Electronic Alarms and Home Security Devices - Basic Motion Detector Question

Greetings and salutations,

I'm looking to get into the basics of alarm systems/home automation. I've acquired an Ademco motion detector, which has three screw terminals ( "+" "-" "R" and "R" ). I hooked it up with a 12v power source, pos and neg. When plugged in the LED bulb lights up indicating the infrared detector is warming up. I understand how relays work, and how the detector closes a circuit, but there are a few things I am unclear on:

1. Lets say I want another led bulb to turn on when the detector is activated. Would I connect the + source to one of the relay terminals with the bulb as a load? I tried this and got nothing. Is this the concept behind it though?

2. Am I right by assuming there is an open circuit that closes, bring the positive to the "R" terminals?

What I am looking to do is whenever the motion detector is activated, it turns on a 120v AC luminaire, for all those drunken nights walking blindly into a dark room. I know that I need a relay and all but first things first.

I apologize if this has been asked many times before, I'm new to all this and am taking my first (and last) year of "BOCES". I'm looking to become an electrician first and want to explore all different aspects of the electrical field.

- Jimbo

A typical Motion Detector has 4 terminals just as you stated "pos,neg,com, N/C" some device have double pole contacts with common, open and closed.
With most residential Motion detectors they detect motion in range of the device and the contacts open. I would doubt that a generally used detector would have a relay that could handle any kind of strong voltage.
I know that some outdoor lighting system come with specially designed motion sensors with strong relays built in for both low light level trigger and motion trips for 110 power lights.

I'm not looking to use it as a relay built in, I just want voltage to come out of the terminals so that it can activate an external relay.

The point he's making is that _most_ security type motions are set up with normally closed contacts that open on detection. There are a few, more expensive models, with a "form-C" contact that has both NC and NO. What you want for your purpose is NO.

That said, the current rating of the contacts in a motion is tiny. Even the actuation current for a lot of small relays are pushing things.

With a careful choice of hardware, you could make the motion do what you want, but for less money and time, I'd just buy one of motion detector wall switches, especially if there is a switched receptical in the room in question, if not, there are ways to make one of those into a freestanding lamp control.

The relay terminals in your motion sensor are typically isolated from the (+) and (-) terminals. The two (R) terminals are simply the contacts of the motion sensor’s internal relay. As for using an LED for concept testing, you may already know this, but LEDs are polarity sensitive. If the LED doesn’t work, try reversing it’s leads. Don’t forget to include a current limiting resistor in series with your LED otherwise it will go PIFFFF! A 1,000 ohm resistor should be a safe value.

The following hook-up should do the trick:

1) Run a jumper wire from the (+) terminal to one of the (R) terminals

2) Run a wire from the remaining (R) terminal to one side of your LED (with current limiting resistor)

3) Run the other side of you LED to the (-) terminal

You can substitute the LED with a low current relay to operate higher current items. Keep in mind the coil of the external relay must be very low current to prevent damaging the 12 volt power source, wiring, and motion sensor’s internal relay contacts.

Have fun!
Pizzazz

It all makes sense now, I'll post how it turned out. Thanks for the explanation

The following hook-up should do the trick:

1) Run a jumper wire from the (+) terminal to one of the (R) terminals

2) Run a wire from the remaining (R) terminal to one side of your LED (with current limiting resistor)

3) Run the other side of you LED to the (-) terminal
The only problem with that is, as was mentioned above, the motion detector is a NC deviced - so the LED would be ON when no motion was detected, and OFF when motion was detected.

Now, that being said, if you connect to an external relay, you could hook it up however you want - NO or NC. So when the motion goes OPEN, it would CLOSE the relay.

The only other thing that hasn't been mentioned is that you have no timer on the system. If you setup the ways that have been mentioned so far, then when the motion detects you, the lights turn on. As soon as you stop moving, or walk around a corner or out of line-of-sight, the lights turn off. Add a simple Timer Module to the circuit and you can turn the lights on for X amount of time when motion is detected.

Or, the better solution, get the motion detector light switch. The above circuit is just too much trouble and is going to cost more than it's worth.

Good luck!