Picture of Xanboo/Homesite Laser Break Beam Sensor
I want a Hollywood style laser beam sensor to play with. The problem is that I have a pile of Motorola Homesight cameras and sensor, but none of them have lasers! This project documents my trials, failures, and successes in building a laser sensor out of spare parts I wasn't going to use while getting the Motorola Homesight software to recognize the homemade sensor. The Motorola Homesight consumer home security products are a rebranded version of the Xanboo products. They are virtually identical.

I will be gutting the camera and using the plastic housing to mount the laser. Since I'll be destroying the camera, I decided to use one of the "wired" cameras. The wireless cameras are still quite useful to me, so I've put them off limits for my projects...for now. The water sensor will be used as a contact/no contact interface into the Homesight system. I used a water sensor rather than a door or temperature sensor because I won't really lose anything if I fry it during my experimentation. I still find the door and temperature sensors useful. The challenge is to build a small circuit that can open or close the sensor's contacts based on presence/absence of the laser light and squeeze that circuit into the battery compartment of the water...er...I mean, laser sensor.

I should mention that I will be using a laser ripped out of a really cheap laser level that I found on clearance for ~$0.50. Cheap. You get what you pay for when dealing with lasers. In this case, that's a good thing. If you hook up a really powerful laser to this, you'll burn through your sensor, your house, your neighbors house, potentially setting fire to your sensor, your house, your neighbor's house. Heck, you might get lucky enough to blind your intruder or slice his legs off at the knee, or burn the hair off the neighbor's cat, etc. The risks outweigh the rewards though, so just go with your typical laser pointer style laser. K?

Actually it's LDR (light dependent resistor), and I think a photo-diode would've been better in this case, the LDR is quite slower than the photo-diode (or even a photo-transistor).
RetroPlayer7 years ago
If you use a phototransistor instead of a CDS sensor, you can eliminate most of the parts in your circuit. The best way to do this (with either the CDS or phototransistor) is to use a comparator like the LM339. You set the negative input for what voltage you want to trigger on, and connect your sensor to the other. The phototransistor changes in voltage (actually a voltage drop across it) and the CDS changes resistance, which changes current. A second resistor in series with the CDS is used to pick off the voltage level. BTW, if this system is like the older Xanboo system that I had (the camera is almost identical) then you really didn't need to use the water sensor at all. The "signal" is just a switch closure and the circuit in the water sensor (or the motion sensor in the camera) just translates the sensor into a switch closure using a comparator like I mentioned above. Also, IIRC, the cable is the same as a PS2 keyboard or mouse. So, you should be able to tear an old KB apart and snag the cable for your project. My camera had the power come in on this cable, though, so there is a 6 or 12V power signal on these wires. Maybe your camera just required more power? Anyway, let me know if you need any help finishing this up.
This explains how to use a phototransistor as a simple switch.

A comparator, however, will allow you to adjust sensitivity so it doesn't trigger on bright light alone.
krich (author)  RetroPlayer7 years ago
Thanks RetroPlayer. I am seriously considering the phototransistor route if my second try using the relay doesn't work well. My goal was to only use parts that I have on hand. Unfortunately all the phototransistors and photodiodes I have in my tacklebox have those visible light filters integrated into the package. I'm going to have to buy the ones I need. I've got a shopping list, so once it gets big enough, I'll be rectifying that situation. I'm out on business for the next two weeks, but I'll be picking this project back up again when I get back in mid-April. Thanks again!
Alright, just PM me if you need some help.
I have found this circuit at:


For a science fair project, I have to activate a 26 LED array to show the power of a magnet array from 4 feet away.

I discovered that my magnet array can bounce a simple compass needle from 4 feet  - -  so why not use a laser diode hacked from like a CD player, with a pinhole cover to give a nice tight beam to shine up through the plexiglass onto the underside of the 3/32nd's compass needle, then have the phototransistor above to catch the laser beam (when the judges show up at the table) and I point the magnet array at the hidden sensor behind a rock and "bounce ithe needle" back and forth midair . . . .

Not looking forward to having to orient, pinout and deal with a LM339 or equiv.

Can't I just use a phototransistor and a micro relay ?

Is there a simplier way to do this with photodetection means ?

My LED array appears to draw about 200 millamps at about 4.5V from three AAA cells  - - that's almost 1 full watt  - -  pretty close to the max limits of a LM 339  - - - maybe I would need a micro relay thrown in anyway ?
Actually, the LM393 would probably be more suited for your purposes. Though Rat Shack usually carries the LM339. Take a look at the datasheet for the "typical applications" to see how to use it.


One reason your circuit may be finicky is that you must completely "saturate" the transistor for it to act like a switch. This means that the base current should be maxed out when you want it to switch. If you are in the linear range, it may trigger just from electrical noise or it might not trigger at all.
PKM7 years ago
Phew.. I thought this was going to be another of those spam advertising 'Ibles about how great this particular home security kit was. By the way, the first sentence of your intro step makes it sound a wee bit like a sales pitch, and that's all you see in the preview so you may want to consider shuffling that around a bit. Anyway, it's a well detailed write up, seems to be fairly practical and informative, and you spell voila correctly so I'm happy :) +
krich (author)  PKM7 years ago
Excellent feedback. I'll make the change to the beginning. Thanks!