View Full Version : Easy concept for breaking a 'beam' to trigger Prop?

T Chap
09-13-2011, 08:16 PM
I have an application where I need to detect a person walking through an area. A regular motion sensor will not work as it will pick up people outside of the area to monitor. The Ping is perfect in most cases, except in this case it cannot be mounted in the cabinets as the two metal parts would not be acceptable cosmetically, nor can the Ping function if recessed. If recessed, the sound reflects back too easily and false triggers. Mounting the Ping on the ceiling is not an option.

Floor mats are not an option. Another idea that has been accepted is to drill a 1/4" hole in both cabinets, and shine a laser diode(pointer) through the opening from one side to the other, and detect the light on the other end.

The use of hands is not an option, it has to detect the person without an effort being made by the person.

Can anyone think of another method to detect a person within 3 feet of some cabinets without having a visible sensor sticking out?

If using the laser diode(LED actually?), what is a good method to detect with the Prop as the controller?


09-13-2011, 08:22 PM
Have you looked at the X-Band Motion Detector (http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/sensor/List/0/SortField/4/catpageindex/4/ProductID/606/Default.aspx)?

It looks like the sensitivity is adjustable. I have not used one yet, but I hear they can be used behind a wall.

09-13-2011, 08:48 PM
One of the Sharp IR rangefinders could work. A bit smaller than a PING. You'd have to recess it and cut a rectangular hole (nibbling tool) in just ONE cabinet. Various units have analog or digital output, both can work with a Prop. See http://www.junun.org/MarkIII/Info.jsp?item=73 and http://www.parallax.com/dl/docs/prod/acc/SharpGP2D12Snrs.pdf

Or, instead of a laser you could have an IR LED pulsing at 38 kHz and a receiver module on the other side for an optical break-beam detector.

Have you tried shielding a PIR sensor in a tube? And/or removing the fresnel lens? Some kind of blinders or shuttering could be applied here.

T Chap
09-13-2011, 09:08 PM
I have tried parallax PIR before with tubes, but it would require more of a hole than is allowed to work properly. My experience with the PIR is that temperature flux can cause false triggers, and this has to be very stable.

I will hook up and test the 38k devices now, I have used them very reliably in short distances ( inches ) to detect a break, but we are talking about 8' in this case, seems the reflections could be an issue. Will let you know in a bit.

09-13-2011, 09:17 PM
I would use a laser pointer to transmit a light source to a phototransistor, photoresistor, or photodetector. This would be very simple to implement using the propeller. Just monitor the pin that the photodetector is located on, it will go high or low depending on the type of detector circuit. All this would cost about $5 bucks

T Chap
09-13-2011, 09:22 PM
38K didn't work well past a few feet. Maybe need to drop the 470R down a bit to give it a little power, but still I don't think the distance will be workable for a reliable break.

BTW, this is 3" off a marble floor, distance between cabinets is 80".

I think the laser idea might be the best idea, very directional, doesn't pick up the door in the vicinity.

09-13-2011, 09:37 PM
Would you like a schematic? I can draw you up one tonight if you like.

T Chap
09-13-2011, 10:15 PM
I found a TIL99 device laying around, a 2 pin device, online it is called both a photo diode and a photo transistor.

Connect the TAB pin to GND. Connect the other side to a 10K, the 10K to 5V. At the junction of the pin to the 10K it reads 5V, and with the pointer on it it reads 0.2V. Seems like a nice solution. Just need to build a mount for the laser/led device to allow pointing it.

Thanks for the advice.

Duane Degn
09-13-2011, 10:27 PM
I'd worry about using a laser. What if someone looked in the hole the laser came out of?

A 38K modulated IR should work at 80". My TV remote does.


09-13-2011, 10:42 PM
38 kHz will easily work for 20+ feet. Depends on your IR signal intensity. Per Duane, millions of IR remotes in use can't be wrong. Note that there are umpteen types of 38 kHz receiver modules, most will NOT accept a continuous (unmodulated) signal for more than a second. If yours won't, try my beacon schematic at http://www.botmag.com/articles/scribbler.shtml which will let you count pulses with a Stamp or Prop.

PJ Allen
09-13-2011, 11:48 PM
It's break-a-beam (http://hackaday.com/2011/09/11/laser-trip-wire-in-an-easy-to-use-form-factor/) time.

09-14-2011, 12:21 AM
I'd worry about using a laser. What if someone looked in the hole the laser came out of?

Well... they'll learn, won't they? :-) No actually, laser pointers in the US are less than .5mw in strength for a reason... it's actually safe! No permant damage will occur if someone happens to look directly into the beam of a typical Class I laser pointer. They might be temporarily blinded while their retina readjusts to the normal light levels, but no tissue damage will result.

Class II and greater lasers require FDA approval for use, so you won't generally find them available and they would certainly be way over powered for this type of application. (In fact, a Class II laser could be used for over 1/2" mile detection, if needed. A Class I laser will work for 50 feet in the daylight.)


Duane Degn
09-14-2011, 03:33 AM
... it's actually safe! No permant damage will occur if someone happens to look directly into the beam of a typical Class I laser pointer.


I take your word for it. So how long can I stare directly into the beam?

I think I've heard that Class I pointer are supposed to be "eye safe". They just seem so darn bright.

I've watched kids who get a new flashlight as a present. One of the first things they do is hold up the light directly in front of an eye (pointing into their eye). (I've seen this happen many times.)

I just worry about kids doing dumb things with a laser (I think it's a good thing I didn't have one as a kid).


T Chap
09-16-2011, 07:58 PM
So I had some laser pointer modules laying around and also this device TIL99, the laser needed a method to precisely adjust it to hit the sensor, so I machined some delrin parts to make an adjustable plate. The oak base will sit inside of some other hardwood in a molding on the floor below some cabinets. I mentioned the circuit already, very simple case of the TIL99 acting like a variable resistor, in conjunction with a 10k the output is 5v or .2v. I thought I'd post the final result of the gadget.

Duane Degn
09-16-2011, 08:35 PM
Wow. That really looks nice.

I like the adjustable laser. Thanks for taking time to post pictures.


T Chap
09-17-2011, 03:21 PM
Question for you all. What would be the best chance of extending the life of the pointer? Pulsing it on and off a few times a second, so it is off 50% of the time. Or leaving it on?