Log in

Next Entry

Penguin Alarm

I work in an office that has a tall divider blocking the entrance. It is a little difficult to explain, but suffice to say that we cannot see people when they enter our office, which can be a big deal. We have a few devious overlings [sic] who like to sneak into our AO (area of operation) to make sure that we are working. But sometimes we are actually too busy to notice when people walk in, and since we can't see them we needed some way to track their entrance. I implemented a closed-circuit camera that has a direct view of the doorway from which people normally enter, but the lights are often off in this area which makes it difficult to see people who are sneaking. And again, sometimes we are too busy to notice when someone walks in, even if they have to pass through camera-view to get to us. What we needed was an alarm to notify us of when someone crossed the threshold. Something like a tripwire would have to be constructed.

The first thing that came to mind was that is obviously had to be IR. We didn't want to use LASERs because not only are they expensive, but they can be seen and have a very small coverage area. IR LED's are low voltage, cover a wide area with light, are outside of the spectrum of human sight, and are ubiquitous since they can be acquired from broken remotes, inside mice and all kinds of other devices.

Next I had to find a way to trigger something using an IR detector. I have lots of IR detectors laying around from broken VCRs and TVs and who knows what, but I had never used on in a circuit before. I had no clue how to identify them since they had no marking on the case, and I didn't know if there was a standard pin-out for each of the legs. I understood that one leg had to be power in, one power out, and one signal out. But which leg is which? After some research I found a website that looked promising:


I constructed a modified version of this circuit using one of the detectors I had which only included the half with the IR detector and L2 (which should be D2 since it is supposed to be an LED). When I shined an IR LED on the circuit I saw no change. After some troubleshooting and testing I was sure that my construction was correct, but the circuit just wasn't working. Maybe it was because the detector I used was a different part number than what was expected. I abandoned this circuit.

After checking out Hack-A-Day for a while hoping for some good ideas, and scrounging the net for a good circuit that I could build that didn't require a 555 timer I finally came across something that I could use. HAD posted a story about breaking open McDonald's toys to get to the goodies inside and use them for other projects. Link:


I had a few of the Avatar toys from McDonald's and thought that I could possibly use the one that sensed light. The female toy had a light detection circuit on it that I could possibly use. Unfortunately I couldn't figure out how the toy works because if I cover it up it shines, if I shine a light on it it shines, if I use IR it shines, so there was no change. I guess maybe the circuit was faulty, or it was supposed to do something that I couldn't make it do. I had to abandon this idea as well.

But I didn't give up on the idea of using toys from McD's. After all, if I make it from scratch it is construction, but if I modify some prefab circuit it is a hack. The next day I made a trip there and got a Happy Meal. As it turns out my luck was positive that day because I happened to come across the absolute perfect solution to my problem. This:


I started by testing the detection circuit with IR LEDs. The reaction was successful and I found that if I increased the voltage to the LEDs and increased the distance the break of line-of-sight would trigger the alarm.

Next I had to construct the IR transmission portion of the alarm. This was achieved by hacking the end off of a 5V power supply and giving it an RCA connector for easy of use:

Next I constructed the IR LED pack using some LEDs recycled from old remote controls. I decided to use 3 in case 1 didn't provide wide enough coverage. The ideal voltage for LEDs is usually somewhere around 3.3VDC @20mA. I tried a 10Kohm resistor in-line with the LEDs to limit the current enough to keep them from burning out but still provide enough current to light up the entire span of the doorway. It didn't work. I decided to use a 100ohm resistor. It worked. I chopped up an RJ45 jack box  and threw an RCA connector on it to connect the power supply.  Yes, I used masking tape to insulate the conductors. I used double-sided tape to mount the pod to the wall:

Now that the transmitter is done, I started working on the receiver. I widened the aperture on the front of the penguin assuming that it would help with receiving more light since the span of the doorway is so wide.  Next I opened the back of the penguin using some precision diagonal cutters and removed the speaker. I soldered a cable to the speaker wires to run back to the office. I added some heavy screws to the penguin with hot glue for weight because when I set the penguin down it would tend to tip over:

I ran the cable back to the office and hooked it up to an externally powered amplified speaker. This was useful because I could change the volume to my liking and could hear the alarm sound in the office, even though the penguin itself was a good 100ft away.

Here's a picture of the penguin in place:

All photos can be seen in larger format on my flickr page:



( 1 comment — Leave a comment )
Apr. 10th, 2010 05:12 am (UTC)
Excellent idea mate- I think I'll friend you
( 1 comment — Leave a comment )


big brother

Latest Month

October 2010

Page Summary

Powered by LiveJournal.com
Designed by Teresa Jones