Guardian Troll

What to do with LEDs and a motion sensor

(Idea mostly by Ania Mitros (ania at klab dot caltech dot edu); built by Ania Mitros and Heath Hunnicutt.)

The problem: The light switch for my living room is across the room from the front door. When I walk in, I have to either leave my door open to the outdoors or stumble around in the dark.

The solution: A motion activated light that turns on when I walk in and stays on for a couple of minutes.

I considered adding a motion-activated switch to the track-lighting on my ceiling. However, redoing the wiring in my apartment seemed like a lot of work, plus the solution wouldn't be transferrable to another apartment. So I asked myself what kind of portable light fixture would enhance the aesthetics of my living room. Several pairs of white LEDs seemed like an ideal solution: nice color, low power, reasonable cost, long lifetime.

I bought a motion sensor from Radio Shack for about $20. This being a practical project, I wanted to use off-the-shelf components as much as possible to minimize the amount of time before completion. This sensor included a circuit with a motion-activated relay; control of how long the light stays on after motion detection (adjustable between 5 sec to 12 min); and control of the maximum light intensity at which the motion sensor will work (so it doesn't have to come on in the daytime).

I removed the light bulb sockets. I also opened up the sensor and soldered two power cables onto the wires intended for attachment to household wiring. One of the cables terminates with a plug to a standard electrical outlet. The voltage in the second cable is switched by the motion sensor and powers a 5V power supply. For safety, all 120V connections are within the plastic housing of the motion sensor or terminated with standard 120V plugs. The photos show the motion sensor mounted on my doorframe; and the wall plug.

The switched 120V power cable from the motion sensor terminates with a standard female power plug, which plugs to a standard 5V power supply. The output wires of the 5V power supply are soldered to eight RCA sockets: the outer conductor of each socket is at GND and the inner conductor at 5V. This allows the system to be modular, wherein I can plug in up to eight light fixtures, and can add or remove additional lights without taking apart nor resoldering anything in the existing system. The eight RCA sockets are epoxied onto the power supply. They were scavenged off the back of a broken audio amplifier. (The LEDs in the snowman's eyes are off in this picture.)

The initial setup included three guardians: a troll, a snowman, and a spider. Each of these has two white LEDs for eyes. A few more have been added, and future plans include a mushroom with little bitty LEDs as spots on the cap.

This circuit generates something around 12mA of current through each LED. A rough search around the Web suggested that white LEDs can happily handle up to about 20mA of current (more causes their lifetime to drop significantly).

I bought this doll in Sarajevo. She was labelled "Bosnianica" and her traditional folk clothing was supposedly sewn by war refugees. The final selling price was lower than what the seller originally claimed he paid the refugees, however, so who knows. In any case, she looks very cute with two super-bright blue LEDs for eyes.

The rat and pig both contain active circuits. Heath filled the rat's belly with a circuit that causes a delay of a few seconds in its eyes turning on. The circuit uses a large capacitor used to generate the necessary time constant. Ania built the pig. Its insides include a circuit based on a 555 timer chip, wired to generate a waveform output that oscillates at about 1 Hz. Since the 555 cannot source enough current to drive an LED, its output goes to the bases of an NPN and a PNP transistor. Each of these transistors drives one ultra-bright red LED for each of the pigs eyes. The result: the eyes blink alternately.

Last updated Nov 24, 2002. © Anna Mitros
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