View Full Version : Sensor Grid

07-27-2009, 02:05 PM

I am working on a dispenser of a vending machine. The prize has to fall through a grid of sensors (24" x 24"). I tried using low angle(10degree) infrared LEDs along with TSOPs. But the problem is that because of the distance the LED rays fall onto more than one TSOP, hence it is difficult to detect the prize.

I also tried using different burst frequencies (100 to 1000 Hz) for each pair, but it could not give accurate results due to interference.

I can use IR lasers, but it would be too costly and the operating life would be limited.

The size of the prize can be as small as 1"x1"x1".

Please suggest me any other method to deal with the problem.


07-27-2009, 02:25 PM
Why not use the cheap red lasers , with a few properly mounted mirrors to create a mesh and a photo transistor , like they show in the movies :) . That is if you wish to detect simple go or no -go. It will not give any other indication.

Should be very cheap.

Problem with TSOP is that if IR is encoded it can bounce around :) . But you can paint the inside black in that case.

07-28-2009, 02:23 PM
Thanks Vikas..

Mirror and laser mounting has to be very precise, thus would be difficult for someone while servicing.

I was thinking that using a laser is a good option. I can use diffraction gratings such that it produces a bright line, and place an array of photodiodes on the other side to detect it.

Do you think this will work??


07-28-2009, 03:38 PM
Mount the LEDs horizontally along one side of the grid. Each LED is individually controllable.

Mount an equal number of receivers horizontally directly in line with the LEDs on the opposite side of the grid. Each receiver is also individually controllable.

All LEDs and receivers are normally off.

Light up each LED and the corresponding opposite receiver. Check if the IR reaches the receiver. Switch it off. Move on to the next LED and receiver pair. And then to the next one... And do this over and over again, with scanning rates to be very high. You can then find out where the object is without any interference between the LEDs. You could also light up pairs which are outside of one another's interference region. For example, the first, fifth, tenth and so on.

Also, instead of one receiver per LED, you could have 3-4 receivers per LED with these mounted vertically. So that if the object falls and you miss it while you were checking some other LED down the line, a receiver below which is lower can catch the object in the next scan. In fact, it would make sense to have a wider beam LED for this arrangement.

Something like:


4 Receivers Object falling LED
Top to bottom

-] o



Receivers Object falling LED
Along the side
-] {-

-] {-
-] {-

-] {-

-] {-

-] {-

-] {-


One downside... you'll need lots of IO pins! But maybe you can multiplex.


07-28-2009, 07:32 PM
Use 2 laser line modules, For ex -

http://cgi.ebay.com/1-x-650nm-5mW-Laser-Line-Module-3VDC-Full-Brass-Case_W0QQitemZ360161897408QQihZ023QQcategoryZ53141 QQcmdZViewItem

Mount them so that they cover almost all the box and in the line make holes for photo transistors , And put all the outputs of these transistors through AND gates ( for ex quad and IC 4081 ) . So when something goes through output will become 0. And itll remain high when its normal.

So just one I/O line to detect , and the System will be Active Low, which is even better.

07-29-2009, 01:14 PM
Thanks again for your posts...

I wanted to ask one more thing, which of the following is a reliable and cost effective receiver:

For IR: Vishay TSOP1738, Sharp GP1UM26XK, Photodiode, Phototransisitor

For Laser/Visible Light: Photodiode, Phototransisitor


07-29-2009, 04:41 PM
1. Lasers are available for Rs. 20 to Rs 50 on the foothpaths. it sis teh best solution.

2. The next best method is to use a single emitter and 2or more Receivers. Use 2 or more encoded signals like the RC5 code from a remote control. The decoder will output the correct bit.
This will make the system foolproof, but with increased complexity.

Stick to the red lasers. You can use anything for the sensor, including an LDR.

Infact , an LDR LED combo works pretty well....... @ 24" :!: