloads and polarity reversals. Current and voltage spikes are usually not even noticed by the relays.
 

Reloadron

Joined Jan 15, 2015
4,517
Relays are far more forgiving of temporary overloads and polarity reversals. Current and voltage spikes are usually not even noticed by the relays.
Absolutely so I guess we could include that as a plus.

I have a little automotive relay lying here. It's a small cube about 1.0" square with a mounting hole. Looks like this one. So I applied 12 VDC to the coil and granted my hearing is not what it once was if I listen real hard I can hear a faint click. Pretty sure if I taped a Kleenex around it it would be inaudible. However, short of a science experiment and dragging out more stuff I can't say how loud it actually is. So if the thread starter has click concerns then by all means go the totally solid state route. I just see a relay as a quick and simple solution. Either way should work just fine. There are literally dozens of PIR sensors available on Amazon just as an example which would easily drive either a relay or MOSFET solution. I guess it's just a matter of which solution trips ones trigger. :)

All of this reminds me I want to add a PIR sensor in the garage and I want to get that done before I am butt deep in snow. I actually have all the parts, just a matter of getting it done.

Ron
 
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