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Sensing Line Voltage

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BrownOut

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I've been reading this forum for the last couple days. I'm sure this question has been asked and answered, but I have yet to find a thread that addresses it.

What I have are security lights that are activated by motion. The motion detector is connected directly to the light, and switches the lights on when motion is detected. I want to modify this scheme to connect the output from the motion detector to some logic, and then activate the lights with the output of the logic. The exact configuration of the logic is not yet determined ( I will post more about that in a follow-up thread when I have a schematic of the conceptual design ) So, what I want is to convert the 120V output of the motion detector to a safe, low-voltage dc. I would like to exploit the small size and economy of a opto-coupler for the conversion. My question is, does anyone know the most effective way to detect 120V to drive the led? I would like to keep the detector completely passive. Thanks.
 

ericgibbs

Well-Known Member
Most Helpful Member
I've been reading this forum for the last couple days. I'm sure this question has been asked and answered, but I have yet to find a thread that addresses it.

What I have are security lights that are activated by motion. The motion detector is connected directly to the light, and switches the lights on when motion is detected. I want to modify this scheme to connect the output from the motion detector to some logic, and then activate the lights with the output of the logic. The exact configuration of the logic is not yet determined ( I will post more about that in a follow-up thread when I have a schematic of the conceptual design ) So, what I want is to convert the 120V output of the motion detector to a safe, low-voltage dc. I would like to exploit the small size and economy of a opto-coupler for the conversion. My question is, does anyone know the most effective way to detect 120V to drive the led? I would like to keep the detector completely passive. Thanks.
hi,
I connect a short length of 3 core cable across the terminal connector inside of the PIR light.

At the other of the short cable I fit a free mains cable socket.

Plug into the free socket a wall wart power supply, voltage to suit the following logic, say 5V regulated.

You can leave the PIR light connected if required

So when the PIR is activated the wall wart psu is powered on.
 

BrownOut

Banned
Thanks

Thanks for your response. I'd like to clarify my question. I want to detect the output of the motion sensor and only output a safe, low dc voltage. I need something small enough to fit inside the small area available in the light's base. Most transformers or wallwarts will be too large. I would like to use a opto-couple for it's small size and low power consumption.

Also, I would like to restate my commnet about being completely passive. What I mean is, I want it to be self-powered. I don't want to have to connect the DC supply to the non-isolated portion of the detector. So, I would be OK using SCR's Triac's, etc. if that is necessary. If I can get away with passives only, then that would be better. Otherwise, I will design something with active devices.
 

ericgibbs

Well-Known Member
Most Helpful Member
Thanks for your response. I'd like to clarify my question. I want to detect the output of the motion sensor and only output a safe, low dc voltage. I need something small enough to fit inside the small area available in the light's base. Most transformers or wallwarts will be too large. I would like to use a opto-couple for it's small size and low power consumption.

Also, I would like to restate my commnet about being completely passive. What I mean is, I want it to be self-powered. I don't want to have to connect the DC supply to the non-isolated portion of the detector. So, I would be OK using SCR's Triac's, etc. if that is necessary. If I can get away with passives only, then that would be better. Otherwise, I will design something with active devices.
hi,
Look at this circuit, it should fit inside the PIR housing.

NOTE: the driving side of the Opto-isolator is NOT isolated from the mains supply voltage.
 

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BrownOut

Banned
That's very cool. I only need the output of the opto isolated. At first I thought c2 might be huge and expensive, but digi-key lists some polyester caps that will just squeeze in, and only a couple bucks a throw. That value gives for rms current:

1/2Πfc = 1/6.28*60*.47µ ≈ 5.6K for the reactance, and for RMS current at the opto:

60/5.6k ≈ 10.7 ma. That should do it. :)
 

BrownOut

Banned
Oh, and BTW, I found this on another thread, in case anyone is interested. It's sort of related to my original problem. This circuit senses and indicates when an appliance it connected and energized!
 

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Leftyretro

New Member
Why not unscrew the lamp and drive the sensor's 120vac lamp output to a 120vac relay coil, then you are to use the relay contacts anyway you wish?
 
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BrownOut

Banned
Why not unscrew the lamp and drive the sensor's 120vac lamp output to a 120vac relay coil, then you are to use the relay contacts anyway you wish?
That's not a bad idea, but...
Because I want to continue to use the lamp. I only want to sense the output of the detector, and then use programmable logic to decide what, if any, other appliances I want to energize. As I said earlier, I will be posting a follow-up explanation of my scheme/idea, but basically, I want to gather input from several sensors, and programmically switch on lights and/or alarm circuits.

I also want to have the option of switching the lamps on for my own activities. I'm getting tired of having to wave my arms around every few mintues when I'm out in my yard at night :)
 
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Leftyretro

New Member
That's not a bad idea, but...
Because I want to continue to use the lamp. I only want to sense the output of the detector, and then use programmable logic to decide what, if any, other appliances I want to energize. As I said earlier, I will be posting a follow-up explanation of my scheme/idea, but basically, I want to gather input from several sensors, and programmically switch on lights and/or alarm circuits.

I also want to have the option of switching the lamps on for my own activities. I'm getting tired of having to wave my arms around every few mintues when I'm out in my yard at night :)
Well you could wire the new 120vac relay's coil to the wires going to the existing lamp's socket so that the relay would activate whenever the lamp is on, and you are still free to use the contacts for whatever and the existing lamp will still function as before.

Lefty
 
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ericgibbs

Well-Known Member
Most Helpful Member
I also want to have the option of switching the lamps on for my own activities. I'm getting tired of having to wave my arms around every few mintues when I'm out in my yard at night :)
hi,
I guess you know that for most PIR security lights if you quickly toggle off/on the mains supply to the PIR, the light will stay on.

To switch back to sense control mains, power down for a few tens of seconds.

You can buy 4 core mains cable, which can be connected to all 4 terminals in the PIR connector strip.
This allows remote detection when the lights is powered on and also allows remote control of the light [4th wire].
Just keep it all on the same mains phase.

This is the method I use, as described I have a small low voltage dc psu inside the house which drives logic and alarms.
 
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BrownOut

Banned
Thanks guys. I guess I need to take the time and draw up a schematic of what I intend to do, so that it will be more clear. My first issue was to convert the output of the motion detector to a safe level that I could route through my house using small wire i.e. intercom wire. I think I have some really good input on how to do that now. The rest is easy; I'm just going to glue up some logic that energizes the lights in the manner that I specify (schematic to follow) As far as keeping the lights on when I want, I just intend to connect the lamps directly to mains, bypassing the detector.
 
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